Page:The Solar System - Six Lectures - Lowell.djvu/135



V. Their planes of rotation nearly coincide with their orbital planes (except Uranus and Neptune).

VI. They rotate also in the same direction that they revolve, counter-clockwise, all of them (except Uranus and Neptune).

VII. Their satellites revolve nearly in the planes of their primaries' equators (so far as we can see).

VIII. And in the same direction.

IX. They rotate in the same plane (so far as we can see).

X. In the same direction (so far as we can see).

Immanuel Kant was the first to suggest something approaching a rational explanation of this very curious and elegant state of things. He made the error, however, of supposing that rotation of the whole could be produced by collisions of its parts; but no moment of momentum can be caused by the interaction of parts of a system, since internal forces occur in pairs and their moments round any line are equal and opposite. We will consider this in detail a little further on. Laplace, who appears not to have known of Kant's writing, himself some years later developed a somewhat similar theory, but with more mathematical foundation. He assumed an original rotation and got the credit for the nebular hypothesis. He had a faculty of getting credit for things which was only second to his ability.