Page:The moon (1917).djvu/10

Rh Now let us look at the Moon itself as it is revealed to us by the telescope. Our first surprise is to find the surface so extremely broken and rugged; the next is that we can see the details of all the features so clearly. Visitors to the Lick Observatory often ask how near the great telescope brings the Moon to us. This, of course, depends upon the magnifying power we use. With a power of 1000, which is as great as can be used to advantage under ordinary conditions in studying the surface of a planet or of the Moon, it is, in effect, brought within about 240 miles of the Earth's surface. But this does not give quite a fair idea of the distinctness with which we see the lunar surface details because when we view an object like a mountain 240 miles distant on the Earth we are seeing it thru a much denser layer of our atmosphere. On a clear winter's day at Mount Hamilton, for example, we can see the Sierras stretching from the far northeast to the far southeast and can readily make out some of the prominent landmarks about the Yosemite Valley 180 miles due east of us without the aid of glasses. But we cannot see them as well defined as we do the Moon's features thru our telescopes. Objects on the Moon having a diameter of 1,000 feet are easily seen and those with half that diameter would hardly escape detection. Smaller inequalities of the surface, or an ordinary house, a single tree, or animal or plant would be invisible. Rugged as the Moon looks to us therefore, its actual surface is probably rougher still.

On that side of the Moon which is visible to us, there are no less than ten mountain ranges of considerable extent, numerous isolated peaks, some 10,000 cracks or "rills" and more than 30,000 "craters" which have been mapped and, for the most part, named. There are also the large dark areas which from Galileo's time have been known as "maria" or seas, tho we have long been aware that they are dry. The system of nomenclature dates back to Riccioli, who, in 1651, published a lunar map on which several hundred mountains and craters were named for distinguished astronomers and mathematicians. The names Alps and Appennines and a few others date back still farther—to 1645, when Hevelius constructed the first satisfactory map of the Moon. The Moon, then is a world, as someone has said, which has no weather and where nothing ever happens.