108 NEW MEASUREMENTS OF DISTANCE OF SUN. 



There were thus three powerful methods which converged upon a 

 vahie close to 8 -80 seconds. But to set against them w^as a method 

 which we have not yet noticed. 



The perturbations in the motions of other planets produced by 

 the earth depend upon the mass of the earth, and from them tliat mass 

 can be determined. There is further a well-known relation between 

 the mass of the earth, the value of the gravitation constant, the length 

 of the year, and the distance of the sun, from which the latter may 

 therefore be derived when we know the others. Professor Newcomb 

 had thus determined the parallax in two different ways, and had 

 found two results agreeing closely among themselves, with mean 8 'TG 

 seconds, but dilfering Avidely from the others. No explanation of this 

 divergence could be found. But the evidence was 3 to 1 in favor of 

 8 -80 seconds, and 8 -80 seconds was adopted in 1896 as the value to be 

 used in all the almanacs from the beginning of this century. 



It might well have been thought that the question would have been 

 allowed to rest there for a while. At the end of a century of labor 

 four principal results had emerged, and there was a majority of 3 to 1 

 in favor of 8 -80 seconds. But there is a phenomenon, known in 

 politics as the swing of the pendulum or the flowing tide, by whose 

 operation a majority hardly won begins immediately to melt away. 

 A like phenomenon appears to affect the solar parallax. We have 

 seen how its adopted value has swung from 8 "57 to 8 -95 seconds, and 

 back again to 8 -85 and 8 -80 seconds. Scarcely had the resolution of 

 the Paris Conference been taken than the majority in favor of 8 -80 

 seconds began to melt away. The beginning of the century had l)een 

 chosen as an auspicious moment in which to make a change, without 

 considering that there were at the end of the preceding century many 

 investigations just then drawing to a close. The value of the aberra- 

 tion constant corresponding to 8-80 seconds is 20 478 seconds. Al- 

 most every determination of that constant published since 189() — and 

 there are many — had come out above 20*50 seconds, many of these a 

 long way above. Further investigation of the parallactic ineciuality 

 of the moon had not only altered the observed value of the inequality, 

 but had modified the theoretical relation by which the parallax i-^ 

 deduced therefrom. The evidence for 8-80 seconds was giving way 

 badly ; and before the 1901 Almanac came into use we had this revised 

 table propounded by one of the chief instigators of the adoption of 

 8-80 seconds. The majority was now 3 to 1 in favor of a value at 

 least as low as 8 -77 seconds. 



Revised table. 

 Solar parallax from — Seconds. 



Gill's lieliometer, minor planets _- S • 802 



Constant of aberration of light 8-77 



Parallactic inequality of moon, probably 8-77 



Mass of earth from secular variations 8 • 759 



