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was in the fluid state, and that her figure became ellipsoidal 
with the longer axis towards the earth under the actions of 
the centrifugal force from rotation, and the ablatitious force of 
the earth’s attraction on the fluid mass. 
The librations of the moon commencing with the diurnal 
libration discovered by Galileo, and the libration in longitude 
discovered by Hevelius, as well as the libration in latitude were 
explained by Newton on the above supposition. Cassini, the 
celebrated French Astronomer Royal, discovered the remarkable 
property of the lunar motions that the nodes of her equator 
coincide with the nodes of her orbit, and that a plane through 
her centre parallel to the plane of the ecliptic lies between the 
planes of her equator and her orbit, so that the poles of the 
ecliptic, her orbit, and her equator are in the same great circle, 
but the two latter on opposite sides of the first. He con- 
cluded that the inclination of the lunar equator to the ecliptic 
was 2° 30’. 
Mayer, in the middle of the last century, found the in- 
clination of the moon’s equator to the ecliptic to be 1° 45’, 
Lalande later found it to be 2° 9’, but recent observations con- 
firm Mayer’s result. 
These are called apparent librations, but Newton discussed 
the existence of a real libration or oscillation of the longer axis 
of the moon’s figure about its mean place. D’Alembert, La- 
grange, and Laplace applied refined analytical methods to this 
problem, but the conclusion of Professor Grant of Glasgow, 
Fellow of the Royal Astronomical Society, in a note in his 
excellent work, the History of Astronomy, has the following: 
“It is natural enough, indeed, to suppose that the illustrious 
author of the Principia did not feel any anxiety to repudiate 
the original equality of the motions of rotation and revolution 
—a relation which, although perhaps difficult to explain by the 
doctrine of chances, becomes very interesting and suggestive 
when it is considered as the result of Supreme Intelligence.” 
