Monday, April 7, 2008

"The Great Debate" [1921]--Shapley & Curtis

"The Great Debate" Between
Harlow Shapley
Herber Curtis
In 1921

Harlow Shapley

Herber Curtis

"Although the 'Great Debate' is important to different people for different reasons, it is a clear example of humanity once again striving to find its place within the cosmic order. In the debate, Shapley and Curtis truly argued over the "Scale of the Universe," as the debate's title suggests. Curtis argued that the Universe is composed of many galaxies like our own, which had been identified by astronomers of his time as "spiral nebulae". Shapley argued that these "spiral nebulae" were just nearby gas clouds, and that the Universe was composed of only one big Galaxy. In Shapley's model, our Sun was far from the center of this Great Universe/Galaxy. In contrast, Curtis placed our Sun near the center of our relatively small Galaxy. Although the fine points of the debate were more numerous and more complicated, each scientist disagreed with the other on these crucial points.

A partial resolution of the debate came in the mid-1920's. Using the 100 inch Hooker Telescope at Mount Wilson, then the largest telescope in the world, astronomer Edwin Hubble identified Cepheid variable stars in the Andromeda Galaxy (M31) . These stars allowed Hubble to show that the distance to M31 was greater than even Shapley's proposed extent of our Milky Way galaxy. Therefore M31 was a galaxy much like our own. In the 1930s, the further discovery of interstellar absorption combined with an increased understanding of the distances and distribution of globular clusters ultimately led to the acceptance that the size of our Milky Way Galaxy had indeed been seriously underestimated and that the Sun was not close to the center. Therefore, Shapley was proved more correct about the size of our Galaxy and the Sun's location in it, but Curtis was proved correct that our Universe was composed of many more galaxies, and that "spiral nebulae" were indeed galaxies just like our own.

Another reason the 'Great Debate' is important is captured nicely in the book Shu, F., 1982, The Physical Universe, An Introduction to Astronomy, (University Science Books, Mill Valley, California) p. 286: "The Shapley-Curtis debate makes interesting reading even today. It is important, not only as a historical document, but also as a glimpse into the reasoning processes of eminent scientists engaged in a great controversy for which the evidence on both sides is fragmentary and partly faulty. This debate illustrates forcefully how tricky it is to pick one's way through the treacherous ground that characterizes research at the frontiers of science.""

Two Interpretations:

"The 'Great Debate': What Really Happened"


Michael A. Hoskin

Editor, Journal for the History of Astronomy

First appeared in J. Hist. Astron., 7, 169-182

Copyright 1976 by Science History Publications Ltd.

The meeting of the National Academy of Sciences in Washington on 26 April 1920, at which Harlow Shapley of Mount Wilson and Heber D. Curtis of Lick Observatory both gave talks under the title "The Scale of the Universe", has passed into the literature as "The Great Debate".[1] It is true that the two resulting papers [2] published in the May 1921 Bulletin of the National Research Council contain the best presentations of the opposing arguments in the current controversy over the dimensions of our Galaxy and the status of the 'spiral' nebulae. But these papers, even if read without comment or discussion, would have taken well over two hours to deliver and therefore cannot possibly represent the proceedings at 'The Great Debate', which took place at 8:15 p.m. with a Conversazione timed to follow at 9:30.[3] Nevertheless, most historians persist in treating these published papers as the verbatim record of a dramatic trial of strength, and so have created an historical romance. In what follows we draw on surviving archives to compile a more accurate account of what actually took place.

The encounter grew out of a remark which George Ellery Hale, founder and Director of Mount Wilson Observatory, made at a Council Meeting [4] of the National Academy of Sciences late in 1919. Hale suggested that an evening of the Academy meeting planned for the following April should be devoted to one of the annual lectures paid from the fund set up in memory of Hale's father, William Ellery Hale.[5] On 3 January C. G. Abbot, the Home Secretary of the Academy, wrote to Hale:

"You mentioned the possibility of a sort of debate, either on the subject of the island universe or of relativity. From the way the English are rushing relativity in Nature and elsewhere it looks as if the subject would be done to death long before the meeting of the Academy, and perhaps your first proposal to try to get Campbell and Shapley to discuss the island universe would be more interesting. I have a sort of fear, however, that the people care so little about island universes, notwithstanding their vast extent, that unless the speakers took pains to make the subject very engaging the thing would fall flat.... Are there not other subjects-the cause of glacial periods, or some zoological or biological subject-which might make an interesting debate?" [6]

It is a little surprising that the island universe theory of spiral nebulae- the claim that the spiral nebulae are galaxies in their own right and independent of our Milky Way star system-was to be defended, not by Curtis but by his Director at Lick Observatory, W. W. Campbell. For Curtis had been engaged for nearly a decade on the photography of nebulae with the Crossley reflector, and for much of that time had been an enthusiastic convert to the island universe theory; only that March he had dined with Hale in Washington within a week of lecturing on "Modern Theories of the Spiral Nebulae" to the Washington Academy of Sciences.[7] And on 8 October, when organizing the observing programmes for the 60in. and the new 100in. reflectors at Mount Wilson, Hale had written to Campbell to say "We are planning an extensive attack on spirals, with special reference to internal motion, proper motion, spectra of various regions, novae, etc., and here again I should be glad to know what Curtis has in hand, so that our work may fit in with it to advantage" (emphasis supplied).[8] Whatever the reason for the initial selection of Campbell as speaker,[9] by the time the question comes up again in correspondence Hale had received from Campbell a copy of the volume on nebulae published by Lick Observatory, in which "three splendid contributions"[10] were the work of Curtis, and thereafter Curtis and not Campbell is the projected speaker. Meanwhile, however, Hale in fact favoured relativity, but on this Abbot had many misgivings:

"As to relativity, I must confess that I would rather have a subject in which there would be a half dozen members of the Academy competent enough to understand at least a few words of what the speakers were saying if we had a symposium upon it. I pray to God that the progress of science will send relativity to some region of space beyond the fourth dimension, from whence it may never return to plague us." [11]

Evidently Abbot's views prevailed, for he cabled Hale on 18 February: "Am wiring Heber Curtis suggesting Debate him and Shapley on subject scale of universe for Academy meeting forty five minutes each suggest communicate Shapley and Curtis and wire if favorably arranged."[12] Curtis accepted, at first with marked reluctance, then with increasing relish at the prospect of battle. Shapley likewise accepted-Hale was his 'boss' and the invitation a compliment -but with deep misgivings, for his career was now at a crossroads. In February 1919 the death of Edward C. Pickering had at last brought to a close his fortytwo- year reign as Director of the Harvard College Observatory. Pickering had been an outstanding administrator. The obvious choice as successor would have been Henry Norris Russell, Shapley's sometime teacher and mentor and the only American astronomer with influence comparable to that of Hale, except that he lacked Pickering's administrative abilities. Kapteyn, writing to Hale from Groningen, thought Shapley perhaps the right candidate; [13] but Shapley, though a brilliant and original astronomer, was as yet only in his mid-thirties.

This handicap did not deter Shapley. In later life he vividly recalled the day he heard of Pickering's death, and decided to "take a shot" at succeeding him.[14]' He promptly wrote to both Russell and Hale to state his claims.[15] Russell was equally frank in reply: "To tell the naked truth, I would be very glad to see you in a good position at Harvard, free from executive cares. . . . But I would not recommend you for Pickering's place; and I believe that you would make the mistake of your life if you tried to fill it." [16] To Hale, Russell remarked that Shapley "would not suffer if he pondered the old fairy tale about the man who got all sorts of good things from a magic fish whose life he had saved-until his wife wanted to be Pope!" [17] Hale warned Shapley: "My advice to any candidate for a position would be never to attempt to take an active part in securing it, as this is the surest way to defeat one's end. [18] Shapley, chastened but secretly unconvinced, wrote to both men to declare himself no longer a candidate.[19]

On 20 December, about the time that Abbot and Hale were considering Shapley for the Washington meeting, A. Lawrence Lowell, the President of Harvard, telegraphed to Mount Wilson: "Is Shapley coming East Xmas time for some scientific meeting? If so could he visit Cambridge? If not when could he come here?"[20] The secretary's reply that Shapley had no such plans led to a mysterious visit to Shapley from a Regent of Harvard. "He evidently sailed under sealed and secret and telegraphic orders," Shapley told Russell on 6 January with some excitement, "for he knew nothing of astronomy or physics or science, or me or anyone here. He asked about the scientific meetings here last June-that A.A.A.S. convention that I managed. . . . I might say that I am naturally very confident that Harvard is not too big for me and that the things I could and would do there would be a credit to American astronomy."[21] The visitor's interest in Shapley's ability as an organizer rather than as an astronomer was no doubt because he was being considered, not for the Directorship, but for a post in support of the new Director. Certainly in June Russell was to be offered the Directorship with "a second astronomer, younger, and with modern ideas, to be called, to act as the Director's right hand main" (Shapley was to Russell the obvious choice), and a third person to act as administrator;[22] and even when Russell eventually declined, Shapley was merely offered the post of Assistant Professor and Astronomer.[23]

Evidently believing he was nominated for the Directorship itself, and eager for the appointment, Shapley viewed the proposed encounter with Curtis with dismay. As ill-luck would have it, Curtis was an experienced and accomplished public speaker who might well put Shapley to rout, whatever the scientific merits of their respective cases, and this-taking place within easy reach of Harvard-could cost Shapley the Directorship. In the ensuing flurry of correspondence between Shapley, Curtis, Hale and Abbot, Shapley tried- half-heartedly-to get an Easterner substituted for Curtis,[24] and-tenaciously- to undermine the seriousness and length of the proposed encounter. Four distinguished and busy men repeatedly discussed whether it should be a 'debate' as originally proposed, or a 'discussion'-"two talks on the same subject from our different standpoints", as Shapley wished .[25] No sooner had Hale been won round by Shapley to a 'discussion' than the latter received from Curtis a letter which reawakened all his anxieties:

"I agree with you that it should not be made a formal "debate", but I am sure that we could be just as good friends if we did go at each other "hammer and tongs". . . . A good friendly "scrap" is an excellent thing once in a while; sort of clears up the atmosphere. It might be far more interesting both for us and our jury, to shake hands, metaphorically speaking, at the beginning and conclusion of our talks, but use our shillelahs in the interim to the best of our ability."[26]

Curtis sent a copy of the letter to Hale. It was 3 March before Hale could talk with Shapley and formulate his reply: "I do not think that the discussion should be called a 'debate', or that Shapley, who is perfectly willing to speak first, should have time allotted him for 'rebuttal'. If you or he wish to answer points made by the other, you can do so in the general discussion." Each should be manifestly a seeker after truth, "willing to point out the weak places in his argument and the need for more results."[27] Not only had Shapley persuaded Hale away from the original concept of a debate, but he had convinced Hale that the proposed 45 minutes for each speaker was too long (on the grounds that this would tax the patience of the audience), and that 35 would be better. Curtis was aghast. The Lick Observatory Journal Club had recently devoted several meetings to the size of the Galaxy and the problem of external galaxies,[28] and Curtis had prepared a paper on the subject which he was circulating to friends for comment.[29] He knew how long he needed to make a serious scientific case. "We could scarcely get warmed up in 35 minutes", he protested to Hale.[30] Again the letters passed back and forth, and eventually a compromise of 40 minutes was imposed.[31]

The next problem concerned the subject matter. In Shapley's view, "The Scale of the Universe" made Curtis's main concern, the island universe theory, an incident to a general discussion of the present guesses as to galactic dimensions and arrangement".[32] On the other hand, both men recognized that if, as Shapley maintained, the Galaxy was much larger than had previously been thought, it would be more difficult for Curtis to sustain the claim that the spiral nebulae were independent island universes; and it was clear from the pictorial slides Curtis proposed to use that he would indeed concentrate on the island universe theory.[33] Shapley welcomed their different but interrelated approaches as offering scope for a partnership instead of the dreaded confrontation: "I shall not be able to get as far as details of nebulae in my half of the talk, but I shall get some of the explanatory, introductory, illustratory requisites out of the way so that you can probably go farther into the details."[34] But he knew that Curtis planned to present a serious scientific argument, summarized on type- written slides which he would show to the audience.[35] Shapley decided to appeal to Russell, his powerful ally, for vocal support, though putting the suggestion as usual into the mouth of a senior colleague:

"I lead off (with pictures), then Curtis presents his views, and then follows general discussion. Mr. Hale is anxious that you lead that discussion in whatever way you see fit, and I believe he plans to ask the presiding officer to call upon you as a starter. . . .

