ON THE STUDY OF DOUBLE STARS BY AMATEUR 



OBSERX'ERS. II. 



Bv G. F. CHAMBERS, F.R.A.S. 



THE LABOURS OF THE HERSCHELS. 



THortvH the name "Struve" occurs so unceasingh- 

 in the following pages, it must never be forgotten 

 that it was Sir \V. Herschel who must be regarded 

 as the "father" of Double Star Astronomy, and. 

 considering the epoch at which he lived, his labours 

 in this field, and their results, deserve our highest 

 admiration. It was Herschel who made the 

 telescopes without which Double Star .\stronomv 

 could never have grown at the beginning of the 

 19th century as it did: it was he who developed 

 their number from a dozen to hundreds which, later 

 on, became thousands : and, more remarkable still, 

 it was he who first suggested that the Newtonian 

 Law of Gravitation was Universal in a sense never 

 before dreamt of. That is to say, that it applied not 

 simplv to our relatively punv Solar svstem, but to 

 systems of celestial bodies distant from us untold 

 and untellable millions and millions of miles. 



The story of Herschel's investigations as to Double 

 Stars has never been fully told, and it would lead me 

 too far afield to deal with it here in the detail which 

 it deserves. The matter was handled by the late 

 Miss Agnes Gierke in her customary- excellent style 

 in her book. The HerscJiels and Modern Astronomy, 

 to which I refer the reader, merely giving here a 

 very bare outline of a few prominent facts. 



Though Sir W. Herschel was not only the first* 

 but one of the greatest of Double-Star observers, he 

 became such by " drifting " into the subject, rather 

 than from any set purpose of studying Double Stars 

 in the abstract. One thing led to another, and his 

 disco\-ery of Uranus and general telescopic work 

 encouraged in him a desire to obtain a further in- 



sight into the constitution of the heavens. 



Parallax 

 of the Stars was his first particular point : and 

 he thought that the study of Double Stars would 

 start him on the road to success in the determination 

 of stellar parallax. His first Catalogue of 269 Double- 

 Stars was completed in 1781, and was published in 

 the following year in the Philosophical Transactions.^ 

 A second Catalogue followed in 1785,* but a long 



inter\al elapsed before the third, which did not 

 appear till 1822, § though some additional notes, not 

 in the shape of a formal Catalogue, appeared in the 

 Philosophical Transactions in 1803 and 1804.* 



The principle of the Double Star method of 

 seeking to ascertain the parallax of a star is thus 

 succinctly stated by Miss Clerke: — "It is perfectly 

 simple, depending upon the perspective shifting, to a 

 spectator in motion, of objects at difterent distances 

 from him. In order to apprehend it. one need onl\- 

 walk up and down before a lamp placed in the 

 middle of a room, watching its apparent change of 

 position relative to another lamp at the end of the 

 same room. Just in the same way a star observed 

 from opposite sides of the Earth's orbit is sometimes 

 found to alter its situation very slightly by comparison 

 with another star close to it in the sky, but 

 indefinitely remote from it in Space. Half the 

 small oscillation thus executed is called that star's 

 ' annual parallax." It represents the minute angle 

 under which the radius of the terrestrial orbit would 

 appear at the star's actual distance. So vast, 

 however, is the scale of the Universe, that this tell- 

 tale swing to and fro is, for the most part, imper- 

 ceptible, even with modern appliances, and was 

 entireh- inaccessible to Herschel's observations. Yet 

 they did not remain barren of results."'* 



Writing in 1781 Herschel said that: — "As soon 

 as I was fully satisfied that in the investigation of 

 parallax the method of Double Stars would have 

 many advantages above any other, it became neces- 

 sary to look out for proper stars. This introduced a 

 new series of observations. I resolved to examine 

 every star in the heavens with the utmost attention 

 and a ver\- high power, that I might collect such 

 materials for this research as would enable me to fix 

 my observations upon those that would best answer 

 my end. 



" The subject has already proved so extensive, and 

 still promises so rich a harvest to those inclined to be 

 diligent in the pursuit, that I cannot help inviting 



'■'' It is hardly a contradiction of this statement to admit that Christian Mayer, of Mannheim, in 1781, collected all the 

 Double Stars then known (89 in number) into a Catalogue which was published in Bode"s AstronouiiscJie Jahrbiich for 

 1784. The details given, such as they were, were of the roughest. 



' Phil. Trans. Vol. LXXII. p. 112. 1772. j .Memoirs. R..A.S.. Vol. I. p. 166. 



* Phil. Trans. Vol. XCIII and Vol. XCIV. 



* Pliil. Trans. \'o\. LXXV. p. 40. 1775. ' Tlie Herscliels and Modern .Astronomy, p. 55. 



359 



