388 W. Harkness—The Solar Parallax. 
may be sufficient to affect the parallax by + 0’07. (CRH, 
1881, t. xcii, p. 658.) As the secular variations of the ele- 
ments of the orbits of Venus and Mars increase continually, 
they will ultimately attain sufficient magnitude to give a very 
exact value of the earth’s mass, and then this method will 
furnish the solar parallax with the utmost precision. 
Parallactic Inequality.—Professor Newcomb found that the 
value of the parallactic inequality of the moon deduced by 
Hansen from the Greenwich and Dorpat observations is 126’”-46. 
(WOb, 1865, App. II, p. 28. 
From 2075 Greenwich lunar observations, made between 
1848 and 1866, Mr. E. J. Stone found the parallactic inequal- 
ity to be 125’°36+0’"4; the probable error being estimated. 
(MNt, 1867, vol. xxvii, p. 27]. 
From the Washington lunar observations, made between 
1862 and 1865, Professor Newcomb found the parallactic ine- 
quality to be 125’46. (WOb, 1865, App. I, p. 24. 
From an extended discussion of the whole subject, pub- 
lished in the MNt, 1880, vol. xl, pp. 386 to 411, and 441 to 
472, Messrs. Campbell and Neison found the observed value of 
the parallactic inequality to be (p. 467) either 125’°64+0'"09, 
or 124’64+0'-25; the difference arising from the admission 
or non-admission into the lunar theory of a certain hypotheti- 
cal forty-five year term. 
By substituting these values of Q in equation (12) the fol- 
lowing values of the solar parallax result : 
Moon’s Mass. se tr dy wT 
GQ — 124°°64 8”°782 8”:780 8”"°778 8”-776 
125°36 833 831 *829 "827 
125°46 "839 837 "835 833 
125°64 "851 "849 *847 *845 
126°46 8°910 8°908 8906 8904 
These parallaxes are but little affected by the assumed mass 
of the moon, and depend almost entirely upon the observed 
value of the parallactic inequality, the relation between small 
changes of p and Q being 
dp = 0°071 dQ (24) 
The original observed values of Q are affected by personal 
equation, irradiation, blurring, and any error which may exist 
in the adopted semi-diameter of the moon. It is difficult to 
estimate how thoroughly these quantities are eliminated from 
the final result, but the remaining uncertainty probably 
amounts to a considerable fraction of a second. 