"Curtis swears by Newcomb and other patriarchs, and will show (?) that my distances are some ten times too big. Now that ten times, as Mr. Hale realizes, is as bad on your hypotheses as on mine; it is a violation of nearly all recent astrophysical theory. So unless Curtis actually bowls us over with the only true truth in these celestial matters, you will be interested in this general assault from the self-styled conservatives.

"Professor Brown is here at the observatory; also Professor Frost. They, as well as the people at Lick and at Mount Wilson, seem to regard that coming discussion as a crisis for the newer astrophysical theories.... But, crisis or not, I am requested to talk to the general public of non-scientists that may happen to drop in. Consequently, whatever answer must be made to Curtis and his school must be made in the discussion.

"I write you this because you may be interested in knowing what the situation is, and so that you may be ready to defend your own views if they are imposed upon by either of us. To make matters worse for me, Mr. Agassiz of the Harvard Obs. Visiting Committee is coming down to the lecture and to eat a lunch with me; and A.L.L. himself has written for an appointment in Washington." [36]

In fact Russell made so substantial a contribution in support of Shapley that the question arose of whether he should be a third author of the published version, for in July Shapley told Curtis: "Russell is probably not coming in the published discussion, according to Hale, so either I should have the come-back or I should know what you are going to do and rebut in advance."[37]

There remained the crucial question of the content and level of Shapley's presentation in Washington. His decision was to treat the National Academy of Sciences to an address so elementary that much of it was necessarily uncontroversial. The typescript he used-covered with pencilled emendations, some in shorthand-runs to some 19 pages.[38] Of these, the last three pages are devoted to the intensifier he had developed to permit the photography of very faint stars -irrelevant to the theoretical argument, but perhaps directed in part to those members of the audience responsible for the future development of Harvard College Observatory. Of the first 16, it takes him more than six to reach the definition of a light year! The remaining ten pages are published below; this, and not the technical paper which appeared over a year later, was what Shapley actually said in Washington.

Although Curtis intended to present his case through a series of typewritten slides, he also had a script of sorts, no longer extant. It was probably by way of introduction to the more technical material on the slides, for he wrote to Shapley the following August: "I am sending with this a copy of my talk at Washington. This will recall to you the general lines of the arguments used.... Unfortunately, most of my actual argument was shown in the form of typewritten slides; I have no copy of these to send on to you at present. . . ."[39] These slides (or some of them) have survived and are reprinted below. They relate closely to the published account, and at Washington must have formed an odd contrast to the elementary talk by Shapley which had preceded. The contrast is echoed in Shapley's letter to Curtis on 9 June, telling him that Hale thought that "in a slightly different form the papers would go to the Proceedings - he favors that, in fact, even if the papers are long, providing the material is suitable in being not too popular (like mine?) or too tabular or technical (like yours?)". Curtis modestly accepted the criticism: "Yes, I guess mine was too technical. I thought yours would be along the same line, but you surprised me by making it far more general in character than I had expected. Had some thoughts of changing entire character of my presentation about five minutes before close of your part, but decided at last minute to go ahead with program as planned."[40]

A referee might have declared 'no contest', but insofar is there was a contest, Curtis was the winner. Shapley in old age recalled: "Now I would know how to dodge things a little better. . . . As I remember it, I read my paper and Curtis presented his paper, probably not reading much because he was an articulate person and was not scared. Curtis, writing to his family on 15 May, reported "Debate went off fine in Washington, and I have been assured that I came out considerably in front".[42] Russell, writing to Hale in June about his invitation to become Director at Harvard, declared: "Shapley couldn't swing the thing alone. I am convinced of this after trying to measure myself with the job, and observing Shapley at Washington"-but if Shapley joined Russell there as his 'second' he ought to offer a lecture course for this "cultivates the gift of the gab, which he needs. [43]

In spite of the disparate performances, the occasion lived in the memory of those directly or peripherally involved.[44] For Curtis it was the climax of his decade of research on the nebulae; by July he was at Allegheny Observatory as Director and his creative years as an observer were over. For Shapley it was the occasion when he (and Mrs Shapley!) were vetted for the Harvard appointment. For the staff of the two great Californian observatories, it was something of a duel between champions.[45] Above all, the time was ripe for an appraisal of conflicting evidence and opposed interpretations on the fundamental question of the nature of the universe in the large; as R. G. Aitken of Lick remarked,

"I would like to hear the debate between Curtis and Shapley. I have read Curtis' paper-a very good one-and have had long talks with Shapley also, and each one has many very good arguments to present. For my own part, I am still "on the fence" on the question. I very greatly doubt the visibility of half-a-million or more 'island universes' on the one hand, and, on the other, I am not ready to accept Shapley's conclusions on the basis of his measuring-rod. It seems to me that its value is not yet sufficiently demonstrated. I am open to conviction."[46]

Curtis went prepared for the fur to fly;[47] his contribution was by common consent well presented and, as the slides show, at a high technical level. Russell, a talker of legendary capacity [48] and an outstanding astronomer, made a substantial reply from the floor, and we may be sure the rest of the session was hard fought. No wonder it was a memorable occasion. But the scientific argument and counterargument between Shapley and Curtis enshrined in the Bulletin papers belong, not to the verbal fisticuffs of Washington, but to their ensuing and protracted correspondence; and that is another story.

This paper results from a study of theories of the Galaxy and the nebulae c. 1920 carried out by the present writer and Professor Richard Berendzen. Dr N. E. Wagman, when Director of Allegheny Observatory, kindly supplied copies of the slides used by Curtis.- Mrs Margaret C. Walters (nee Curtis), Mrs Margaret Russell Edmonson, and the Shapley family generously allowed use of materials by Curtis, Russell and Shapley respectively. Grateful thanks are also due to Dr H. W. Babcock (Director, Hale Observatories), Dr Clark A. Elliott (Associate Curator, Harvard University Archives), Mr Alexander P. Clark (Curator of Manuscripts, Princeton University Library) and Miss Jean R. St. Clair (Archivist, National Academy of Sciences).
... Now that we have a satisfactory unit of sidereal distance [the light-year], let us go rambling about the universe. We see at one edge of this Milky Way field a Cluster of stars-a typical phenomenon in the galactic system. To the unaided eye we see the Pleiades as such a cluster; stars of the constellation Orion also form a real physical system of stars moving together and probably all of common origin. We know from special investigations that even the big dipper is a stellar organization. We may, indeed, trace by continuous steps the denseness and richness of the clustering motive from the richest of globular clusters to the poorly organized nearby systems. Messier 11 is a society of a few hundred stars, forming an open cluster in a rich part of the galactic clouds. In the same region is Messier 22, a transition type from open to globular clusters. It is one of the nearest systems of its class-only 25,000 light-years away- nearer and brighter than the great cluster in Hercules, Messier 13, but not so well known because far south and less condensed. This eleven hour exposure of Messier 13, made by Ritchey with the 60-inch reflector, probably shows the faintest stars ever photographed with that telescope. Since all globular clusters are very much alike except in distance, this picture is suitable for an illustration of the dimensions and physical properties of a typical system.

We do not know how many stars are in Messier 13, probably not less than 50,000; about 30,000 have been counted, and the bottom has not yet been reached. The distance of the cluster, I find, is some 35,000 light-years; its linear diameter is therefore more than 350 light- years, and its total radiation is 300,000 times that of our Sun.

The Hercules cluster has been extensively studied. We know for example the positions, magnitudes, and colors of all its brightest stars, and many relations between color, magnitude, distance from the center and star density, We now have the spectra of many of the individual stars, and their radial velocities; and the velocity and spectral type of the cluster as a whole. We know the types and periods of light variation of its variable stars, the colors and spectral types of these variables, and something also of the absolute luminosity of the brightest stars of the cluster from the appearance of their spectral lines. With knowledge of all this structural and historical detail, is it surprising that we venture to determine the distance of Messier 13 and similar systems with more confidence than was possible ten years ago when not a one of these facts was known or seriously thought about?

I shall not impose upon you the dreary technicalities of the methods of determining the distance of globular clusters. That would involve discussion of parallactic motion, probable errors, Cepheid variation, giant stars, and such matters. I think it will suffice to sketch briefly the principles involved.

For nearby stellar objects we can make direct trigonometric measures of distance, using the earth's orbit or the path of the Sun through space as a base line. For many of the more distant stars the spectroscopic method is available, using the appearance of the stellar spectra and the readily measurable brightness of the stars; for certain types of stars, too distant for spectroscopic data, there is still a chance of obtaining the distance by means of the photometric method. This simple device, which is one particularly suited to studies of globular clusters, consists in determining, by some indirect means, the real light power of a star, that is, its so-called absolute magnitude, and then measuring its apparent magnitude. Obviously, if a star of known real brightness is moved away to greater and greater distances, its apparent brightness decreases; hence, for such stars of known absolute magnitude, the apparent magnitude gives, by a simple formula, the distance from the observer.

As I have suggested before, it is because within recent years we have advanced our knowledge so greatly that we can use these powerful spectroscopic and photometric methods of measuring distance. The advance is in two directions; first, in the study of the nearby stars we have learned of the uniformity in absolute brightness and spectroscopic characteristics of various types; and second, we have shown that in the clusters we have exactly the same kinds of stars as those around the sun-the same except that the cluster stars appear to be very faint. This apparent faintness of the cluster stars is due to distance, and is a measure of it. For instance, the very extensive studies by Plummer, Kapteyn, and Charlier, have shown that stars of spectral type B in the solar neighbourhood (the blue stars) are closely restricted around an average absolute luminosity about 200 times that of the Sun. In Messier 13 we have the same types of stars. This we know from direct observations of spectrum with the 60-inch and 100-inch telescopes, supplemented by thousands of measures of color. But these blue cluster stars, which are actually about 200 times as bright as the Sun, appear to be only one five-thousandth as bright as the Sun would be if it were 33 light-years away; the distance of the blue cluster stars is therefore readily found to be some 35,000 light-years, and their distance is of course that of the whole cluster.

You may ask, however, is it not likely that these stars in the cluster, just because they are in a globular cluster, are of it different magnitude from our standards, even though comparable in color and spectrum? May they not be dwarfs in luminosity, and therefore not so far away? All the evidence, theoretical and observational, argues to the contrary. There should be little doubt in this matter of comparability for several good reasons. For instance, one reason-an all important one-- is that these nearby stars are themselves cluster stars. I have already pointed out the continuous sequence from the densest globular cluster to the constellation Orion; and the stars in such open nearby clusters as the Hyades, Orion, Scorpio, are the standards.

