On account of the vividness of the shadows, moon-mountains are best observed during the first and last quarter of the moon. A good time to begin observing with a glass is when the moon is a narrow crescent in the western sky. [Figure 162] The most prominent object in the narrow crescent is the Mare Crisium or the Sea of Crises, 380 miles long. South of Mare Crisium is Mare Fecunditatis, the Sea of Fecundity and Mare Nectaris, the Sea of Nectar. When these three "Seas" are visible with Mare Tranquillitatis just coming into view, the moon is between 5 and 6 days old, that is, 5 or 6 days after new moon. The Leibnitz mountains may then be seen near the southern pole. Watch carefully evening after evening as the light creeps over new regions and brings them into view. The best place to select for observation is the boundary line between the illuminated and unilluminated portions, called the "terminator," for the shadows that are cast make the irregular features stand out more distinctly and bring out the individual beauties of the lunar scenes. Since we are looking down upon the moon, a mountain peak, at this distance, resembles a point of light, and a mountain range whose base is still in the shadow, but whose peaks are lighted by the rays of the sun, a straight or curved row of lights. Besides beautifying the country of the moon and revealing to us the peculiar sharpness of the mountain peaks, the shadows have proved of great practical value to astronomers, for by measuring the length of their shadows, the heights of the mountains are calculated. THE CRATERS ON THE MOON Craters are so numerous in some localities on the moon that one might walk for hundreds and hundreds of miles and step on nothing but crater rims. On the earth a crater is usually a rare object situated on the top of a mountain, but on the moon they are scattered all over the lunar plains, a state of landscape which permits of no short cuts. If, however, in the dim, distant past, life had ever developed on the moon, a moon-being would have been %th as heavy and able to jump six times as high as an earth-being for the surface gravity of the moon is only about ⅙th that of the earth. Thus if a lunarian had no other mode of transportation, he might at least have been able to surmount many of the minor obstacles by jumping over them. A map of the moon resembles a big mud-ball with all the little pebbles picked out. [Figure 163: NORTHERN PORTION OF THE MOON. Note the "Sea of Showers," the "Bay of Rainbows," Copernicus and Plato, the three craters north of the lunar "Apennines," the Caucasus Range, the Alps and the Valley of the Alps. Refer to the maps. Photograph by Mount Wilson Observatory through the 100-inch Hooker telescope.]