128 ANNUAL OF SCIENTIFIC DISCOVERT. 



astronomers already begin to talk of preparing for them, Encke declares 

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

 unfavorable that nothing short of perfection in the construction of in- 

 struments, and in the art of observing, can compensate for the natural 

 disadvantage ; so that the reduction of the possible error in the sun's 

 parallax within the limit of one-hundredth of a second is hopeless 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 1 740, 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 decli- 

 nation), the horizontal parallax of Mars was computed, and consequent- 

 ly that of the sun. The solar parallax thus found was 10' .20, with a 

 possible error not exceeding 0".55. 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 portion of her orbit, and especially at 

 the stationary points, in conformity with a method suggested by Dr. 

 Gerling; the whole to be compared with simultaneous observations at 

 northern observatories. Although the observations at Chili were made 

 on two hundred and seventeen nights, covering a period of nearly three 

 years, the cooperation of northern astronomers was so insufficient that 

 only twenty-eight corresponding observations were made. On this ac- 

 count, the second conjunction of Venus was useless : the other conjunc- 

 tion 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 univer- 

 sal 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 rel- 

 ative 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". 8. In 1824, Burg calculated this parallax, from better observa- 

 tions, at 8^.62. Laplace gives it at 8''. 61. Fontenelle had said that 

 Newton, without getting out of his arm-chair, found the figure of the 

 earth more accurately than others had done by going to the ends of the 

 earth. Laplace makes a similar reflection on this new triumph of theory : 

 " It is wonderful that an astronomer, without going out of his observa- 

 tory, should be able to determine exactly the size and figure of the 

 earth, and its distance from the sun and moon, simply by comparing his 

 observations with analysis, the knowledge of which formerly demanded 



