106 NEW MEASUREMENTS OF DISTANCE OF SUN. 



up of the atmosphere of Venus ; there was no instant when tangency 

 was perceptible, and, to be frank, the transit of Venus as a means of 

 determining tlie distance of the sun was a failure. The photographic 

 and heliometer observations had for various reasons met with no 

 better success than the observations of contacts; there was no con- 

 sistency about the results. 



But just as the jJi'^pa rat ions for the transits were beginning in 

 18()T Prof. Simon Xewcomb had published an elaborate discussion 

 of the solar parallax based ujion several dilferent methods. AVith 

 some of these we are already familiar, and I will call attention to one 

 ojily, the last, which we have not as yet discussed. 



Goiiiponoits of Neicco))ib's value. 



It 



Newconib, "Observation of Mars, 18()2 " seconds- _ 8 -855 



Hall, "Observation of Mars. 18G2 " do 8-842 



Hansen. Stone, and Newcomb. from " Parallactic ineqnality of 



moon " seconds 8 -838 



Newcomb, "Lunar equation of the earth" do 8-809 



Powalky. "Transit of Venus. 17G0 " do 8-832 



Foucaulfs "Velocity of light." and Struve's "Aberration const." do 8-8G0 



Weighted mean seconds__ 8 :848 



It is an effect of aberration that every star describes yearly in the 

 sky an ellipse of which the semimajor axis is about 20*5 seconds, and 

 this number is called the constant of aberration. It is the ratio of 

 the velocity of the earth in its orbit to the velocity of light. When 

 the constant is known and the velocity of light is known, the velocity 

 of the earth in its orbit is known; and since the time of describing 

 that orbit is also known, the size of the orbit and the distance of the 

 earth from the sun follow immediately. 



In 1870 it appeared then that there was strong evidence against the 

 value 8-05 seconds; and Avithout waiting for the results of the tran- 

 sit of Venus expeditions, the Nautical Almanac adopted for the while 

 the value 8 •848 seconds found by Newcomb from this galaxy of results 

 which looked so accordant; and that value was first used in the 

 Almanac for 1882, the year of the second transit. 



But meanwdiile Sir David Gill, who had observed the transit of 

 1871 at Mauritius and had made up his mind very definitely that no 

 good would come out of the transit of 1882, had borrowed Lord 

 Lindsay's heliometer and established himself on the island of Ascen- 

 sion to observe with the heliometer the opposition in 1877 of the 

 planet Mars. Every night the observing station in Mars Bay was 

 carried some six or seven thousand miles by the rotation of the earth 

 and the planet thereby displaced among the stars by some forty sec- 

 onds. The heliometer is by far the most refined instrument for the 



