136 



NATURE 



[ June 5, 1884 



opposite side of the sun — that is, from two points 186 millions 

 of miles apart — we might expect to find a considerable change 

 in their apparent places. 



But previous to 1S32 astronomers could not discover with any 

 certainty that such changes were sensible — that, in other words, 

 the red and the white strings met at a point so distant that, as 

 far as they were able to measure, the two strings were practically 

 parallel — or, putting it another way, the stars were so distant 

 that the diameter of the earth's orbit viewed from the nearest 

 star subtended a smaller angle than their instruments could 

 measure. Bradley felt sure that if the star 7 Draconis were so 

 near that its parallax amounted to 1" of arc he would have 

 detected it — that is, if the earth's orbit viewed from 7 Draconis 

 measured 2" in diameter, that is, if it looked as big as a globe 

 one foot in diameter would look if viewed at forty miles distant, 

 he would have detected it. But the real distances of the stars 

 were greater than that. 



The time at last arrived when the two great masters of 

 modern practical astronomy, Bessel and Struve, were preparing 

 by elaborate experiment and study for the researches which led 

 to ultimate success. After vain attempts to obtain conclusive 

 results by endeavours to determine the apparent changes in the 

 absolute direction of a star at different seasons of the year, both 

 astronomers had recourse to a method which, originally proposed 

 by Galileo in 1632, was carried out first on a large scale by Sir 

 William Herschel. I shall refer in the first place to the re- 

 searches of the great Russian astronomer Struve. 



Astronomers had sufficiently demonstrated that the distances 

 of the stars were very great, and it was reasonable to argue that 

 as a rule the brighter stars would be those nearest to us. If, 

 therefore, two stars are apparently near each other — the one 

 bright, the other faint — the chances are that in reality they are far 

 apart, though accidentally nearly in a line. 



Fig. 



If two such stars are represented by s s in Diagram I., they 

 would appear near each other viewed from one side of the earth's 

 orbit at A, but not so near each other viewed from B, the opposite 

 side of the earth's orbit, the red lines obviously indicating the 

 apparent angle between the stars when they are viewed from 

 A, and the black lines the apparent angle when they are viewed 

 from B. Struve selected for the star s the bright star Vega 

 (o Lyra:). From its brilliancy he considered it probably one of 

 our nearest neighbours amongst the stars, and a faint star ap- 

 parently near it seemed to afford a suitable representative of the 

 really distant star s. Struve was careful to ascertain that this 

 comparison star was not physically connected with a Lyras, and 

 he was able to prove this from the fact that whilst a Lyra: has a 

 small annual motion relative to all neighbouring stars, this 

 motion is not shared by the faint comparison star. Struve was 

 provided with a telescope driven by clockwork to follow the 

 diurnal motion of a star, and thus the hands of the observer 

 were free to make the necessary measures. These were accom- 

 plished by an instrument such as I hold in my hands applied to 

 the telescope. This micrometer contains two parallel spider 

 webs each attached to a slide, one slide being moved by one 

 screw, the other by the other screw. The screws are provided 

 with drum-heads divided into 100 parts. One web was placed 

 on the image of a Lyra:, the other upon that of the faint com- 

 parison star, and the angle between the stars was thus read off 

 in terms of the number of revolutions and decimals of a revolution 

 of the screws. A number of such observations was made on 

 each night, and the result for each night depended on the mean 

 of the numerous observations made each night. 



By observations on ninety-six nights between November 1835 

 and August 1838, he showed that the distance between o Lyra 

 and the faint comparison star changed systematically with a 

 regular annual period, and that the maxima and minima of those 

 distances corresponded with the times of the year at which these 



maxima and minima should occur if the brighter star were really 

 much nearer than the fainter one. 