It is thus because of this comparability of local B stars with those of the distant open and globular clusters, and because the absolute luminosities of the local stars are based upon thousands of good measures of proper motion and radial velocity, that the blue stars give us the strongest evidence for the great distances of globular clusters.

Similarly the giant red stars Of Our local clusters are found to be comparable with the many red stars in globular clusters, and qualitatively at least, they give through the photometric method the same distances for remote systems as the blue stars give.

Another class of stars, the Cepheid variables, have been used extensively, and in much the same way, in exploring not only the system of globular clusters but the star clouds of the Milky Way. By determining the light-curve of such a star in any cluster, the distance may be known with remarkable accuracy. The particular luminosity that corresponds to a given period is found, as for the B stars and red giants, from studies of nearby examples of the class. Professor Curtis may tell you more of the photometric method of getting at the distances of the nearby Cepheid variables-he may question the sufficiency of the data or the accuracy of the methods of using it. But this fact remains: we could discard the Cepheids altogether, use instead the thousands of B-type stars upon which the most capable stellar astronomers have worked for years, and derive just the same distance for the Hercules cluster, and for the other clusters, and obtain consequently the same dimensions for the galactic system.

To conclude, in the face of these results, that the Hercules cluster is not approximately at the distance derived for it photometrically, is to avoid the most direct, and simple, and conservative interpretation of the data. To suggest, as I believe Prof. Curtis may do, that the clusters are only one-tenth as remote as I place them, is equivalent to subscribing to views so radical in several departments of astronomy and physics that we instinctively hesitate.

If the distances I have assigned must be decreased to one-tenth, then the light-emitting power of distant cluster stars must be only one-hundredth that of local cluster stars of exactly the same types. As a consequence I believe Russell's illuminative theory of spectral evolution must be largely abandoned, and Eddington's brilliant theory of gaseous giant stars must be greatly modified or given up entirely. Now both of these modern theories have their justification, first in the fundamental nature of their physical concepts, and second, in their great success in fitting observational fact. Similarly, the period-luminosity law of Cepheid variation would be meaningless; Kapteyn's classic researches on the structure of the local cluster would need new interpretation, because his luminosity laws could be applied locally but not generally; and a very serious loss to astronomy would be that of the generality of the spectroscopic method of determining star distances, in fact, the whole application of that method as an independent tool, for it would mean that identical spectral characteristics may indicate stars differing by five magnitudes, depending merely upon whether the star is in the solar neighborhood or in a distant cluster.

With so many satisfactory methods and theories at stake-the very foundations of modern astrophysics-is it any wonder that we hesitate and argue against revolutionary interpretations ? I believe I cannot follow Professor Curtis in calling those older, long-established interpretations conservative.

Suppose, therefore, we admit the obvious comparability of local cluster stars with those of distant clusters; is there not, however, a possibility that the distant stars appear faint through the loss of light in travelling through space? The Mount Wilson photometric studies show, however, no effect on star colors of such loss-a result checked by Hertzsprung and others. Two Swedish students find no suggestion of it in their studies of the very distant spiral nebulae. In the plane of the Milky Way, to be sure, we have dark nebulosity that may obscure more distant objects without affecting the color. But outside the galactic regions, and Messier 13 and most of the other globular clusters are far outside, we have in the distinct correlation of the angular size of a cluster with the brightness of its stars a fairly good proof of the absence of general light absorption. That is, if a cluster were removed to twice the present distance, its stars would be fainter, and also its area would be one-fourth as large. That is just what we observe-the faint clusters small, the small clusters faint-an obvious indication that the small faint clusters owe both of these characters to distance. Whereas, if the faintness of the cluster were due to obstruction of light, we should expect the angular diameter to be little affected; moreover, we should also expect to find, in such obstructed clusters, wholesale irregular variations, lopsidedness, and other curious effects that are not observed, unless, miraculously, the hypothetical obstructing matter were exactly at rest with respect to the cluster beyond, or exactly uniform.

When we accept that the distance of the Hercules cluster is such that its stellar phenomena are all harmonious with local stellar phenomena, then it follows that fainter, smaller clusters arc still more distant. Thirty of the 86 known are more distant than 100,000 light-years; the most distant is more than 200,000 light-years away, and the diameter of the whole system of globular clusters is about 300,000 light-years. Since the affiliation of the globular clusters with the Galaxy is shown by their concentration to the plane of the Milky Way and their symmetrical arrangement with respect to it, it also follows that the galactic system of stars is as large as this subordinate part. During the past year we have found Cepheid variables and other stars of high luminosity among the fifteenth magnitude stars of the galactic clouds; this can only mean that some parts of the clouds are more distant than the Hercules cluster. There seems to be good reason, therefore, to believe that the star-populated regions of the galactic system extend at least as far as the globular clusters.

One consequence of the cluster theory of the galactic system is that the sun is found to be very distant from the center of the Galaxy. It appears that we are near the center of a large local cluster or cloud of stars, but that cloud is at least 60,000 light years from the galactic center. Twenty years ago Newcomb remarked that the sun appeared to be in the galactic plane because the Milky Way is a great circle-an encircling band of light-and that the sun also appears near the center of the universe because the star density falls off with distance in all directions. But he concluded as follows:

"Ptolemy showed by evidence, which, from his standpoint, looked as sound as that which we have cited, that the earth was fixed in the center of the universe, May we not be the victim of some fallacy, as he was?"

The answer to Newcomb's question is: Yes, we have been victimized by the chance position of the sun near the center of a subordinate system, and misled by the consequent phenomena, to think that we are God's own appointed, right in the thick of things. In much the same way ancient man was misled, by the rotation of the earth, and by the consequent apparent daily motion of all heavenly bodies around the earth, to believe that even his little planet was the center of the universe, and that his earthly gods created and judged the whole. If man had reached his present intellectual position in a later geological era, he might not have been led to these vain conceits concerning his position in the physical universe, for the solar system is going rapidly away from the center of the local Cluster. If that motion remains unaltered in direction and amount, in a hundred million years or so the Milky Way will be quite different from an encircling band of star Clouds, the local cluster will be a distant object, and the star density will no longer decrease with distance from the Sun in all directions.

Remembering these delusions, relative to his physical status in the universe, may we not appropriately ask if man is also biologically blindfolded? Does he, perhaps, hold his self- assumed and self-defined position at the peak of animal development as a victim of psychological fallacy?

Another consequence of the Conclusion that the galactic system is 300,000 light-years or more in greatest diameter, is its baring on the problem of the spiral nebulae. I shall leave the description and discussion of this debatable question to Professor Curtis. We agree, I believe, that if the galactic system is as large as I maintain, the spiral nebulae can hardly bc comparable galactic systems; if it is but one-tenth as large, there might be a good opportunity for the hypothesis that our galactic system is a spiral nebula, comparable in size with the other spiral nebulae, all of which would then be "island" universes of stars. On one other point I think we also agree, or at least we should agree, and that is that we know relatively so little concerning the spiral nebulae and we are soon going to know relatively much because of the increasing activity in the nebular field, that it is professionally and scientifically unwise to take any very positive view in the matter just now.

But to summarize my view, which I hope is not positive and which is certainly subject to change if future data justify, the spiral nebulae are distant objects, not members of our galactic system, except that the nearer brighter ones have some sort of a relation to the Galaxy, not only in distribution, but also in motion. This relation to the Galaxy may be transitory for any given nebula, for with their enormous speeds of recession, they may eventually pass out of our domain of space. I prefer to believe that they are not composed of stars at all, but are truly nebulous objects. For instance, these two photographs of the typical spiral, Messier 51, by Mr. Scares indicates that the nebulosity is not composed of stars-as required by the island universe theory. The picture on the left is taken with a plate sensitive to yellow light, the one on the right records the blue light. The exposures are adjusted so that the superposed stars are of the same intensity. If the arms of the spiral were composed of stars the two pictures should be comparable in intensity throughout. If, however, the spiral is truly nebulous, it would appear faint on the photograph in yellow light. That, you see, is the actual condition in this system. No type of star is known with anything like as large a negative color index as is shown by the nebulosity in this spiral. But even if the spirals are stellar, they are not comparable in size with our stellar system, and our system is not comparable in constitution with the spiral nebulae. Professor Curtis, I hope, will have time to go farther into this interesting question....



Studies of the distribution of the stars and the ratios between the numbers of stars of successive magnitude have led a number of investigators to fairly accordant dimensions for the galaxy.

Wolf; about 14,000 light-years in diameter.
Eddington; about 15,000 light-years.
Shapley (1915); 20,000 light-years.

"That the maximum radius of the Milky Way is probably not greater than ten thousand light-years and may be somewhat less has been deduced from many lines of evidence, the most important of which is the color of the faint stars." (Mt. Wilson Contr., No. 116, 1915.)

Newcomb; not less than 7,000 light-years; later-perhaps 30,000 light-years in diameter and 5,000 light-years in thickness.

A maximum galactic diameter of 30,000 light-years will be assumed as representing sufficiently well the older view; it is perhaps too large.


Studies of the distribution of the stars over the entire sky, with investigations based on the ratios between the numbers of stars of successive magnitudes, have given the following results:

1. The stars are not infinite in number, nor uniform in distribution.

2. Our Galaxy, delimited for us by the projected contours of the Milky Way, contains possibly a billion suns.

3. Our Galaxy is shaped much like a lens, or a thin watch, the thickness being perhaps one-sixth of the diameter.

4. Our sun is located fairly close to the center of figure of the Galaxy.

5.The stars are not distributed uniformly through this galaxy. A large proportion may be actually in the ring structure suggested by the appearance of the Milky Way. There is some slight evidence for a spiral structure. Our position near the center of figure of the Galaxy is not a favorable one for a determination of the actual galactic structure.


From evidence to be referred to more fully later, Dr. Shapley has derived very great distances for the globular star clusters, 220,000 light-years for the most remote.

The apparent distribution of these globular clusters shows incontrovertibly that they are an integral feature of our galactic system.

This evidence has formed the main reason for Dr. Shapley's adoption of a diameter of 300,000 light-years for our galactic system, fully ten times greater than that accepted hitherto.


The smaller postulated dimensions for the Galaxy require stars whose absolute magnitudes are in fair accord with those of known distance. The larger dimensions require a very large proportion of giant stars.

Apparent Corresponding absolute magnitudes Magnitudes for distances of 10,000 l.y. 100,000 l.y. 8 - 4.4 - 9.4 10 - 2.4 - 7.4 12 - 0.4 - 5.4 14 + 1.6 - 3.4 16 +3.6 - 1.4 18 +5.6 +0.6 20 +7.6 +2.6


The conditions of star concentration obtaining in the Magellanic Clouds and in the globular clusters appear to render these regions of space unique as regards variable stars.

The Magellanic Clouds contain 1800 variable stars

Total of all variables in the rest of the sky, excluding those in globular clusters 1686

The globular clusters contain numbers of variable stars ranging from 137 in N.G.C. 5272 to 0 for N.G.C. 3293 and 4755. Practically all are shorter than one day in period. Total... 509

Short period cluster-type variables discovered to date in the rest of the sky 45


As island universes

The spectrum of the average spiral nebula is indistinguishable from that given by a star cluster.

It is such a spectrum as would be expected from a vast congeries of stars.

In general type it resembles the integrated spectrum of our Milky Way.

The spectrum of the spiral nebulae offers no difficulties in the island universe theory of the spirals.

As galactic phenomena.

If the spiral nebulae are an integral part of our Galaxy, we must assume that they are some sort of finely divided matter, or of gaseous constitution.

If galactic, we have no adequate and actually existing evidence by which we may explain their spectrum.

