Page:The evolution of worlds - Lowell.djvu/148

116 nearly at the Earth, and its tropical features, the places where the belts lie, are wholly hidden or greatly fore-shortened from our point of view. As the planet's year is eighty-four of our years long, it is only at intervals of forty odd years that the disk is well enough displayed to bring the belts into observable position.

The planet is attended by four satellites,—Ariel, Umbriel, Titania, and Oberon,—a midsummer night's dream to a watcher of the skies. They travel in a plane inclined 98° to the ecliptic, so that their motion is nearly up and down to that plane and even a little backward. Whether their plane is also the equatorial plane of the planet, we do not know for certain. The observations as yet are not conclusive one way or the other. If the two planes should turn out not to coincide, it will open up some new fields in celestial mechanics. The belts have been thought to indicate divergence, but the most recent observations by Perrotin on them minimize this. They suggest, too, a rotation period of about ten hours, which is what we should expect.

Its albedo, or intrinsic brightness, is, according to Müller, 0.73, or almost exactly that of cloud. This tallies with the lack of pronouncement of the belts and is another argument against the reality of the recent diametral measurements, as all Müller's values are got by dividing the amount of light received by the amount of surface sending it. If the diameter were