and the Sun's Distance. 533 



with that of 1769, the next two transits will be so unfavourable 

 that nothing short of perfection in the construction of instru- 

 ments, and in the art of observing, can compensate for the natu- 

 ral disadvantage; so that the reduction of the possible error in 

 the sun's parallax within the limit of too^ 1 °f a second is hope- 

 less for at least two centuries more. 



2. The solar parallax may also be derived from the parallax 

 of Mars, when this planet is in opposition. In 1740 the French 

 astronomer Lacaille was sent to the Cape of Good Hope, and 

 from the parallactic angle observed between the direction of 

 Mars as seen from that station and from the observatory of 

 Paris (deduced from observations of declination), the horizontal 

 parallax of Mars was computed, and consequently that of the 

 sun. The solar parallax thus found was 10"*20, with a possible 

 error not exceeding ,/# 25. Henderson, by 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 United States Naval Astronomical expedition to Chili, 

 under the charge of Lieut. J. M. Gilliss, during the years 1849- 

 1852, had for its object the advancement of our knowledge of 

 the solar parallax, partly by observations of Mars at opposition, 

 and partly by observations of Venus during the retrograde por- 

 tion of her orbit, and especially at the stationary points, in con- 

 formity with a method suggested by Dr. Gerling — the whole to 

 be compared with simultaneous observations at northern obser- 

 vatories. Although the observations at Chili were made on 217 

 nights, covering a period of nearly three years, the cooperation 

 of northern astronomers was so insufficient that only 28 corre- 

 sponding observations were made. On this account the second 

 conjunction of Venus was useless; the other conjunction of 

 Venus and the second opposition of Mars were of little value; 

 and even the first opposition of Mars led to no significant result. 

 Dr. B. A. Gould has computed the solar parallax from the first 

 opposition of Mars, observed at Chili, at 8 //# 50. 



3. The solar parallax can also be computed from the law of 

 universal gravitation. The principle may be thus stated. The 

 motion of the moon round the earth is disturbed by the unequal 

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

 disturbance will be 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 parallaxes of the sun and moon. By selecting one of 

 the perturbations in the moon's longitude particularly adapted 

 to this purpose, Mayer, as early as 1760, computed the solar 

 parallax at 7"'S, In 1824 Burg calculated this parallax from 

 better observations at 8 ,,, 62. Laplace gives it at 8"*61. Fon- 



