On the Precession of the Equinoges. 141 
solitary moon continually situated on the side nearest the 
sun. 
It. therefore appears that the sun may have some effect 
upon the equatorial ring arising “ merely from the different 
distances” of its perihelion and aphelion. I shall now pro- 
ceed to mention how great an effect it has been calculated 
to have. Newton proved that the sun’s attraction should 
bring the equator sooner to the ecliptic so as to produce a 
mean precession in the equinoxes.of 9", 07!’ only, for he 
assumed as true that the motion of the nodes of a rigid ring 
of moons is just equal to the motion of the nodes of a soli- 
tary moon. Frisius demonstrated that the motion of the 
nodes of a rigid ring of moons must be double that of a soli- 
tary moon. Consequently taking Newton’s data he proved 
that the sun alone might cause a precession of 182". Vince 
shows that the sun acting upon the equatorial ring may 
cause a precession of 21”. 6”. The European astrono- 
mers all agree that the sun alone may cause an annual pre- 
cession from 18” to 21”. The mean annnal precession of 
the equinoxes is 501”. It is caleulated that the moon alone 
will produce a mean annual precession of about 30”. ‘The 
action of the moon upon the equatorial ring is not so uni- 
form as that of the sun, because its orbit is inclined to the 
equator 10° more at one time than at another. If the pre- 
cession is caused by the joint action of the sun and moon 
it should be variable. Facts.prove that it does vary ac- 
cording to the inclination of the moon’s orbit, and accord- 
ing to its place in the orbit. The moon when in the most 
favorable situation will produce a precession of 35” nearly. 
The annual precession should vary, in order to agree with 
calculations, from about 45” to 55”. ‘This agrees with fact. 
This calculation makes the influence of the sun about one 
half as great as that of the moon. Thisis as we should ex- 
pect, for in producing tides the sun exerts very near one 
half as much influence as the moon. 
3. Itis said that “the precession of the equinoxes if caused 
by the equatorial ring must arise”’ ‘‘ from a diminution of the 
angle of the equator with the ecliptic, or from a change 1 in the 
direction of the line of the equatorial nodes.” The writer 
goes on to state that we have no evidence of such a regular 
diminution of the angle. and immediately after says that the 
nutation of the earths poles is produced by the diminution of 