The diffuse nebulosities of our galaxy give a bright-line gaseous spectrum. A few, associated with bright stars, agree with their involved stars in spectrum, and are well explained as a reflection or resonance effect.

Such an explanation is untenable in the case of a large proportion of the spirals.


The spiral nebulae are found in greatest numbers just where the stars are fewest (at the poles of our Galaxy), and not at all where the stars are most numerous (in our galactic plane). No spiral has as yet been found actually within the structure of the Milky Way.

As island universes.

It is most improbable that our galaxy should, by mere chance, be placed about half-way between two great groups of island universes.

So many of the edgewise spirals show peripheral rings of occulting matter that this dark ring may be the rule, rather than the exception.

If our Galaxy, itself a spiral on the island universe theory, possesses such a peripheral ring of occulting matter, this would obliterate the distant spirals in our galactic plane, and explain their peculiar distribution.

There is some evidence of such occulting matter in our galaxy.

Additional observations on the spirals south of the galactic plane may remove this recession excess. Part of this may also be due to the motion of our Galaxy in space.

As galactic phenomena.

If the spirals are galactic objects, they must be a class apart from all other known types.

Their abhorrence of the regions of greatest star density can only be explained on the hypothesis that they are, in some manner, repelled by our Galaxy.

We know of no force adequate to produce such a repulsion, except perhaps light pressure.

Why should this repulsion invariably have acted at right angles to our galactic plane ?

Why have not some been repelled in the direction of our galactic plane?

The repulsion theory is given some support by the fact that most of the spirals observed to date are receding from us.


Within the past few years some twenty-five novae have been discovered in spiral nebulae, sixteen of these in the Nebula of Andromeda, as against about thirty in historical times within our own galaxy.

Apparent Magnitudes.

Thirty galactic Seventeen Novae in Novae Neb. Andromeda At maximum + 5 about + 17 At minimum +15 perhaps +27 ?

Absolute Magnitudes.

Novae in Nebula of Andromeda, Four galactic Novae if at distance of of known distance 20,000 l.y. 500,000 l.y. At maximum + 3.1 - 3.9 - 3.4 At minimum +13.1 +6.1 +7.2



1. On this theory we avoid the almost insuperable difficulties involved in the attempt to place the spirals in any coherent scheme of stellar evolution, either as a point of origin, or as a final evolutionary product.

2. On this theory, it is unnecessary to attempt to coordinate the tremendous space-velocities of the spirals with average star velocities. .

3. The spectrum of the spirals is like that given by a star cluster.

4. A spiral structure for our own Galaxy has been suggested, and is not improbable.

5. If island universes, the new stars observed in the spirals seem a natural consequence of their nature as galaxies. Correlations between the new stars in spirals and those in our Galaxy indicate a distance ranging from perhaps 500,000 light-years in the case of the Nebula of Andromeda, to 10,000,000, or more light-years for the more remote spirals.

6. At such distances, these island universes would be of the order of size of our own Galaxy of stars.

7. Very many spirals show evidence of peripheral rings of occulting matter in their equatorial planes. Such a phenomenon in our own Galaxy, regarded as a spiral, would serve to obliterate the spirals near our galactic plane, and would furnish an adequate explanation of the peculiar distribution of the spiral nebulae.


1. Discussions include: Otto Struve, "A Historic Debate about the Universe", Sky and telescope, xix (1959-60), 398-401 ; Norriss S. Hetherington, "The Shapley-Curtis Debate", Astronomical Society of the Pacific Leaflet no. 490 (April 1970); Otto Struve and Velta Zebergs, Astronomy of the twentieth century (New York, 1962), chaps 19 and 20, passim; and "The Great Debate", chap. 6 of Harlow Shapley, Through rugged ways to the stars (New York, 1969).

2. H. Shapley and H. D. Curtis, ','The Scale of the Universe", Bulletin of the National Research Council, ii, Part 3 (May 1921).

3. As shown by the official programme of the Academy meeting.

4. The meeting took place on 19 December. There is no reference in the minutes to Hale's suggestion.

5. W. E. Hale had used his wealth to support his son's projects, notably by providing the disc for the 60in. telescope eventually erected at Mount Wilson.

6. Archives of the National Academy of Sciences.

7. As shown by the letters from Curtis to his children, 8 February and 9 March 1919 (Michigan Historical Collections, University of Michigan), The script of Curtis's lecture is in the archives of Lick Observatory.

8. Hale microfilm.

9. Not surprisingly, in 1968 Dr Abbot did not recall the reason for the choice of Campbell, but remarked that Campbell was of course "a more important astronomer than Curtis" (personal communication). Mr Robert Smith points out that Campbell had supported the island universe theory in "The Nebulae", Science, xiv (1917), 513-48.

10. Letter of Hale to Curtis, 24 February 1920 (Hale microfilm).

11. Letter of Abbot to Hale, 20 January 1920 (Hale microfilm).

12. Hale microfilm. Hale cabled at once to Shapley and Curtis, offering each an honorarium of $150.

13. Letter of 7 February 1919 (Hale microfilm): "In America you have Russell and Shapley. Shapley is a brilliant man and personally I, who know him mainly only through his scientific work, would think him the best fitted for the position. Meanwhile I do not know him sufficiently to know how he would do as an organiser at the head of such a large and complicated Institution as the Harvard Observatory."

14. Shapley, Through rugged ways to the stars, 82.

15. Shapley to Russell, 12 February 1919, to Hale 13 February 1919 (Shapley Archives, Harvard University).

16. Russell to Shapley, 19 February 1919 (Shapley Archives, Harvard University).

17. Russell to Hale, 19 February 1919 (Russell Archives, Princeton University).

18. Hale to Shapley, 27 February 1919 (Hale microfilm).

19. Shapley to Russell, 27 February 1919; to Hale, 7 March 1919 (Shapley Archives, Harvard University).

20. Hale microfilm

21. Shapley to Russell, 6 January 1920 (Russell Archives, Princeton University).

22. Russell to Hale, 13 June 1920 (Hale microfilm).

23. Invitation dated 10 November 1920 (Shapley Archives, Harvard University).

24. Shapley to Hale, "Sunday", i.e. 22 February 1920 (Hale microfilm).

25. Shapley to Hale, 19 February 1920 (Hale microfilm). On the 24th Hale wrote to Shapley, Curtis and Abbot approving the concept of a 'discussion'.

26. Curtis to Shapley, 26 February 1920 (Shapley Archives, Harvard University).

27. Hale to Curtis, 3 March 1920 (Shapley Archives, Harvard University).

28. Curtis to E. E. Barnard, 28 January 1920 (Archives of Yerkes Observatory). Curtis reports that "most of us here find it impossible to subscribe to some of the recent theories on these points".

29. Ibid. On 23 February Curtis requested from Barnard the return of this paper as a matter of urgency.

30. Curtis to Hate, 9 March 1920 (Hale microfilm).

31. Abbot to Hale, 18 March 1920 (Hale microfilm).

32. Shapley to Abbot, 12 March 1920 (Shapley Archives, Harvard University).

33. Curtis to Shapley, 14 March 1920. Since Shapley maintained to the end of his life that Curtis did not address himself to the subject of the title (Shapley, Through rugged ways to the stars, 79), it is worth recording the synopsis Curtis proposed to Hale (Curtis to Hale, 20 February 1920, Shapley Archives, Harvard University) and which Hale welcomed: "Dr Shapley will discuss recently secured evidence pointing to dimensions of our galaxy about ten times greater than held in the older theories of the Milky Way, i.e., a diameter of about 300,000 light-years, with the spiral nebulae regarded as a galactic phenomenon. Dr Curtis will defend the older view that our Milky Way is approximately of the dimensions suggested by Newcomb, i.e., about 30,000 light-years in diameter, with the spiral nebulae regarded as very probably individual galaxies, or 'island universes'."

34. Shapley to Curtis, 18 March 1920 (Shapley Archives, Harvard University).

35. Curtis to Shapley, 14 March 1920 (Shapley Archives, Harvard University).

36. Shapley to Russell, 31 March 1920 (Shapley Archives, Harvard University).

37. Shapley to Curtis, 27 July 1920 (Shapley Archives, Harvard University).

38. Shapley Archives, Harvard University.

39. Curtis to Shapley, 2 August 1920 (Shapley Archives, Harvard University).

40. Shapley to Curtis, 9 June 1920; Curtis to Shapley, 13 June 1920 (Shapley Archives, Harvard University).

41. Shapley, Through rugged ways to the stars, 79-80. In private conversation Shapley was much more emphatic as to his disappointing performance.

42. Michigan Historical Collections, University of Michigan.

43. Russell to Hale, 13 June 1920 (Hale microfilm).

44. Writing to Shapley on 10 July 1922, Curtis spoke of "our memorable set-to" (Archives of Allegheny Observatory). C. D. Shane of the University of California (Berkeley) wrote to Curtis on 3 December 1923 about "the famous debate", and Curtis in reply on the 10th again referred to "our memorable set-to" (Michigan University Archives). Campbell, writing on "Do we live in a spiral nebula ?" in Popular Astronomy for 1926, speaks of the "memorable discussion" of 1920 (p. 175).

45. Robert G. Aitken later wrote of Curtis that "For a time only his colleagues at Mount Hamilton, and a few other astronomers agreed with him in his views" (National Academy of Sciences Biographical Memoirs, xxii (1943), 280), and this may be true. although Aitken had forgotten that he himself had been "on the fence" (cf. ref. 46 below).

46. Aitken to Barnard, 27 April 1920 (Archives of Yerkes Observatory); emphasis in original.

47. "... some fur ought to fly, on both sides", Curtis to W. J. Hussey, formerly of Lick Observatory, 15 April 1920 (Michigan Historical Collections, University of Michigan).

48. "He sure is a talker.... I never saw any man better qualified to teach the unwashed astronomy then he", "Benny" writing to Shapley of a talk by Russell to the general public at Mount Wilson, 30 June 1921 (Shapley Archives, Harvard University).

49. This rough typescript, now in Box 1 of the Shapley Archives at Harvard University, contains pencil amendments in longhand, and occasionally in shorthand. Those in longhand have been incorporated in this printed text. Of the typescript, the first one- third is too elementary to justify reprinting; likewise, the final three pages dealt with Shapley's intensifier, and, however significant this might have been as an instrumental advance in the study of faint stars, it was not directly relevant to the theoretical discussion and is omitted here as it was in the printed version of the proceedings (ref. 2).

50. The slides survive at Allegheny Observatory. Of the nine, eight are represented in modified form in the printed version of the proceedings, as indicated, and the slides have accordingly been arranged in a probable order. The title of slide H suggests that Curtis may have used other slides no longer extant, but surely very few additional slides could have been fitted into a 40-minute talk.



Virginia Trimble

Physics Department and Astronomy Department

University of California

University of Maryland

Prepared for the 1995 75th Anniversary Astronomical Debate and for publication in Publications of the Astronomical Society of the Pacific


No one now living attended the original lectures by Curtis and Shapley, and the scientific and other worlds in which they moved are connected to ours only by the written record and second-hand stories. Depending on which corners you choose to peer into, those worlds can seem remarkably modern or remarkably ancient. As is often the case for classic dichotomies, the wisdom of hindsight reveals that each of the speakers was right about some things and wrong about others, both in choosing which data to take most seriously and in drawing conclusions from those data. Modern (mostly casual) discussions of the 1920 event leave the impression that Shapley was, on the whole, the winner. But the two men's reactions to Hubble's discovery of Cepheids in the Andromeda galaxy makes clear that both felt that the issue of existence of external galaxies (on which Curtis had been more nearly correct) was one of greater long-term importance than the size of the Milky Way (on which Shapley had been more nearly correct). Shapley is much the better known today and is generally credited in text books with the Copernican task of getting us out of the center of the galaxy. Under modern conditions, he would probably also have gotten most of the press notices. Curtis's repeated theme, "More data are needed," is remarkably difficult, then as now, to turn into a headline.


