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As a consequence, while Enceladus moves through its slightly elliptical orbit, Saturn's gravitational field rocks the moon back and forth - or 'librates' it by tiny amounts.

Cause of librations.—That the moon should librate is by no means so remarkable a fact as that it should at all times turn very nearly the same face toward the earth.

At this period the balance of tropic and pole librates, and the vast atmospheric tides pour their flood upon one hemisphere and their ebb upon another.

He proceeded to see if by making the planet librate, or the plane of its orbit tilt up and down, anything could be done.

Whence the objects appear to librate or circulate according to the motions of our heads, which is called dizziness; and we lose the means of balancing ourselves, or preserving our perpendicularity, by vision.