Laplace perceived the significance
of the fact that all the planets revolved in the same direction
around the sun; he noticed also that the movements of rotation of the
planets on their axes were performed in the same direction as that in
which a planet revolves around the sun; he saw that the orbits of the
satellites, so far at least as he knew them, revolved around their
primaries also in the same direction. Nor did it escape his
attention that the sun itself rotated on its axis in the same sense.
His philosophical mind was led to reflect that such a remarkable
unanimity in the direction of the movements in the solar system
demanded some special explanation. It would have been in the highest
degree improbable that there should have been this unanimity unless
there had been some physical reason to account for it. To appreciate
the argument let us first concentrate our attention on three
particular bodies, namely the earth, the sun, and the moon. First
the earth revolves around the sun in a certain direction, and the
earth also rotates on its axis. The direction in which the earth
turns in accordance with this latter movement might have been that in
which it revolves around the sun, or it might of course have been
opposite thereto.
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