The suggestion came originally from George Ellery Hale, whose father had endowed a lecture series for the National Academy of Sciences. After some initial hesitation, the NAS Home Secretary, C.G. Abbot, agreed that the 1920 William Ellery Hale lectures would be a discussion on "The Distance Scale of the Universe," with Harlow Shapley of Mt. Wilson Solar Observatory and Heber Doust Curtis of Lick Observatory as the discussants. Both the published versions of their presentations (Curtis 1921, Shapley 1921) and the notes from which they spoke (Hoskin 1976) are now available, as is a good deal of information on the lead-up to what much later came to be called "the great debate" and on its scientific aftermath.

We first examine the cultural and scientific environments in which the 1920 event occurred, then the event and its participants, ending with an examination of the scientific issues as then perceived and as now understood. It is not clear whether any very useful lessons for the case of gamma ray bursters can be drawn. As is frequently (but not always!) the case in scientific disputes, Shapley and Curtis each had hold of portions of the correct elephant.


At the time of their Academy encounter, Heber Doust Curtis and Harlow Shapley were employed respectively at Lick and Mt. Wilson Observatories. A born Californian, I thought first of probing their world by comparing the road maps they would have used with the ones that now guide us to the observatory sites. At first glance, the differences seem small. The main north-south route into the Oakland - San Francisco area, then as now, split to go both ways around the Bay. And a motorist striving to get over the mountains surrounding Los Angeles had a choice of two routes, one now called the Hollywood Freeway and one the Golden State Freeway, which follow the routes then called Cahuenga Pass and San Fernando Road, the latter nearly the old Spanish El Camino Real from Mission San Gabriel to Mission San Fernando.

The speed limit on the open road was, however, 35 mph (30 mph in Oregon), and the driving instructions rejoiced in stretches that were "paved all the way" and presented "no grades steeper than 12%." Alum Rock Road, where one begins the modern climb out of San Jose to Lick was on the maps, but petered out within a few miles into randomly-places images of hillocks and mountains that might almost have been labeled "here be dragons." The Mt. Wilson road was both better marked and more often traveled by casual visitors, but the site of Palomar Observatory was simply a random part of northern San Diego County, between the Pala Indian Reservation and "Nellie Warner's Hot Springs." The modern access road, from the south, was built by San Diego County much later. According to a contemporary hand-drawn map, the site could, however, be reached from the west, via a route later called Harrison Grade (and then carrying a name so politically incorrect that I dare not mention it) past landmarks like "Doane's old cabin" and "Elbow Creek Telephone Line."

The auditorium in which we meet had existed for about seven years and contained seats made of materials suitable for the pre-microphonic age. Curtis and Shapley necessarily filled the room with their own voices.

Politics, History, and Demographics:

The 9:30 PM Conversazione following the 1920 William Ellery Hale lectures took place without the customary glasses of wine, for the 19th amendment to the US constitution took effect on January 16th, ushering in "the great experiment" of prohibition (which, though it had the desired effect of considerably decreasing ethanol consumption, is nevertheless generally held to have failed).

That year also American women went to the poles nationwide for the first time, increasing voter turn out nearly 25% over the previous two elections and helping to elect Warren Gamliel Harding and Calvin Coolidge over James M. Cox and Franklyn Delano Roosevelt by 16.1 to 9.1 million votes (by modern standards an overwhelming majority). Eugene V. Debs, running for the Socialists, also lost, for the fifth and last time. Only Norman Thomas, his successor, with six defeats, ever equaled or beat his record. Levi P. Morton (vice president under Benjamin Harrison) died at the age of 96, and I mention it because he was born in 1824 and so overlapped by two years the lives of Thomas Jefferson and John Adams. We are a young country! (My grandmother, dying at 98 in 1984, had lived through more than half the life of our Constitution.)

Outside the US, the League of Nations was established (fatally, without the US); Austria held her first elections; and the Communist party completed taking control over the newly-named USSR. Two Georges ruled England (David Lloyd as prime minister and "V" as king); Poland retook Wilno/Vilna from Lithuania (with long-term implications for the universities and demographics of the region); and Benedict XI was pope, in succession to Pius X, the last occupant of the chair of St. Peter so far elevated to sainthood (we hope in spite of, not because of, his abolition of solos in liturgical music).

World population was roughly 2 Gigapersons, with 108 million of them resident in the US. Within the US, only 4.7% of the population was over 65, and the male:female ratio was 1.04 (and greater than unity even for the over-65's, the last census for which this was true). The foreign-born fraction was about 13%, higher than at any time since. Women made up 22% of the labor force, and unemployment was 5.2%, quite close to the current level.

Our national debt, left from the first world war (and the first one never significantly repaid) stood at $24.3 million, or $228.32 per person. This was something like 10 weeks salary for a semi-skilled craftsman and so not so very different either from the current level.

Among the 300,000 people who graduated from high school, women outnumbered men by 50%, but men outnumbered women nearly 2:1 among the 48,000 college graduates. Another legacy of the "great war," Spanish influenza, wound down after killing roughly 20 million people in three years, compared to about 8.5 million in WWI itself (insert your own best estimate for AIDS fatalities to date).

Sports and Culture:

Somehow these items seem to present the most striking contrast of ancient and modern. The Cleveland Indians (their name not yet threatened by the forces of political correctness) defeated the Brooklyn Dodgers (long gone) in the 1920 World Series. Harvard edged out Oregon 7-6 in the Rose Bowl, in striking contrast to the "Fight Fiercely, Harvard" image we inherit from Tom Lehrer. Jack Dempsey was 7th world heavyweight champion, while Emanuel Lasker of Germany, the first man ever declared world chess champion, still held the title. At the 7th Olympiad, Pavlo Nurmi won his first gold medals (one of a large number of Finnish track and field winners). The American men raking in gold for swimming events carried Hawaiian surnames, a reminder of the time when swimming was a survival skill rather than a recreation.

The winner of the Kentucky Derby (Paul Jones - horse, not rider) had a winning time only marginally longer than current records, though the purse at $30,375 sounds small till you inflate it. But the winner of the Indianapolis 500 (Gaston Chevrolet, driving a Monroe) had an average speed of 88.62 mph, slower than many of us have driven our production models. Bill Tilden (from the US) won Wimbledon, and the Ottawa Senators carried away the Stanley cup.

The Academy Awards (Oscars) had yet to be invented, but Eugene O'Neill won the 1920 Pulitzer for Beyond the Horizon. It was not a great year for the Nobel Prizes, several of the winners inviting a "hoo hee" response from non-experts. Physics went to Charles Guillaume, Peace to Leon Bourgeois, Literature to Knut Hamsun, Physiology or Medicine to August Krogh, and Chemistry to Walter Nernst (who illustrates the advantages of having a theorem named after you).

Enrico Caruso sang his last performance (La Juive) - which feels infinitely long ago, and Agatha Christie published her first murder mystery The Mysterious Affair at Styles - which was obviously only yesterday, since she brought out new volumes long enough to see many of us through high school and beyond. Sinclair Lewis published Main Street, which remains a classic (something everybody wants to have read, but nobody wants to read).

The first regular transcontinental airmail opened between Boston and San Francisco. Deaths during the year included artist Modigliani and explorer Admiral Robert E. Peary -- both controversial figures down to the present. An incomplete list of those born in 1920 includes Ravi Shankar, Isaac Stern, Nat "King" Cole, Alex Hailey, Isaac Asimov and Ray Bradbury, Mickey Rooney, Federico Fellini, Yul Brynner, Eileen Farrell, Lana Turner, Tony Randall, David Brinkley, Dave Brubeck, Jack Webb, Stuart Udall, Walter Mathau, and Patti Andrews. Christmas came on a Saturday, and the 26 April debate on a Monday.

Finally, what would prove to be the trial of the year or even the decade began with the arrests of Nicola Sacco and Bartolomeo Vanzetti, the good shoemaker and the poor fish peddler, for a murder most now think they never committed (though they died for it in 1927), but really for the crime of not being upper middle class WASPs. The drawing of modern analogies is left to the reader.

Astronomy in 1920:

The Astrophysical Journal was already a quarter of a century old and under the joint editorship of Hale, Frost, and Gale. They had just added abstracts to the standard paper format and admitted that page charges were here to stay, owing to the numbers of overseas subscribers not having recovered after the War, at least for authors or institutions who incurred more than $200 of production expenses in any one volume (of which there were two per year, with fewer than 30 papers each).

Very few of our current "best buy" theories were yet in place (Russell, Dugan, and Stewart 1926; Eddington 1926). The Chamberlin-Moulton (dynamic encounter) hypothesis for the origin of the solar system was in favor, largely because the sun seemed to have too little angular momentum to have come from a "nebular hypothesis." The solar wind eventually resolved that issue. Elements common in the earth (silicon, iron, oxygen) were supposed also to dominate the stars, giving them (with ionization) a mean molecular weight close to 2.1. It took Cecilia Payne's 1925 Harvard thesis on K giants and H.N. Russell's later work on the sun to sort this one out.

Not surprisingly, the source of stellar energy was unknown. The 2 Gyr age of some earth rocks (found by Rutherford and his colleagues) and the stability of Cepheid pulsation periods had already demonstrated that neither gravitational potential energy nor radioactivity was sufficient. New ideas in the air were "subatomic energy" that might power the sun for 1010 years without much changing its mass (advocated by Eddington) and some form of total annihilation of electrons and protons (the only known particles) that would suffice for 1012 years (favored by James Jeans because he thought that much time was needed to allow star clusters to relax). The only picture of stellar evolution sufficiently developed for comparison with observations was Russell's giant and dwarf theory, whose imprint lingers today in the use of "early" and "late" for spectral types. The idea was that stars begin bright and red, contracting toward the main sequence until they had used up all their "giant stuff", whatever it was, and then move diagonally down the main sequence, living on their "dwarf stuff" for a much longer time, fading out as red or white dwarfs. The debaters were both more or less subscribers to this point of view, and Shapley invokes it as part of the theoretical argument for his point of view.

Events of 1920 within the astronomical community included the deaths of Lockyer (discoverer of helium and founder of Nature), Brashear (of the process), and Hermann Struve. The Royal Astronomical Society marked its centenary, with Frank Dyson (whose successor is our moderator) as astronomer royal. Warner & Swazey Observatory was dedicated, and installments of the Henry Draper Catalogue (spectral types) and Wolf Catalog of proper motions were published. The International Astronomical Union, the first of the international scientific unions established under the Treaty of Versailles, which specifically abolished all international organizations of the pre-war period, came into being. The losers in the recently-ended conflict were specifically barred from membership, and Germany did not adhere to the Union until another war had come and gone.

Publications during the year relevant to "the scale of the universe" included Shapley on globular clusters, Haber claiming that Cepheids were eclipsing binaries (a well known crank in his day, now nearly forgotten), Kapteyn and van Rhijn arguing for a small, nearly sun-centered galaxy on the basis of star counts, and H.N. Russell demonstrating that the large positive velocities of the spiral nebulae could not be caused by radiation pressure from the Milky Way. Shapley apparently thought this a possible mechanism while he was preparing his manuscript. That anyone could have entertained the idea for more than five minutes suggests a painful shortage of envelope backs. The Thompson cross section and the momentum carried by light were already old ideas.

