12 



can we obtain a more correct knowledge of those physical laws 

 which influence the phenomena observed in our solar system. 



The parallax of Venus is known to be almost four times 

 as great as the solar parallax, which causes a very sensible 

 difference between the times that Venus will be seen to pass 

 over the sun by observers situated at different parts of the 

 earth's surface ; if the observations are correctly made, the 

 sun's parallax can be determined in this way to a small jjart of 

 a second, and will furnish an universal standard of astronomical 

 measure for all the planets in the system. 



The older astronomers endeavoured to ascertain the sun's 

 distance, or in other words the sun's parallax ; but in practice 

 this is a very delicate problem, as the sun's parallax forms a 

 very small angle. Aristarchus of Samos, 260 years B.C., 

 thought that as the centre of the sun,the centre of the moon, 

 and the eye of the observer, form the three summits of a 

 triangle, in which one angle is a right angle when the moon is 

 at a quadrature, or when the light and dark portion of the 

 lunar disc are separated by a perfectly straight line. Aris- 

 tarchus measured the angle subtended and deduced from it 

 the ratio of the distance very much too small. The same may be 

 said of the plan employed by Hipparchus,and after him Ptolemy, 

 which consisted in measuring the distance of the diameter of the 

 earth's shadow in eclipse of the moon. The parallax of the 

 sun at this time was set down at 3', which is much too great. 

 Kepler, Riccioli, Hevelius, and Vendelini, being provided with 

 better means, reduced the parallax of the sun successively to 

 2' 28"', and finally to 15", the latter figure still being nearly 

 twice as large as it should be. 



By the three well-known astronomical laws of Kepler, 

 which govern the motion of all the planets round the sun, and 

 establish a connection between the duration of revolutions and 

 mean distance, the sun's parallax was found to be by Cassini 

 10", by La Hire 6", by Maraldi 10^ by Pond and Bradley 9 

 to 12'^, and by Lacaille 10"5". The approximate parallax now 

 known is 8-9", which gives for the sun's distance 91,308,642 

 miles ; until recently 95,000,000 miles has been adopted. 



The earth's parallax taken from 8-578'" to 9-343'" gives a 

 difference of 0*765", and every tenth of a second is equal to 

 1,130,000 miles, so that this difference will bring the sun in 

 the northern hemisphere about 3,000,000 miles nearer to the 

 earth in winter, and in the southern hemisphere 3,000,000 

 nearer in the summer season. 



Laplace arrived at a very striking result in this way from his 

 researches on the value of the solar parallax ; among the 

 equations in longitude involving that element and varying with 

 the angular distance between the sun and moon, the co- 



