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 in action. The walls of Aristarchus are also conspicuously terraced and rise about 4000 feet above its floor. [Figure 165] South of Aristarchus and east of Copernicus may be seen _Kepler_, a 22-mile crater noted for its extended system of glistening streaks. Its surrounding wall, like that of Tycho and Copernicus, seems to be covered with the same shining substance. The wall of Kepler is very low but its crater is about 10,000 feet below the exterior plain. Southeast of the two bright craters, Kepler and Copernicus, and almost at the edge of the moon, lies the darkest and also one of the largest of the moon's craters, or walled-plains, as the large craters are now more frequently called. This huge crater, _Grimaldi_, extends north and south for a distance of 148 miles and covers an area of 14,000 square miles. It is very noticeably dark in comparison to all surrounding objects. _Gassendi_, a little southwest of Grimaldi, contains a great variety of detail both on its rim and on its floor. Its plain is 54 miles in diameter and includes a number of central peaks.