THE VISIBLE STARS. 559 j 



In such a state of things, it will easily be imagined that astronomers have ' 

 diligently directed their observations to the discovery of some chance of appa- 

 rent position, however small, produced upon the stars by the earth's motion \s 

 those stars most likely to be affected by the motion of the earth are those which 

 are nearest to the system, and therefore probably which are brightest and lar- 

 gest, it has been to such chiefly that this kind of observation has been directed. 

 Since it was certain, that if any observable effect was produced by the earth's 

 motion at all it must be extremely small, the nicest and most difficult means 

 of observation were those alone from which the discovery could be exp. 

 Among the many expedients used for the detection of such effects, we shall se- 

 lect as an example one which was adopted at the Royal Observatory at Green- 

 wich. A telescope of great length was attached to the side of a pier of solid 

 masonry erected upon a foundation of rock. This instrument was scrcunl 

 into such a position that particular stars as they crossed the meridian would 

 necessarily pass within its field of view. Micrometric wires were in the usual 

 manner placed in its eye-piece, so that the exact point at which the stars passed 

 the meridian each night could be observed and recorded with the greatest pre- 

 cision. The instrument being thus fixed and immoveable, the transits of the 

 stars were noted each night, and the exact places where they passed the merid- 

 ian recorded. This kind of observation was carried on through the year, and 

 if the earth's change of position, by reason of its annual motion, should produce 

 any effect upon the apparent position of the stars, it was anticipated that such 

 effect would be discovered by the'se means. After, however, making all allow- 

 ance for the usual carses which we knew to affect the apparent position of the 

 stars, such as refraction or aberration, no change of position was discovered 

 which could be assigned to the earth's motion. 



Within the last few years, however, Professor Bessel has directed his scien- 

 tific labors to this inquiry, and has succeeded in detecting a small effect on one 

 of the stars in the constellation of the Swan. In a communication, made in 

 1838 by that astronomer to Sir John Herschel, he says : " After so many un- 

 successful attempts to determine the parallax of a fixed star, I thought it worth 

 while to try what might be accomplished by means of the accuracy which 

 my great Fraunhoffer heliometer gives to the observations. I undertook to 

 make this investigation upon the star 61 Cygni ; which, by reason of its great 

 proper motion, is perhaps the best of all, which affords the advantage of being 

 a double star, and on that account may be observed with greater accuracy, and 

 which is so near the pole that, with the exception of a small part of the year, 

 it can always be observed at night at a sufficient altitude." 



These observations were continued for four years, and the result was the 

 discovery that the position of the star in question was affected by the earth's 

 motion to the extent of a little less than one third of a second. From this may 

 be calculated the distance of the star from the solar system. 



To render intelligible the spirit of the method by which the distance of the stars 

 may be inferred from their discovered parallax, let us suppose two lines, drawn 

 from a star to opposite ends of a diameter of the earth's orbit, or to two positions 

 which the earth occupies after an interval of six months. The angle formed 

 by these two lines is, in fact, the amount of the apparent change of position of 

 the star by reason of the earth's motion, and it is technically called the parallax. 

 We may in this case consider the diameter of the orbit as a portion of an enor- 

 mous circle, the centre of which is at the star, and the radius of which is the 

 distance of the star from the earth. It is known, in geometry, that an arc 

 circle which measures one second is in length the 206,265th part of the rnd 

 and if it measures one third of a second, it will, of course, be the 618,79 

 part of the radius. 