Some of the less relevant papers were remarkably prescient. Albert Michelson was advocating use of the 60" and 100" telescopes (the latter only 3 years old) at Mt. Wilson for interferometry. Eleanor Seiler suggested the use of photoelectric cells as photon detectors for astronomy. And Walter S. Adams and Coral Burall pointed out that novae must really be ejecting material. Other 1920 authors who are part of our folklore include Joel Stebbins (who, with a 64 year history of publications in ApJ, 1901-64, may be the longest-productive astronomer ever, Abt 1995), E.O. Hulbert, Francis G. Pease, Karl T. Compton, F.H. Seares, Edwin Hubble, Robert Millikan, Leigh Page, R.S. Dugan, Gustave Strömberg, and Seth Nicholson. Among those who lived long enough that I (and undoubtedly many of you) had a chance to meet them were Alfred H. Joy, Ira S. Bowen, Harold D. Babcock, Bancroft W. Sitterly, and Paul Merrill. The proportion of women authors was not so very different from the current mix. In addition to Burall and Seiler just mentioned, I spotted Mary Fowler (on eclipsing binaries), Mary Ritchie and Helen David (both Shapley co-authors in the Harvard tradition of measuring project lengths in woman-years).

Shapley and Curtis were not the only well known scientists to speak at the 1920 Academy meeting, though the usual difficulties of travel were compounded by, as secretary Abbot described it, "Washington [being] still somewhat congested" in the aftermath of the war. What would he think of the place now, when a change in power structure means that the Republicans arrive but the Democrats don't leave (or conversely)? There were no parallel sessions, but a good many speakers were allotted only 5, 10, or 15 minutes.

In any case, Frank Boas spoke on "growth and development as determined by environmental issues." He meant of people, and the issue is still (or again) a burning one. Robert Yerkes presented the results of a psychological study of Army doctors. Robert H. Goddard proposed "possibilities of the rocket in weather forecasting." Hale described recent results from the 100" telescope (as old then as Keck is now), Edward Kasner discussed "geodesics and relativity," Millikan "reflection of molecules from surfaces," Michael Pupin "wave balance," whatever that is, and Arthur Noyes (brother of the poet Alfred) the direct combustion of nitrogen and chlorine. Some of these would be perfectly possible titles or subjects for this year's academy meeting. Some definitely would not.

Topics whose presenters are less familiar to our selective memories were a similar mix of ancient and modern -- "conservation of nature resources," "rate of growth of the population," "Indian tribes of the Klamath River region," "common foods as sources of vitamines" (but note the spelling; they were all still thought to be true amines), "specific heat of powder gases," "alternating current for submarine transmission," "improvements in telegraphy," and two presentations on the properties of Springfield rifles! Yes, American militia units really carried the black powder, smoky "trapdoor" right up to, and occasionally beyond, the moment we went "over there." (Sweeney 1995; Pinckney 1995)


The background and circumstances of the 1920 lectures have been described by Struve and Zebergs (1962), Whitney (1971), Jaki (1972), and Berendzen et al. (1976), among others, on the assumption that the printed versions of the talks (Curtis 192l, Shapley 1921) were a close approximation to the material presented orally. Hoskin (1976) has shown that this is not the case, and his discussion therefore takes precedence.*

William Ellery Hale I, having presciently moved his family out of the center of Chicago shortly before the 1871 fire, made his fortune by constructing elevators for the buildings that grew up afterwards as well as for the Eiffel tower and other structures (Wright 1966, Osterbrock 1993). Some of the profits of these ventures bought his elder son, George Ellery, his first microscopes and telescopes, and, eventually, much of the Mt. Wilson 60". In addition, he endowed a fund for the National Academy of Sciences to be used, among other purposes, for invited lectures at annual meetings. Shapley and Curtis each received $150 honorarium (plus, presumably, travel expenses).

Not surprisingly, G.E. Hale (elected to the Academy in 1902) had some considerable say in how these funds were expended. In late 1919, he spoke to Charles G. Abbot, Home Secretary of the NAS, proposing that there be a William Ellery Hale Lecture at the 1920 April meeting in the form of a debate or discussion on either general relativity or the distance scale of the universe. Abbot's reaction was that it might be difficult to stir up interest in so specialized a topic as the existence of island universes, and that everyone would be heartily sick of relativity by then. He counter proposed causes of the ice ages or some topic in zoology or biology. Hale had originally suggested that the discussants on island universes and the distance scale should be W.W. Campbell (director of the Lick Observatory), presenting the conventional view, and Harlow Shapley (Hale's junior associate at Mt. Wilson), putting forward his new, larger distance scale, based on variable stars in globular clusters and other considerations.

When the dust settled, they had agreed on two talks, by Harlow Shapley and Heber D. Curtis (of Lick) on "the distance scale of the universe," and Hale sent out telegrams of invitation on 18 February. Shapley's invitation still exists, in the possession of Vera C. Rubin, who found it in a book she bought from Shapley's collection. After some discussion, the lecturers agreed to exchange their ideas in advance and each to give a single talk, with Shapley going first, and to include responses to each other's viewpoints therein, rather than to adopt a debate format, with rebuttals. The participants in the 1995 commemorative even similarly considered several possible formats, but made a different choice, opting for a formal debate structure.

* The mistake of placing the debate in 1921 is curiously common. Bok (1972) does it in his obituary of Shapley, as do several of the secondary accounts of the debate. And Florence (1994) manages to make several chapters out of the events of "April 1921." The cause is, presumably, the date of the publication and the fond belief that refereeing didn't take so long in those days!

Table 1 presents some aspects of the lives and works of the four people most closely associated with the 1920 debate: Hale who suggested it, Shapley and Curtis who carried it out, and Edwin Hubble who, a few years later, collected the data that settled the issue of island universes. All were born in the midwest, within 21 years of each other, and all had doctorates of some sort, though Hale's were all honorary. I mention their activities during WWI because at least part of the source of the life-long coolness between Hubble and Shapley was that Shapley, remaining at Mt. Wilson, carried out some project that Hubble had intended to pursue as soon as he could take up his profered position there after returning from active duty overseas (Hoffleit 1995). Hubble had volunteered immediately after defending his thesis and apologizing to Hale for not being able to accept the Mt. Wilson position immediately. We was wounded in France and rose to the rank of major. Correspondingly, during the second world war, while Shapley remained at Harvard (helping resettle refugees), Hubble moved to Aberdeen Proving Ground to direct its ballistics lab.

Of our four protagonists, Hale was far the most wide-ranging in his activities (Wright 1966; Osterbrock 1993). Astronomers know him as the founder and initial fund raiser for Yerkes, Mt. Wilson, and Palomar Observatories. A strong believer in international cooperation, he was among the prime movers in establishing the International Union for Cooperation in Solar Research in the years before WWI. Not easily discouraged in those days, he reacted to its abolishment by the Treaty of Versailles (which dissolved all pre-war scientific and cultural international organizations) by starting over with a still larger vision and persuading into existence the entity now called the International Council of Scientific Unions, as well as the International Astronomical Union under it.

During the war years, Hale was the first pure scientist to try seriously to persuade President Woodrow Wilson (awkwardly stuck with the slogan "He kept us out of war") that the services of his colleagues would be needed to win the war and the peace that followed. The organization he founded with that goal in mind is now the National Research Council. The Yerkes Primate Lab at Chicago is another of his inspirations. Curiously, the Robert Yerkes for whom it is named was not a close relative of the industrial magnate whose name the observatory bears. Hale early encouraged psychologist Yerkes to turn his attentions from people to other primates. Under the circumstances, one can only be astounded that Hale also made fundamental contributions to our understanding of the solar spectrum, magnetic field, and activity cycle, though he failed in a life-long ambition to photograph the solar corona outside of eclipse.

Curtis, too, was interested in the sun and participated in 11 eclipse expeditions between 1900 and 1932 (McMath 1942). His years at Lick were, however, devoted primarily to photographing spiral nebulae with the Crossley telescope, the work that resulted in his being asked to face off with Shapley in 1920. Curtis moved later the same year to the directorship of Allegheny Observatory (having already served as president of the Astronomical Society of the Pacific in 1912) and later to the corresponding position at the University of Michigan. He was an important force in the transformation of Hulbert-McMath Observatory from a private endeavor to a serious research facility. His own research days essentially ended when he left Lick, though his name continued to grace the astronomical journals with papers on subjects as unlikely as "A Voyage to the Moon." Curtis guided to their PhDs Helen Dodson Prince (1934), Ralph B. Baldwin (1937), and K.O. Wright (1940) among others. The University of Michigan had, incidentally, been admitting women students to its astronomy graduate program since before 1920, when Julia May Hawkes received her PhD for work on the positions of stars and nebulous knots in the Great Nebula of Andromeda (Sears 1955). Curtis died in the observatory director's house in Ann Arbor, with his directing, if not his observing, boots on.

Ralph Baldwin (1955, whose thesis was on the spectrum of Nova Cygni 1920 and its relationship to that of Nova Herculis 1934) remembers Curtis as "a small, quiet man with a remarkable sneeze." Curtis was "not full of wild enthusiasm for Einstein's theory," to which he had a long list of objections, and once ended a graduate course by throwing out the final exams of the five of so students on the grounds that if he hadn't given them enough work over the whole semester to get to know them, "the three hours isn't going to tell me anything new." The grades were all A's. He described the 37 inch telescope at Michigan as "focusing like a dish pan," and had great expectations for the 98 inch mirror he had cast at Corning in 1936 (while Corning was in the process of learning to produce the 200" blank for Hale and Palomar). Unfortunately, the money to turn it into a telescope never materialized, and the 98 inch sat next to the observatory parking area for many years until it became the primary of the Isaac Newton Telescope, and so sat next to Herstmonceux Castle for an additional number of years (contributing at least slightly more to astronomy in the latter location).

Curtis, like Hubble, was a confirmed pipe smoker, who sporadically set his wastebasket on fire. It was a search for the correct pronunciation of his middle name that eventually led to my making contact with Baldwin. The correct answer is "to rhyme with soused." And if you think you have heard of Baldwin in some other context, you probably have. He was one of the very first and most vocal proponents of impact cratering as the explanation for The Face of the Moon (Baldwin 1949).

Shapley, too, spent more of his life as an observatory director than as a research astronomer, taking up the reins of Harvard College Observatory shortly after the 1920 debate as successor to Pickering (and handing over to Donald H. Menzel more than 30 years later). He brought Harvard firmly into the 20th century, though he retained always a preference for relatively small telescopes with wide fields of view (Kopal 1972). In the post-war years he served as president of the American Astronomical Society, the American Association for the Advancement of Science, and the honorary scientific fraternity Sigma Xi, and was a firm opponent both of the communist witch hunts in the US and of the nonsense propounded by Velikowski.

Cecilia Payne Gaposchkin (at Harvard from 1923 to her death in 1979) described Shapley's style of leadership as "divide and rule" (Haramundanis 1984, p. 224). His decision that she must switch from spectroscopy after her thesis work (which was the first clear demonstration that stars consist mostly of hydrogen) to variable stars, leaving the spectroscopy to Menzel, hurt her deeply without in the least making Menzel dislike Shapley less (Hoffleit 1995). On the other hand, it was Shapley who persuaded Hoffleit to go on for her PhD (on spectroscopic parallaxes), though it would take her away from the work she was doing for him, and he welcomed her back at Harvard from war work at Aberdeen, though it had been done under the supervision of Hubble. Shapley's commitment to international cooperation rivaled that of Hale, and he is generally credited as the man who put the S in UNESCO.

