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planet Neptune revolves on its axis in the same direction as the satellite moves. But this cannot be determined, because it is so distant and its disk so faint and diffuse that no markings can be detected upon it. Indeed, if we reflect that the rotation of a planet so near us as Venus has never been certainly determined, we may easily see how hopeless is the prospect of determining that of Neptune.

But, in spite of this, there is remarkable evidence that the planet has a rapid rotation. It is found that the orbit of the satellite is very slowly changing its position from year to year. During the half century since observations commenced, this change amounts to several degrees. The only way in which it can be accounted for is by supposing that Neptune, like the earth and the other rapidly rotating planets, is an oblate ellipsoid, and that the plane of the planet's equator does not coincide with that of the orbit of the satellite. In time the astronomer will be able to learn from this motion the position of the poles and equator of the planet Neptune, but this may require a century of observation, or even several centuries.