194 ON THE MAGNITUDE OF 



Of course, the results obtained in 1672 produced a keen desire 

 on the part of astronomers for further evidence respecting the true 

 value of the parallax, and as Mars comes into a favourable position 

 for such investigations only at intervals of about sixteen years, 

 they had recourse to observations of Mercury and Venus. In 

 1677 Halley observed the diurnal parallax of Mercury, and also a 

 transit of that planet across the sun's disc, at St. Helena, and in 

 1 68 1 J. D. Cassini and Picard observed Venus when she was on 

 the same parallel with the sun ; but although the observations of 

 Venus gave better results than those of Mercury, neither of them 

 was conclusive, and we now know that such methods are inaccu- 

 rate even with the powerful instruments of the present day. 

 Nevertheless, Halley's attempt by means of the transit of Mercury 

 ultimately bore fruit in the shape of his celebrated paper of 17 16, 

 wherein he showed the peculiar advantages of transits of Venus 

 for determining the solar parallax. The idea of utilising such 

 transits for this purpose seems to have been vaguely conceived by 

 James Gregory, or perhaps even by Horrocks ; but Halley was the 

 first to work it out completely, and long after his death his paper 

 was mainly instrumental in inducing the governments of Europe 

 to undertake the observations of the transits of Venus in 1761 and 

 1769, from which our first accurate knowledge of the sun's dis- 

 tance was obtained. 



Those who are not familiar with practical astronomy may 

 wonder why the solar parallax can be got from Mars and Venus, 

 but not from Mercury, or the sun itself. The explanation depends 

 on two facts. Firstly, the nearest approach of these bodies to the 

 earth is for Mars 33,870,000 miles, for Venus 23,654,000 miles, 

 for Mercury 47,935,000 miles, and for the sun 91,239,000 miles. 

 Consequently, for us Mars and Venus have very much larger 

 parallaxes than Mercury or the sun, and of course the larger the 

 parallax the easier it is to measure. Secondly, even the largest of 

 these parallaxes must be determined within far less than one-tenth 

 of a second of the truth, and while that degree of accuracy is 

 possible in measuring' short arcs, it is quite unattainable in long 

 ones. Hence, one of the most essential conditions for the suc- 

 cessful measurement of parallaxes is that we shall be able to com- 

 pare the place of the near body with that of a more distant one 



