“ 
W. Harkness—The Solar Parallazx. + 387 
1 
| 81°5° 
sible that this estimate may be in error by one part in a hun- 
dred. The precession-nutation method is considered one of the 
best for obtaining the moon’s mass, but equations (21) and (22) 
show that neither it, nor the method by the fall of the moon 
in its orbit, is likely ever to furnish the mass within one part 
ina thousand. Throughout all his lunar work Hansen adopted 
‘Probably the moon’s mass is about but it is quite pos- 
a mass of rr and in what follows I wil] assume that the true 
a 1 
mass lies between the limits — and —. 
etween tf imit, 30 83 
Parallax from Gravitational Methods. 
Mass of the Earth—In 1872 LeVerrier obtained the mass of 
the earth from the inequalities in the motions of Venus and 
Mars, and the secular variations in the elements of their orbits, 
produced by it; and from the mass thus found he derived the 
solar parallax by means of an equation similar to (10). (CRH, 
1872, t. lxxv, pp. 165-172; MNt, 1872, vol. xxxii, pp. 322- 
328.) He gave the resulting parallaxes without directly stat- 
ing the masses, but it is readily seen that his values were as 
follows : 
(A). From the latitudes of Venus at the moments of the 
transits in 1761 and 1769, earth’s mass = 395,166 ‘ 
(B). From a discussion of the meridian observations o 
Venus in an interval of one hundred and six years, earth’s 
Mass = 
324,575 oe 
(C). From observations of the occultation of ¢* Aquarii by 
Mars, October 1st, 1672, earth’s mass = oe 
746" 
Substituting these values in equation (10), the resulting 
values of the solar parallax are 
A 8”°862 
B 8°868 
C 8°875 
Taking the earth’s mass as unity, the change in the parallax 
produced by a change of one thousand units in the mass of the 
sun is given by the expression 
dp = 000912 dS (23) 
It is difficult to estimate the probable error of the above 
values of the earth’s mass, but Tisserand seems to think it 
