Page:The Music of the Spheres.djvu/325

 Chacornac mentions a walled-plain called Schickard, on the southeast portion of the moon, which is 134 miles in breadth, and, although encircled by a mountain wall which in one place is nearly 10,000 feet high, a spectator centrally located on the floor of the crater would think himself on a boundless desert, for this encircling wall, due to the curvature of the surface of the moon, would lie entirely beneath his horizon.

One of the most magnificent craters on the moon is the great crater Copernicus. Copernicus lies in an isolated position a little southeast of the range of the Apennines, on the darkened space called Mare Imbrium. Being not far from the center of the lunar disk, it is an easy object for the eye to find and to use as a guiding point from which to locate other craters. A circular range of mountains much terraced on the inner side surrounds the great plain of Copernicus, while in the very center of its 56 miles of floor stands a solitary mountain with an altitude of 11,000 feet. It is believed by many astronomers that such terracing as is shown in the interior of Copernicus, is mainly due "to the repeated alternate rise, partial congelation, and subsequent retreat" of a great flow of lava which rose up from the floor during a period long past. A number of other craters also show such terracing. The bright appearance of Copernicus is caused by an interesting system of bright streaks which radiate from the circular rim of its mountains. Tycho, near the southern pole, has also such a system of radiating streaks, but so enlarged and intensified are the rays of Tycho's system that they run for hundreds of miles and dominate the whole scene when the moon is full.

Aristarchus, northeast of Copernicus, is the brightest single point on the moon. The peak in the center of its 29 miles of brilliant interior shines so brightly that it has often been seen on the dark side after new moon. Indeed, when Sir William Herschel first observed it through a telescope he mistook it for a volcano