Page:The Solar System - Six Lectures - Lowell.djvu/147

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We cannot be certain of Uranus and Neptune because we cannot see their surfaces well enough to be sure of the position of their axes, but the planes in which their satellites revolve makes the value given altogether likely.

The tidal friction explanation of this would make Neptune very much the oldest planet, Uranus very much the next so, and so on. But the explanation is not satisfactory.

Our solar system has, as I have said, a very small relative moment of momentum; only the one thousandth part of what it might have as exemplified in the system of α Centauri.

One supposition will account for the small moment of momentum of the system, without supposing the individual motions so nearly balanced at the start. The moment of momentum would be small if the principal mass were initially collected in the centre of the nebula. Now this