22 



best iletormined is by solar parallax, or the angle botwoon two or moro 

 stations ufc considerable distances from each otlier, in the noi-thcrn and 

 southuru hemispheres, whose geo<,Tai)hical positions are well known, and from 

 which stations two ;istronomers point their telescopes, when looking at thesun, 

 at the same moment. As the distance of the object increases, as for sun, or 

 star, the biuso line is to be enlarged, and the angle thus obtained is the means 

 by which solar parallax is associated with the sun's distance. In the case of 

 the SMIL, the base line is measured on the earth's surface ; and in the case of a 

 star from the extremes of the earth's orbit. The astronomer then makes use 

 of Kepler's third law, which establishes a relation between the distances of the 

 different planets from thesun,and their periods of revolution ; if either distance 

 is found by observation, the other can be computed by this law. 



The choice lies between Venus, at inferior conjunction, and Mars at opposi- 

 tion. The parallax of Mars may vary from 20" "7 to 19' "1, according to the 

 position of Mars and the earth with respect to the perihelion of the orbit at 

 the time of opposition. The parallax of Venus at conjunction may vary for 

 the same reasons from .33" '9 to 29" "9. Venus, therefore, may be nearer to the 

 earth than Mars, and the parallax more favorable. But Venus cannot be seen 

 at conjunction, except when its latitude is so small that a transit across the 

 sun's disc occurs. Then the two observers refer its place not to a star but to 

 the sun, and the quantity determined is the difference of parallax between 

 Venus and the sun, which will vary from about 21" to 25". The difference of 

 parallax is not measured directly, but through the influence it produces on the 

 duration of the transit at the two stations, and consequently upon a much 

 enlarged scale. 



The solar parallax may be derived from, the parallax of Mars, when this 

 planet is in opposition ; Lacaille was sent in 1740 to the Cape of Good Hope 

 for the purpose, and the parallactic angle observed between the direction of 

 Mars as seen from that station and from the Observatory at Prais. The solar 

 parallax then found was 10" '20, with a possible error not exceeding 0"' 20. 

 Henderson comparing his own observations of the declination of Mars at its 

 opposition in 1832 with corresponding observations at Greenwich, Cambridge, 

 and Altona, computed the solar parallax at 9" "028. 



The solar parallax is also computed from the law of imiversal gravitation by 

 means of the disturbed motion of the moon round the earth, and the unequal 

 attraction of the sun on the two bodies. The magnitude of the disturbance is 

 in some proportion to the distance of the disturber when compared with the 

 relative distance of the two disturbed bodies ; and this ratio of distances is 

 the inverse ratio of the parallax of the sun and moon. By selecting one of 

 the perturbations in the moon's longitude adapted to this purpose, Mayer, in 

 1760, computed the solar parallax at 7" "8, In 1824, Burg calculated this 

 parallax at 8" '62, Laplace gives it at 8" "61. 



The following table:]: gives the values of solar parallax and the sun's distance 

 by the different methods of astronomy and by experiment : — 



t This table, together with some other datum, is taken from a recent article on th« 

 •ulqect compUed by Professor Joseph Lovering, of Harvard College. 