Hubble, in contrast, was primarily a research astronomer all his life. He never directed an observatory or held an AAS office, though he served two 3-year terms as President of the IAU Commission now called Galaxies. While we remember him here for the discovery of Cepheids in NGC 6822, M33 and M31, which settled the issue of the existence of external galaxies, he is at least as well known for helping to draw the distinction between emission and reflection nebulae, discovering the linear redshift-distance relation that bears his name, classifying galaxies into their "Hubble types," and demonstrating that virtually all spiral galaxies rotate in the same direction, with their arms trailing. That Hubble was not personally known more to us is a consequence of his having been the shortest-lived of our protagonists. I have not attempted to assemble any personal impressions of him, but suggest that readers should take the one presented by Florence (1994) with some reservations, based on his treatment of Hale (Osterbrock 1993, 1995).

Of course, a very large number of other astronomers contributed relevant data and ideas before, during, and after the epoch of the "great debate." Vesto Melvin Slipher (1875-1969) measured the first wavelength shifts of spiral nebulae. Johannes C. Kapteyn (1851-1922) was a life-long proponent of a small Milky Way, centered nearly on the sun, and his "Kapteyn universe" continued to bedevil attempts to picture the large scale distribution of stars for decades after his death (both Trumpler and Shapley trying to picture Kapteyn's star cloud as part of the disk of some larger structure traced out by the globular clusters).

Adriaan van Maanen (1884-1946) was responsible for most of the measurements of apparent rotation of spiral galaxies that prevented Shapley from considering the possibility of their being at large distances until very late. Van Maanen's plates and equipment were not at fault. Although the instrument at Mt. Wilson bore the legend "Do not use the stereocomparator without consulting A. van Maanen", Knut Lundmark (1889-1958), visiting from Sweden, actually used it a few years after the debate to remeasure van Maanen's plates. He found no rotation, and, while the non-existence of the rotation is no longer in question, nobody has ever been quite sure what van Maanen did wrong. Lundmark was also the first to write, in 1920, that some novae might be so bright as to be detectable even at millions of light years from us. He advocated a quadratic relationship between redshift and distance (as expected in a de Sitter universe) before Hubble promulgated his law. Though van Maanen's sign remained on the blink comparator through my own graduate days (1964-68) and down to the time Berendzen photographed it (1972), I and others did eventually use it without consulting him. A minor point of possible confusion: "Mt. Wilson" was long used to mean, indifferently, the Mountain site and the administrative offices on Santa Barbara Street in Pasadena (the blink comparator was in the basement at Santa Barbara Street). Both places still exist, though the latter has undergone name changes to "Mt. Wilson and Palomar Observatories," "Hale Observatories," "Mt. Wilson and Las Campanas Observatories," "and Las Campanas Observatory," and "Observatories of the Carnegie Institution of Washington." And I may have forgotten one or two.


For more than a century after Herschel (1785), astronomers lived essentially at the center of a galaxy not much more than 6000 LY across (illustration 18 in Jaki 1972). Herschel arrived at his result by counting stars as a function of apparent magnitude in various directions ("star gauging") and, according to Kopal (1971) increased the diameter to 20,000 LY in 1806. The issue of whether the spiral nebulae might constitute other island universes was discussed sporadically through the 19th century, but was not the focus of anyone's research. Simon Newcomb (1882; illustration 19 in Jaki 1972), for instance, put "the region of the nebulae" immediately above and below a Herschel-like disk. It is widely believed that Newcomb was Walt Whitman's "Learned Astronomer," but this should probably not be held against either of them.

Cornelius Easton's (1900; Berendzen et al. 1976 p.56) galaxy was also small and sun-centered, but he was the first to give the Milky Way spiral arms. An honest examination of the sky forced him to displace the center of the spiral pattern away from us by more than half the galactic radius in the direction of Cygnus, and his drawing gives the impression of a man struggling with the truth and losing. Parsecs gradually replace Light Years as the unit of choice between 1900 and 1920. Karl Schwarzschild's (1910) galaxy was 10 kpc across, 2 kpc thick, and sun centered, while Arthur S. Eddington (1912, picture p. 196 in Whitney 1971) put us 60 LY above the center of the galactic plane. Hugo von Seeliger, the most thorough counter of stars since Herschel, and many others, concurred (Seeliger 1911).

Shapley (1918, 1919 and earlier references therein) shows a certain youthful exuberance in his distances - 67 kpc for NGC 7006 and 13.9 kpc even for M3. The centroid of his distribution slid from 13 to 25 kpc, with the 1919 paper settling on 20 kpc and a total diameter at least three times that. Shapley's universe had precious little room for anything outside this enormous galaxy, and he attempted at one point (Shapley 1930) to describe the Milky Way as more like the Coma-Virgo cloud of galaxies than like a single spiral or disk system. This is also the purport of his remark, quoted in Russell, Dugan, and Stewart (1926) that, if the spiral galaxies are islands, the Galaxy is a continent. Anton Pannekoek (1919) agreed with Shapley in placing the sun far off center but in a smaller galaxy (Ro = 40-60,000 LY, d = 80-120,000 LY).

At the time of the debate, Curtis's Milky Way was only 10 kpc across, with the sun at Ro = 3 kpc. Meanwhile, Kapteyn and van Rhijn (1920, Kapteyn 1922) were counting stars more precisely than they had ever been counted before, but with no allowance for absorption by dust. Their first result was Ro = 0 and d = 24 kpc; the second Ro = 3 kpc, d = 17 kpc (shown on p. 24 of Berendzen et al. 1976). But Shapley's numbers dominated people's thinking very quickly. Sir Harold Spencer Jones (1923, General Astronomy), Sir James Jeans (1927, Astronomy and Cosmology), as well as Russell et al. 1927, vol. 2) place the galactic center 20 kpc away. Jeans describes the Milky Way and other spirals as having the relationship of a cake to a bunch of bisquits. All attempt to fit Kapteyn's "universe" in somewhere as a local stellar subsystem.

Trumpler (1930, reproduced p. 93 of Berendzen et al. 1976) made a valiant attempt to declare all parties correct. His drawing shows a coordinate system centered at the sun in the middle of a slightly-tilted 10 kpc Kapteyn universe, but globular clusters scattered over an 80 kpc spheroid, centered about 18 kpc away from us.

Jan Oort's discovery of galactic rotation quickly led to a new calibration of distance scales. His first version (shown in Oort 1927) reported Ro = 6300 +/- 2000 kpc, soon revised upward to 10 kpc (shown in Oort 1932). This value was widely used over the next 20 years and incorporated in many images (see, e.g., Bok 1937).

Walter Baade (1953), however, looked again at the globular clusters and their RR Lyrae variables and settled on Ro = 8.16 kpc. This value, rounded off to 8.2 kpc, was generally accepted as the standard for reducing galactic rotation curves over the next decade (as shown by Westerhout 1956 and Kerr 1962). Nancy Grace Roman (pr. comm. 1992), who attended the symposium where Baade shrank the galaxy, described herself as having gone to college at 10 kpc and to graduate school at 8.2 kpc.

The present author did precisely the opposite; for in 1963 Oort (1964, cf. Schmidt 1965) moved us back out to 10 kpc. And there the official IAU set of galactic rotation constants kept us until the 1985 General Assembly in Bangalore, where the Commission on Galactic Structure (cf. Kerr and Lynden-Bell 1986) voted to reduce Ro to 8.5 kpc. This number is the average of along table that includes numbers between 6 and 11 kpc. Subsequent trends have perhaps been toward the small end of the range. Thus our present distance from the galactic center is quite close to the geometric mean of the numbers advocated by Shapley and Curtis in 1920.


Shapley and Curtis disagreed to some extent on at least 14 astronomical issues. These are presented in the following paragraphs in roughly the order in which they occur in the printed texts (Shapley 1921, Curtis 1921), which is neither in order of importance nor according to any other pattern a modern reviewer would be likely to choose. According to the actual texts reproduced by Hoskin (1976), no other additional scientific points were made during the main talks, though some may have arisen during Russell's rebuttal or other parts of the discussion, no record of which has been preserved. Each paragraph indicated an issue, what each disputant thought (or anyhow wrote or said), what we think now and sometimes why, and who should be counted the winner on each issue.

1. Resolved F, G, and K stars in globular clusters. Shapley believed they were giants like local F-K giants, with absolute magnitudes near -3, placing average globular clusters 10-30 kpc from us. Curtis said they were like the commonest sorts of stars around us, F-K dwarfs, with average visual magnitudes of about +7, putting the clusters at a kpc or two. As became unambiguously clear when the first 200" color-magnitude diagrams of globulars reached the main sequence turn-off (e.g. Sandage 1953), Shapley was essentially right on this one.

2. B stars in globular clusters. Shapley said they should have absolute magnitudes near 0, like nearby main sequence late B and early A stars. Curtis responded that something very strange must be going on, since the brightest blue stars in the solar neighborhood are brighter than the brightest red stars, while the opposite is true in the clusters. It took the insight of Walter Baade and his data gathered during the black outs of WWII to sort this one out, with the concept of two stellar populations. Each of the speakers was right about the particular point he emphasized.

3. Cepheids as distance indicators. Shapley used the relative period-luminosity relation found in the Large Magellanic Cloud with its zero point calibrated on a handful of Milky Way disk examples using statistical parallax. He noted that the nearby Cepheids of the cluster type (that is, RR Lyrae stars) are high velocity objects and must not be used for the calibration. Curtis responded that there was no evidence for a period-luminosity relation in the Milky Way, and that a larger sample, including some stars with geometric parallax measurements, even ruled it out. This was the point on which he said most firmly "more data are needed." When they came, Milky Way Cepheids did display a P-L relation, based both on secular parallaxes (or statistical) and on open cluster members. But the zero point was offset from the globular cluster one by more than a magnitude. This also was the work of Baade, who knew something was wrong the day (or rather night) he turned the 200" toward Andromeda and saw no RR Lyrae stars. Curtis was right about "more data" but wrong about what they would show -- he had placed too much faith in tiny geometric parallaxes, though he had more sense (paragraph 14) than to be misled by tiny proper motions. Shapley was right that Cepheids are generally good distance indicators.

4. Spectroscopic parallaxes in general. Shapley believed these could be trusted as long as you could see any of the line ratios indicative of giant surface gravities in nearby stars. Curtis believed they should be trusted only in the region of less than 100 pc where they had been calibrated. Errors and omissions expected (like some high latitude B stars), Shapley was right on this, though one shudders to think of the faith of eye required to see luminosity indicators like the ratio of 4215 (Sr II) to 4454 (Ca I) in spectra of individual globular cluster giants taken before 1920.

5. Interpretation of star counts. Curtis said, correctly, that star counts, straightforwardly interpreted, require a small Milky Way. His idea that spiral nebula dust existed as a ring around the stellar disk prevented him from suggesting absorption as relevant to the problem. Shapley did not address the issue, presumably because his use of globular clusters had already committed him to the "negligible absorption" camp, and he could, therefore, say nothing to rebut the point. Robert Trumpler (1930), by correlating apparent diameters of open star clusters with their apparent brightnesses revealed the importance of interstellar absorption (though Jesse Greenstein and others had come very close to discovering it earlier).

