A crater only six miles in diameter may be easily seen in a small telescope, although there are thousands less than a mile in diameter which may be seen in a large telescope. Craters from 16 miles to 60 miles in diameter are common and a few even measure as much as 100 to 150 miles. These craters have deeply sunken floors and are usually surrounded by gigantic mountain walls which rise like a fringe of rock above the general level of the region outside. Near the center of many of the craters with the largest brims, one and sometimes two or three sharp-peaked mountains tower, casting a long, weird shadow over the crater's floor. The surrounding brim is almost invariably higher than the central mountain, yet many of these central mountains are a mile or so high. Probably the deepest of the moon's craters is _Theophilus_, on the edge of the circular "sea" Mare Nectaris, and it is best seen while the moon is still a slender crescent. The encircling walls of this crater rise from 14,000 to 18,000 feet above its floor, the floor itself being 10,000 feet below the general level of the moon outside the wall. The walls of this amazing cavity, in the main, slope gently to the outer plain but rise very steeply from the depths of the inner floor which measures 64 miles across. Three lofty cone-shaped mountains rest in the center of this floor, their sharp, black shadows stretching to the left. One of these mountains rises 6000 feet, yet its summit is some 4000 feet lower than the level of the outside plain! Imagine standing on the rim of Theophilus and watching the darkness flow into its huge bowl until the mountain peaks in the center look like sunny isles in a dense, black sea! This is a _real_ crater, for the word "crater" means "cup." Most of the large craters on the moon, however, are not so "cup-like," but, to pursue the simile still farther, are more like platters and saucers and individual butter plates, for their diameters range from five times to more than fifty times their depth. [Figure 164: THE LUNAR CRATER COPERNICUS, FIFTY MILES IN DIAMETER. Photograph by Mount Wilson Observatory through the 100-inch Hooker telescope.] 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.