Assuming that the fainter star is at a practically immeasurable 

 distance, Struve showed that a Lyra: had a parallax that amounted 

 to about a quarter of a second of arc, which is equivalent to the 

 statement that a globe whose diameter is equal to that of the 

 earth's orbit — that is, to iS6,ooo,ooo of miles — would at tin 

 distance of a Lyra; present an apparent diameter of half a 

 second of arc. If you wish to realise this angle, place a globe- 

 one foot in diameter at a distance of eighty miles, or look at a 

 coin half the diameter of a silver threepenny piece at a distance 

 of one mile from the eye, and try to measure it. 



The great German astronomer, Bessel, was simultaneously 

 engaged in like work at Konigsberg. He selected as the object 

 of his researches a very remarkable double star — 61 Cygni. 



This star had already been the subject of similar researches on 

 his part with much inferior means. He now attacked the 

 problem with the splendid heliometer which had been made for 

 him by Frauenhofer for the purpose. The principle of this 

 instrument I shall presently explain. His reasons for choosing 

 61 Cygni were that the two components of this star, though not 

 remarkable for brightness — they are just visible' to the naked eye 

 — yet have this peculiarity, that they have a remarkably large- 

 proper motion, the largest then known, though now surpassed 

 by that of two other stars which I shall afterwards mention. 

 They have an apparent angular motion relative to other stars of 

 more than five seconds of arc per annum. 



Struve had argued that if the stars were on the average of 

 similar brightness, those stars which were brightest would 

 probably be those nearest to us, and Bessel, in like manner, 

 argued that if the absolute motions of the stars were similar 011 

 the average, those motions which appeared the largest belonged 

 to stars which on the average were nearest to us — just as the 

 motion of a snail could be easily watched at the distance of two 

 or three feet from the eye, but could not be detected except 

 after a long interval, if the animal were a good many yards 

 distant. 



Bessel employed two faint comparison stars at right angles to 

 each other with respect to 61 Cygni, and he made two separate 

 series of observations, the first extending from August 1837 to 

 October 1838, the second from October 1838 to March 1840. 



Both series confirm each other, and the results deduced sepa- 

 rately from the measures of the two comparison stars also agree 

 within very narrow limits. From all the observations com- 

 bined Bessel found the parallax of 61 Cygni to be 35/100 of a 

 second — a quantity which has been shown by the modern re- 

 searches of Prof. Auwers and Dr. Ball to be more nearly half a 

 second of arc. Thus at 61 Cygni the diameter of the earth's 

 orbit round the sun would appear of the same size as a globe a 

 foot in diameter viewed at forty miles distance, or of a silver 

 threepenny piece a mile off. But whilst these great master, ol 

 astronomy — Struve and Bessel — had been exhausting the re- 

 sources of their skill in observation, and that of the astronomical 

 workshops of Europe in supplying them with the most refined 

 instruments, a quiet and earnest man had been at work at the 

 Cape of Good Hope, and, without knowing it at the time, had 

 really made the first observations which afforded strong pre- 

 sumptive evidence of the existence of the parallax of any fixed 

 star. 



Henderson occupied the post of Her Majesty's Astronomer at 

 the Cape of Good Hope in 1832 and 1833, and during his brief 

 and brilliant tenure of office there he made, amongst many others, 

 a fine series of meridian observations of a Centauri — a bright and 

 otherwise remarkable double star. When, after his return to 

 England, Henderson reduced these observations, and compared 

 them with the earlier observations of other astronomers, he found 

 that a Centauri had a large proper motion ; he was therefore led 

 to examine and see whether his observations gave any indication 

 of an annual parallax. He found that they did so, and not of 

 a small parallax but of one amounting to nearly a second of arc. 

 But it was not till this was confirmed, not only by the observa- 

 tions with the mural circle but by those of the transit instrument 

 also, not only by his own observations but by those of Lieut. 

 Meadows, his assistant, that Henderson ventured to publish his 

 remarkable result. 



In the year 1842 it was felt by the astronomical world at large 

 that the problem which hitherto had baffled astronomers had 

 begun to yield, that some approximation to the truth had at last 

 been arrived at with regard to the distance of a fixed star, 

 and it was fit and proper that the Royal Astronomical Society 



