410 Lord Kelvin on the Formation of 
and ocean depths, have been produced by long continued 
geological actions of upheavals and erosion. 
§ 29. Immediately after the separation, the Moon, about 
-g^ of the Earth's mass, would begin moving from the 
perigee of a somewhat approximately elliptic orbit round 
the centre of inertia of Earth and Moon, much disturbed on 
account of the great and violently changing deviations from 
sphericity of the two masses. The period of this orbital 
motion of the two bodies round their centre of inertia would 
be longer than the rotational period of the Earth, which 
would be but little changed by the catastrophe. In becoming 
rounded into a spheroidal form, the Moon would come to 
rotate round its own axis in a somewhat shorter period than 
that of the whole mass before the separation. Thus, in the 
beginning of the new regime, we have three different 
periods ; the shortest being the rotational period of the Moon 
round an axis through her centre of inertia ; somewhat longer 
than this the rotational period of the Earth ; and considerably 
longer tban it, the orbital period of the two bodies round their 
centre of inertia. 
§ 30. The changes of shape of the two semiplastic spheroids 
in their subsequent motions under the influence of mutual 
gravitation between all their parts, would give rise to a loss 
of energy : while the total moment of momentum would 
remain unchanged. The main action would be loss of kinetic 
energy of the Earth and Moon, by transformation into heat 
of quasi-tidal work within the two bodies. Essentially con- 
comitant features would be augmentation of the distance 
between them, involving "work done against mutual gravity, 
and gradual transformation of the moment of momentum of 
their rotations into augmentation of the moment of momentum 
of their orbital motions round the centre of inertia of the 
two. The energy of the initial rotation of the Moon would 
be small compared with that of the Earth. The whole kinetic 
energy of the rotations, and the motions of centres of inertia, 
of the two bodies, at the present time exceeds by a relatively 
small quantity the present kinetic energy of the Earth's 
rotation. The work done in separating the Moon to its 
present distance from the Earth, and in giving it the kinetic 
energy of its orbital motion, has been wholly drawn from the 
Earth's rotational kinetic energy at the time of the disruption, 
with the exception of a small contribution derived from the 
Moon's initial kinetic energy of rotation. A comparatively 
early result of the motions of the two bodies must have been 
to bring the Moon to keep always the same face to the Earth 
as she does at the present time. 
