31. 



REMARKS ON MR STONE'S EXPLANATION OF THE LARGE AND IN- 

 CREASING ERRORS OF HANSEN'S LUNAR TABLES BY MEANS OF A 

 SUPPOSED CHANGE IN THE UNIT OF MEAN SOLAR TIME. 



[From the Monthly Notices of the Royal Astronomical Society, Vol. XLIV. (1883).] 



IN some recent communications to the Royal Astronomical Society Mi- 

 Stone contends that the mean solar day in use before 1864 when Le 

 Verrier's Solar Tables were substituted for Bessel's in calculating the sidereal 

 time at mean noon given in the Nautical Almanac differs from the mean 

 solar day adopted since that time. 



In the Monthly Notices, Vol. XLIII. p. 403, Mr Stone states that the 

 consequent error in our present reckoning in time is increasing at about 

 the rate of l"'46 per annum, and in the same volume, p. 335, he adduces 

 this supposed error in explanation of the increasing errors of Hansen's Lunar 

 Tables. 



That this view of Mr Stone's is erroneous may, I think, be shewn by 

 very simple considerations. 



The only mean Sun known to astronomers is an imaginary body which 

 moves uniformly in the equator at such a rate that the difference between 

 its Right Ascension and that of the true Sun consists wholly of periodic 

 quantities. 



These periodic terms are due to the obliquity of the ecliptic, the 

 eccentricity of the Earth's orbit, and also to the small perturbations of the 

 Earth's motion about the Sun. 



332 



