104 NEW MEASUKEMKNTS OF DISTANCE OF SUN. 



One is an absolute method, the other a relative, and it is scarcely 

 necessary to emphasize the dilTerence in accuracy between the two. 

 We shall see that, various as are the kinds of measurement which 

 may be made to contribute to a knowledge of the solar parallax, they 

 are all of them relative measurements. Vov example, one may ob- 

 serve the displacement of the planet Mars among the stars, as seen 

 from a northern and a southern station — say Greenwich and the 

 Cape — or one may observe the displacement of the place of Venus in 

 transit across the sun from stations suitably chosen. In each case we 

 are measuring the displacement as viewed from different stations of a 

 near object with respect to one farther off, the displacement of Mars 

 lunong the stars or of Venus against the sun. We have secured the 

 advantages that the parallactic displacement to be measured is 

 greater than that of the sun itself; that the objects to be observed, 

 Mars or Venus, are better adapted for observation, and that the meas- 

 ures are relative. 



In the middle of the eighteenth century Lacaille made observations 

 of Mars at the Cape of (xood Hope, which were compared with others 

 made at various observatories iji Europe, and he deduced a parallax 

 of about ten seconds. In the same century there occurred the two 

 famous transits of Venus of 17G1 and 1769, which were very exten- 

 sively observed, among others by Captain Cook on his celebrated ex- 

 pedition for that purpose to the South Seas. Many and various were 

 the results obtained by different discussions of the observations, lying 

 lietween eight and one-half aud nine and one-half seconds, but decid- 

 edly less than the parallax found from Mars, and we find that at the 

 beginning of the nineteenth century the Nautical Almanac adopted 

 a value in round numbers, nine seconds, as the best that could be 

 made of them. 



Values of tJie solar parallax used in the Xaiitical Almanac during the ■nineteenth 



cent ur II. 



1801-1833 U" 



18.31-1869 8-5776" 



Encke. from transits of Venus, 1761 and 17<)!). 

 1870-1881 8 -95" 



Leverrier, from parallactic inequality of moon (1858). 

 1882-1900 8-848" 



New comb, from general mean of many methods (1867). 



In 1824 the German astronomer Encke submitted to a very search- 

 ing examination the collected results of the transit of 17(59 and 

 deduced the result 8 "5776 seconds, which, with its imposing train of 

 decimals intact, was incorporated in our Xautical Almanac for 1S84, 

 survived until 18G9, and was responsible for the statement, which 

 many of us can remember in the schoolbooks, that the distance of 

 the sun is 95,000,000 miles. 



