Many theories have been advanced to explain the origin of the lunar craters which have no counterpart on our own planet. They are saucer-like depressions in the surface of the moon, frequently of such great size that an observer standing in the center would not be able to see either side of the crater owing to the curvature of the moon's surface. Craters fifty, sixty or one hundred miles in diameter are by no means uncommon, while there are thousands between five and fifty miles in diameter. A characteristic feature of many craters is a central peak, and the surrounding walls are often a mile or more high and in some instances are symmetrically terraced. New craters have been formed on the sides or floors of old craters, and these are always more clear-cut and sharper in outlines than the old formations, and generally much smaller. A number of craters are surrounded by a system of light streaks or rays of unknown origin that extend in some instances to enormous distances on all sides of the crater. The most conspicuous system is the one surrounding the lunar crater Tycho near the south pole of the moon. The rays originating in this crater extend in all directions for hundreds of miles without turning aside for any obstructions, passing over mountains, craters and plains in their course in practically straight lines like spokes in a wheel. This ray system of Tycho is the most noticeable marking on the moon's surface at the time of full moon. As these streaks cast no shadow they are apparently cracks in the surface that have filled up with some light-colored material from below. Their origin has never been satisfactorily explained. As to the origin of the lunar craters, some believe that they were produced in past ages by a bombardment of the lunar surface by huge meteoric masses; but there are many objections to this theory that we will not take up here. It is more generally believed that the lunar craters are a result of volcanic activity on an enormous scale which took place on the moon many ages ago and which has now practically ceased, its only manifestations now taking the form of a quiet emission of gases from small volcanic vents or fumaroles which exist all over the lunar surface but which are to be found in greatest numbers on the floors and sides of craters. XXII COMETS The orbits of comets are inclined at all angles to each other and to the orbits of the planets which, on the other hand, lie very nearly in the same plane. The larger members of the sun's family, the planets and their satellites, revolve from west to east around the sun. Comets on the contrary frequently retrograde or back around the sun in the opposite direction--from east to west.