of momentum of Neptune 8
Rotational moment of momentum of Sun 2
--
100

The contributions of the other items are excessively minute. The orbital
moments of momentum of the few interior planets contain but little more
than one thousandth part of the total amount. The rotational
contributions of all the planets and of their satellites is very much
less, being not more than one sixty-thousandth part of the whole. When,
therefore, we are studying the general effects of tides on the planetary
orbits these trifling matters may be overlooked. We shall, however, find
it desirable to narrow the question still more, and concentrate our
attention on one splendid illustration. Let us take the sun and the
planet Jupiter, and, supposing all other bodies of our system to be
absent, let us discuss the influence of tides produced in Jupiter by the
sun, and of tides in the sun by Jupiter.

It might be hastily thought that, just as the moon was born of the
earth, so the planets were born of the sun, and have gradually receded
by tides into their present condition. We have the means of enquiry
into this question by the figures just given, and we shall show that it
is impossible that Jupiter, or any of the other planets, can ever have
been very much closer to the sun than they are at present. In the case
of Jupiter and the sun we have the moment of momentum made up of three
items. By far the largest of these items is due to the orbital
revolution of Jupiter, the next is due to the sun, the third is due to
the rotation of Jupiter on its axis. We may put them in round numbers as
follows:--

Orbital moment of momentum of Jupiter 600,000
Rotational moment of momentum of Sun 20,000
Rotational moment of momentum of Jupiter 12

The sun produces tides in Jupiter, those tides retard the rotation of
Jupiter. They make Jupiter rotate more and more slowly, therefore the
moment of momentum of Jupiter is decreasing, therefore its present value
of 12 must be decreasing. Even the mighty sun himself may be distracted
by tides. Jupiter raises tides in the sun, those tides retard the motion
of the sun, and therefore the moment of momentum of the sun is
decreasing, and it follows from both causes that the item of 600,000
must be increasing; in other words, the orbital motion of Jupiter must
be increasing, or Jupiter must be receding from th