6. Stellar evolution theory. Shapley claimed that if and only if the globular clusters were put at large distances would their stars fir the Russell giant and dwarf theory and Eddington's models of gaseous giants. Curtis opined that spiral nebulae as a phase of stellar evolution didn't fit anywhere in any reasonable theory (remember protostellar nebulae were Out for solar system formation and encounters were In that year, and Jeans' idea that they were places where new stuff was pouring into the galaxy from Elsewhere had yet to be espoused and modified by Victor Ambarsumyan and others). While both points were true enough, we have to count Curtis the winner on this one, since we no longer adhere to the giant and dwarf theory!

7. Distribution of spiral nebulae on the sky. Shapley doesn't really mention this, but for a "single system" man, it was no more unreasonable for spirals to avoid the galactic plane than for OB stars to favor it. Curtis was forced to deal with the problem and concluded that it was "neither impossible nor implausible" for the Milky Way to have an occulting ring around it, as many edge-on spirals seem to, so that we would not be able to see nebulae in the plane. Curtis was closer to the truth than Shapley, but missed the critical point that stars and absorbing material are mixed together.

8. Nova brightness at maximum light. Both speakers agreed that "new stars" had been seen in the Milky Way and in several spiral nebulae. Shapley felt strongly that the implied real brightnesses would be totally ridiculous if the spirals were separate galaxies. Curtis said that, for four events with estimated distances in the Milky Way and a handful of novae in spirals, peak luminosity would be the same, provided the Milky Way had his preferred small size and the spirals were separate systems of similar physical diameter. He agreed that S Andromeda in 1885 was much brighter than this general run of events, said that Tycho's nova probably had been too, and concluded "a division into two classes is not impossible." One of the participants in our modern debate presumably feels the same way about the gamma ray bursters. Notice that Curtis was willing to trust a calibration based on four examples when he liked the answer, but not for the Cepheids, where he didn't. Two classes was, of course, the solution. Lundmark (1920) hinted at it, and Baade and Zwicky (1934) said it firmly from December 1933 onward, dubbing the brighter class super-novae (the hyphen disappeared the year Hale died; not causal). Curtis gets the points for this topic.

9. Nova mechanisms. Shapley suggested, seemingly with a straight face, that both the star and the nebulosity had existed to begin with, and that nebulae (with their large velocities) overtook and enveloped stars, producing nova events. He claimed to get the right rate of a few per year in the Milky Way from the numbers of stars and nebulae in his model universe. Curtis countered that the proposed mechanism would yield a rate of 1 per 500 years in Andromeda, where several had already been caught in the last 20 years. Once again, Curtis 1, Shapley 0.

10. The large, positive average velocities of the spiral nebulae. Shapley suggested the cause might be repulsion by radiation pressure from the Milky Way (a mechanism Russell showed to fail by many orders of magnitude the same year). Curtis simply proposed that large (mostly) positive wavelength shifts might somehow be intrinsic to the nebulae, and a large velocity also characteristic of the Milky Way. There are cases where "I haven't a clue" is the correct answer. It took the combined force of observations by Hubble, Milton Humason, and others and theoretical advances by Einstein, Alexander Friedmann, and others to come up with expansion of the universe as the explanation. Curtis over Shapley again, though perhaps not full marks. Incidentally, in case I forgot to mention it elsewhere, Einstein did not attend the 1920 debate, pace Florence (1994) and could not have, being still in Europe.

11. Properties of Galaxies, I. Shapley pointed out that the observed central surface brightnesses of spiral nebulae are much larger than anything seen in the Milky Way and the radial distributions of colors and surface brightnesses are different. Curtis remained silent on the issue. The answer, of course, is absorption and reddening, so Shapley was right about the data, but wrong about the interpretation. Love-love.

12. Properties of Galaxies II. According to Curtis, spiral nebulae have colors and line spectra a lot like those of star clusters, implying that the nebulae are mostly large assemblages of stars. Shapley did not mention this, and Curtis was right.

13. Central location of the sun. Shapley claimed this was an illusion, caused by the local star cloud now called Gould's belt. Curtis said it was God's own truth, and that our location kept us from readily seeing our own spiral arms. Once again, dust is an important part of the picture, but Shapley was nearly right.

14. Rotational proper motions of spirals as measured by van Maanen. Shapley said these were "fatal to the comparable galaxy theory." Curtis fully agreed, but said that you should never trust a proper motion of less than 0.1/yr for fuzzy things measured from a base line of 25 years or less. A round of applause for Curtis and sympathy for Shapley, who said later that van Maanen was his friend, so of course he believed him.


Immediate reaction to the two lecturers was undoubtedly driven by the two men's styles of public speaking. Comments have come down to us indicating that Curtis was by far the more experienced lecturer and expounder to the public. He had, at any rate, taught at Detroit High School, Napa College (California), and College of the Pacific (first Latin and Greek, later mathematics and astronomy) for about five years before seeking his PhD (McMath 1942, Stebbins 1950). Russell's private reaction (Hoskin 1976) was that Shapley ought to be persuaded to offer a lecture course to hone his skills in this direction. From our modern vantage point, it is hard to see things this way. Shapley springs to mind as the man for whom, rightly, the AAS Shapley Lectures are named, while Curtis is the man with the rimless glasses (and without the hair) who was prone to describe astronomical hypotheses as "not impossible" and "neither implausible nor impossible," while intoning a refrain of "more data are needed." Shapley, however, appears literally to have read his paper (from a typewritten text with long- and short-hand corrections), while Curtis had his lecture notes on slides. He might have even used overhead plastics, like the 1995 debaters, if they had existed in his time.

Although the participants continued to speak and write among themselves about the "famous debate," "memorable set-to," and "memorable discussion" for several years after 1920 (Hoskin 1976), the event seems to have attracted very little attention in the popular or scientific press. Berendzen et al. (1976) were able to locate only one contemporary report (given by a historian of science, Peter Doig, at the December 1921 meeting of the British Astronomical Association). Several contemporary reviews of distance scales refer to the work of one or both debaters, but not to the debate, and conclusions drawn are essentially those held by writers before April 1920. A splashy headline on a May, 1921 issue of the Boston Sunday Advertizer (p. 72 of Berendzen et al. 1976) refers only to Shapley's work on the distance scale and seems to have been featured primarily because he was, by then, a "Harvard astronomer." As noted in Sect. 4, Shapley's structure for the Milky Way was rapidly adopted by writers of astronomical textbooks, but without any reference to the 1920 event or 1921 publications.

At the time of Curtis's death, the discussion at the NAS was sufficiently forgotten that McMath's (1942) obituary makes no mention of it. Shapley, in his 1969 autobiography, similarly averred that it had for long escaped his memory. The first commemorative account I have seen is Stebbins' (1951) talk at the dedication of the Curtis Memorial Telescope in 1950. Otto Struve (1960), writing on the 40th anniversary, spoke of "a historic debate," and described (William) Albert Whitford (PhD U. Michigan 1942) and his students at Wisconsin as having restaged the event several times around 1950. Following Struve's account, others (based largely on the published texts) appear in many books and articles. Struve could not have been at the original debate. Stebbins (who was there to welcome Curtis to Lick in 1902) could have been, but apparently was not.

Each debater at some point expressed the opinion that he had won, perhaps not surprising, given the near-equality of their scores (Sect. 5). The question of correct distance scales within (and without) the Milky Way has been iterated on many times since 1920. According to recent rounds, Shapley's galaxy was too big and Curtis's too small, but, more seriously, centered far too close to the sun. The sort of sketch map most of us would draw has not changed much since that of Plaskett (1939, shown as fig. 202 in Berendzen et al.)

The question of the existence of separate, external galaxies, island universes, or whatever you want to call them, was resolved much more cleanly. A contribution by Edwin Hubble to the December/January 1924-25 meeting of the AAAS (read by H.N. Russell) announced the presence of Cepheids in the nebulae at apparent brightnesses that put them firmly outside even Shapley's bloated Milky Way. The result had actually been published in the 23 November 1924 New York Times, without attracting much attention. But Stebbins, Russell, and others who were at the AAAS meeting felt that the issue had been fully resolved.

The debaters apparently agreed. Curtis (quoted in Berendzen et al. 1976, p.138) wrote in April, 1925 "I have always held this view [that spirals are separate galaxies], and the recent results by Hubble on variables in spirals seems to make the theory doubly certain." This sounds like a calm, reasoned reaction, appropriate to a scientist who had distrusted earlier conclusions based on Cepheids (and I cannot say whether the disagreement between subject and verb was Curtis's or accidentally introduced by Berendzen et al.).

Shapley's predictably much more flamboyant reaction was recalled long after by Cecilia H. Payne-Gaposchkin, who had come as Harvard's first PhD student in astronomy in 1923 (Haramundanis 1986, p. 209). She was in Shapley's office when a letter arrived from Hubble, describing the period-luminosity relation for Cepheids in M31. "Here is the letter that destroyed my universe," said Shapley, holding it out. She also recalled him saying, "I believed in van Maanen's results . . . after all, he was my friend." And she herself resolved (Haramundanis 1986, p. 227) that she would "not accept the conclusions of another astronomer simply because I am fond of him, or reject them because I dislike him (though I admit there is a temptation here)."


It is possible to discern, or perhaps imagine, several patterns in the 1920 debate and long-term repercussions. First, Curtis and Shapley each seem to have got things more or less right when they relied on data they had collected for themselves (Shapley's photometry of stars in globular clusters; Curtis's images of spiral nebulae), and to have gone astray when they attempted to make use of data assembled by others. This is not a happy omen for the 1995 debate.

Second, conclusions that they drew by attempting to rely on astrophysical theory did not have a very high batting average. You can argue about just what belongs in this paragraph, but Shapley invoking the giant-dwarf theory of stellar evolution, radiation pressure for the large redshifts of spiral nebulae, and an encounter hypothesis for nova events, and Curtis attempting a sort of generalized Copernican approach to stellar populations strikes me as good examples. Most of us would, of course, say that correct theories do not get you into this kind of trouble, but rather add strength to observational conclusions by enabling you to understand them. This is what Eddington had in mind when he said he refused to believe an observation until it was confirmed by theory. But then, how much confidence should we have that any of the astrophysical theory brought to bear on the bursters so far is of this type?

Third, each of the 1920 protagonists had hold of part of the truth and so could claim partial victory. Other famous scientific disputes have ended this way, for instance that between the 18th century neptunists (who believed only in sedimentary rocks, laid down under Neptune's oceans) and the plutonists (who believed only in igneous rocks, rising up from Pluto's underworld). Both, of course, exist. I am inclined to suspect that the claims and counterclaims of star bursts vs. monster central engines to model active galaxies will prove to be like this.

Not all scientific disputes can end in such mergers or compromises. There is no middle ground between a planetary system co-forming with the sun and one dragged out of an already-established star by an intruder. Nor have various attempts to combine the virtues of standard hot big bang cosmology with those of steady state succeeded.

What about the gamma ray bursters? Could both galactic and very distant (and perhaps even "other") sources lurk among the classical events, with perhaps very different physical mechanisms at work in each? Since I have always felt that the popular image of the Curtis-Shapley debate gave the elder astronomer rather short shrift, I would like him to have the last word, taken from his comments on novae, "A division into two magnitude classes is not impossible."


First and foremost, we all thank Robert Nemiroff for the enormous amount of work he has put in to organize this event, comprising among the wishes of people at least as discordant as their scenarios. I am personally deeply grateful to Ralph Belknap Baldwin, Dorrit Hoffleit, Vera Cooper Rubin, and Richard Sears for sharing their memories and records that still connect us, tenuously, to the era of the original Curtis-Shapley debate, and to Katherine Gaposchkin Haramundanis for an invitation to write the introduction to the second edition of her mother's biographical volume, which led to my rereading it at just the right time.


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