Page:The Moon (Pickering).djvu/14

vi equator. Whether there remain any more readily observed but still unknown facts in the solar system we cannot of course say, but if there are any it seems probable that they will be found in connection with the Moon, which has been less studied under favourable circumstances than some other bodies, such, for instance, as Mars, and which presents, moreover, a far greater wealth of detail than any other of the celestial objects.

To understand better what is meant by the terms a "steady" atmosphere and good "seeing," let us imagine that our observations are all made at the bottom of a pool of water. As long as the weather remains calm we get along very well, but as soon as a breeze springs up we had best pack up our telescopes and go home. That is the case with the astronomer interested in planetary research. The breeze represents the air currents engendered by the continuous succession of cyclones and anti-cyclones forever sweeping through northern latitudes. These air currents of varying temperature are quickly recognized by the astronomer and send their disturbing ripples far toward the equator. Even in Jamaica, when the Sun went south in winter and the northern anti-cyclones began, our definition became at once inferior to what we had found it when the Sun was nearly overhead in the summer season and the northern barometer was comparatively quiet. In Cambridge, few observations on the Moon or planets can ever be made to advantage. If anything new is to be learned we must go south, and those who remain at home have little chance at the new discoveries. What is now needed is a large telescope located near the equator, but just how near it is necessary or desirable to go has not as yet been definitely determined.

The earlier selenographers mapped and studied the whole surface of the Moon. Later observers confined themselves to smaller areas. The author's plan has been to devote his attention exclusively to a few very small selected regions. Occasionally a whole crater has been observed, but more frequently especial attention has been directed merely to a few square miles of surface, thus obtaining a complete and accurate record of all the very finest detail visible in that particular region under all conditions of lighting. A comparison of many such drawings, all made on a nearly uniform scale, extending sometimes through a period of several years, but each described with full details upon a uniform system, will occasionally bring to light a new fact. It is believed that this will be the method of advance of the selenography of the future.