174 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



the same angular size (viz, about one one-hundredth part of the 

 distance in each case), we know that the sun must be about 1,000,000 

 miles in diameter and the moon only about 2,500 miles, using round 

 numbers throughout for simplicity. 



Thus the sun is a veritable giant compared with the moon, in spite 

 of the similar apparent size of the disks ; but this we discover only 

 when we have measured the distances. 



The sun and moon present to us large disks which we can study 

 in detail, and the study of the disk of the sun by means of the spectro- 

 heliograph has had new triumphs which, owing to the war, have 

 not yet been seen in this room, so that I may be pardoned for exhibit- 

 ing one. 



But when we come to the stars there is no disk. If one seems to 

 see disks for these objects, the appearance comes through the im- 

 perfections of the telescope. Hence it would seem to be superfluous 

 to inquire about their distance. When Mark Twain had been roughly 

 handled at Niagara Falls, and the doctor reported that only 16 of his 

 wounds were mortal, he said " he did not care about the others." In 

 the same way we might argue that since the stars have no appreciable 

 disks we need not care about their distances. 



That, however, was not the attitude of men of science. They went 

 to work to measure their distances, and though the difficulties were 

 heart breaking they were attacked and overcome. Here is a table 

 showing a sensible fraction of the life work of an eminent Scotsman, 

 Sir David Gill, observing at the Cape of Good Hope. It includes the 

 famous a Centaur i, the first star to have its distance measured at all, 

 which again was by a Scotsman, Henderson, also observing at the 

 Cape of Good Hope. 



Gill's paraUa.rc* of bright stars. 



Star. 



Sirius 



Canopus. . . 



Rigel 



a Centauri 

 a Eridani. 



Parallax. 



0.37 

 .00 

 .00 

 .75 

 .04 



Distance in 

 light-years. 



(?) 



(?) 



4 



80 



Star. 



Centauri... 



a Crucis 



Spica 



a Pise. Austr 

 fi Crucis 



Parallax. 



0.03 

 .05 

 .00 

 .13 

 .00 



Distance in 

 light-years. 



100 

 64 



(?) 

 25 



(?) 



Sir David Gill was accustomed to describe the difficulties of de- 

 termining that distance by saying that it was like trying to measure 

 the size of a 3-penny bit 2 miles off; and we remember his delight 

 when his chairman, on one occasion asked, "Who but a Scotsman 

 would care about a 3-penny bit 2 miles away?" 



But you will see that this star is the nearest and therefore easiest 

 of all; while some even of these brightest stars gave no result even 



