PROCEEDINGS 
OF THE 
NATIONAL ACADEMY OF SCIENCES 
Volume 7 JANUARY 15, 1921 Number 1 
INTERNAL MOTION IN THE SPIRAL NEBULA MESSIER 5»J 
preliminary results 
By Adriaan van Maanen 
Mount Wilson Observatory, Carnegie Institution of Washington 
Communicated by G. E. Hale, November 10, 1920 
One of the most interesting questions in astronomy concerns the rela- 
tion of the spiral nebulae to the galactic system. Are they or are they not 
separate stellar systems comparable with our own? Especially during 
the last few years have many arguments been advanced both for and 
against. Real internal motions of the order indicated by the measures 
on Messier 101, communicated to these Proceedings in 1916, 1 would raise 
a strong objection to the "island-universe" hypothesis. If comparable 
with our own system, spiral nebulae would be so distant that no ordinary 
velocities of their constituent parts would yield measurable displacements 
in the short interval of a few years. 
The measures on Messier 101 were, however, based principally on two 
plates taken by Mr. Ritchey at the 25-foot focus of the 60-inch reflector 
with an interval of only five years, and required confirmation before a 
general acceptance of the results could be expected. This was afforded 
in part by measures of three photographs of the same object taken at the 
Lick Observatory, which were kindly loaned by Director Campbell. 
Measures on Messier 81 also pointed in the same direction; while Lamp- 
land 2 found analogous motions for Messier 51 in 1916, and Kostinsky 3 
in 1917. 
Meanwhile another five years has elapsed, which would have doubled 
the interval for the 60-inch reflector photographs of Messier 101; but, 
unfortunately, bad weather during the spring made it impossible to ob- 
tain a satisfactory plate. In August, 1920, however, Mr. Duncan secured 
a fairly good photograph of another spiral, Messier 33, for which an 
excellent plate, taken by Mr. Ritchey in 1910, was also available. Messier 
33 is, moreover, better adapted for such measures than Messier 101, on 
account of its numerous condensations and their close approach to star- 
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