184 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1913. 



by Bessel on the star 61 Cygni, 1837 to 1840. Bessel's result was 

 0.35'". This vahie is in close accord with recent measurements. It 

 was a great feat to measure such a small angle as this. In modern 

 practice the efforts to measure parallaxes absolutely have practically 

 been discontinued, for it is found that the very faint stars are so 

 immensely distant from us as hardly to be displaced at all in their 

 apparent positions by the motion of the earth in its orbit. Hence, for 

 stars which are near enough to be observed for parallax it suffices to 

 compare their positions with respect to the faint stars in their neigh- 

 borhoods at two epochs separated from one another by six months. 



Many of the parallax determinations of great weight have been 

 made by use of the heliometer, which is a telescope with its objective 

 lens cut in half, with the two parts movable with respect to one 

 another by a fine screw. With this instrument the images of two 

 celestial objects, one formed by one-half of the lens and the other 

 by the other may be brought into coincidence by shifting the two 

 parts of the lens with respect to one another, and the scale of the 

 instrument gives thereby an indication of the angular distance be- 

 tween the two objects in the heavens. Thus the relative positions of 

 the stars may be observed for parallax purposes. 



Stellar parallax measurements by means of the heliometer have 

 been the main work of the Astronomical Observatory of Yale 

 University. A volume of the Transactions of the Observatory 

 has been issued recently containing the results. The parallaxes of 

 19,5 stars have been ascertained with an average probable error of 

 0.015". The stars investigated at Yale naturally do not include 

 stars visible only in the southern hemisphere. The parallaxes of 

 some of the southern stars have been observed with the heliometer 

 from the Observatory of the Cape of Good Hope. Of the parallaxes 

 determined at Yale the largest pertaining to a bright star is 0.33" 

 for the star Procyon, whose magnitude is 0.6, and which has a proper 

 motion in the sky of 1.23" per century.^ According to the little table 

 just given it will be seen that this corresponds to a distance of 

 10 light-years. The largest parallax found for any star by the 



1 The stars have long been arranged with respect to their brightness, by " magnitudes." 

 A number of the brightest stars in the heavens are regarded as of the first magnitude. 

 The Polar Star is of the second magnitude, and the brightness of the faintest of the six 

 readily visible stars in the constellation Pleiades is 4.4 magnitudes. An increase of five 

 magnitudes corresponds to a decrease of a hundredfold in the brightness of the stars. It 

 was found when the measured distances of the stars were arranged in the order of the 

 magnitudes of the stars observed that the brighter stars were on the whole nearer to 

 us than the fainter ones. 



The " proper motion " of a star is ordinarily given as the angle through which the 

 star moves in a century in the heavens, after allowances are made for all effects of 

 nntation. precession, aberration, etc., but not for the motion of the solar system toward 

 the con.stellation Hercules. Proper motion therefore includes the star's real motion 

 in space with reference to the whole system of stars and, in addition, the star's apparent 

 motion, really due to the motion of the solar system toward the constellation Hercules. 

 Proper motions tend of course to diminish the greater the distances of the stars considered. 



