As a matter of fact the two agree. The moon in
its monthly revolution around the earth follows also the same
direction, and our satellite rotates on its axis in the same period
as its monthly revolution, but in doing so is again observing this
same law. We have therefore in the earth and moon four movements,
all taking place in the same direction, and this is also identical
with that in which the sun rotates once every twenty-five days. Such
a coincidence would be very unlikely unless there were some physical
reason for it. Just as unlikely would it be that in tossing a coin
five heads or five tails should follow each other consecutively. If
we toss a coin five times the chances that it will turn up all heads
or all tails is but a small one. The probability of such an event is
only one-sixteenth.
There are, however, in the solar system many other bodies besides the
three just mentioned which are animated by this common movement.
Among them are, of course, the great planets, Jupiter, Saturn, Mars,
Venus, and Mercury, and the satellites which attend on these
planets. All these planets rotate on their axes in the same
direction as they revolve around the sun, and all their satellites
revolve also in the same way. Confining our attention merely to the
earth, the sun, and the five great planets with which Laplace was
acquainted, we have no fewer than six motions of revolution and seven
motions of rotation, for in the latter we include the rotation of the
sun.
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