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JUL 20 1921

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Mount Wilson Observatory, Carnegie Institution of Washington

By Heber D. Curtis
Director, Allegheny Observatory

Part I

Evolution of the idea of galactic size
Surveying the solar neighborhood ..
On the distances of globular clusters

The dimensions and arrangement of the galactic system
Part II

Dimensions and structure of the galaxy
Evidence furnished by the magnitude of the stars.
The spirals as external galaxies .

Part I

171 176 180 191

194 198 210


EVOLUTION OF THE IDEA OF GALACTIC SIZE The physical universe' was anthropocentric to primitive man. At a subsequent stage of intellectual progress it was centered in a restricted area on the surface of the earth. Still later, to Ptolemy and his school, the universe was geocentric; but since the

*This address and the following one by Dr. Heber D. Curtis are adapted from illustrated lectures given on the William Ellery Hale Foundation before the National Academy of Sciences, April 26, 1920. The authors have exchanged papers in preparing them for publication in order that each might have the opportunity of considering the point of view of the other.

The word "universe" is used in this paper in the restricted sense, as applying to the total of sidereal systems now known to exist.

time of Copernicus the sun, as the dominating body of the solar system, has been considered to be at or near the center of the stellar realm. With the origin of each of these successive conceptions, the system of stars has ever appeared larger than was thought before. Thus the significance of man and the earth in the sidereal scheme has dwindled with advancing knowledge of the physical world, and our conception of the dimensions of the discernible stellar universe has progressively changed. Is not further evolution of our ideas probable? In the face of great accumulations of new and relevant information can we firmly maintain our old cosmic conceptions?

As a consequence of the exceptional growth and activity of the great observatories, with their powerful methods of analyzing stars and of sounding space, we have reached an epoch, I believe, when another advance is necessary; our conception of the galactic system must be enlarged to keep in proper relationship the objects our telescopes are finding; the solar system can no longer maintain a central position. Recent studies of clusters and related subjects seem to me to leave no alternative to the belief that the galactic system is at least ten times greater in diameter—at least a thousand times greater in volume—than recently supposed.

Dr. Curtis, on the other hand, maintains that the galactic system has the dimensions and arrangement formerly assigned it by students of sidereal structure—he supports the views held a decade or so ago by Newcomb, Charlier, Eddington, Hertzsprung, and other leaders in stellar astronomy. In contrast to my present estimate of a diameter of at least three hundred thousand lightyears Curtis outlines his position as follows:2

As to the dimensions of the galaxy indicated by our Milky Way,till recently there has been a fair degree of uniformity in the estimates of those who have investigated the subject. Practically all have deduced diameters of from 7,000 to 30,000 light-years. I shall assume a maximum galactic diameter of 30,000 light-years as representing sufficiently well this older view to which I subscribe though this is pretty certainly too large.

I think it should be pointed out that when Newcomb was writing on the subject some twenty years ago, knowledge of those special factors that bear directly on the size of the universe was extremely fragmentary compared with our information of to-day.

See Part II of this article, by Heber D. Curtis.
2Quoted from a manuscript copy of his Washington address.

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