MOON'S SURFACE The surface of the moon has been a subject of careful and continuous telescopic study from the time of Galileo. The early observers seem to have been under the impression that the dark regions might be oceans; but this impression must have been cor rected as soon as the telescope began to be improved, when the whole visible surface was found to be rough and mountainous. The work of drawing up a detailed description of the lunar sur face and laying its features down on maps has from time to time occupied telescopic observers. The earliest work of this kind, and one of the most elaborate, is the Selenographia of Hevelius, a magnificent folio volume. This contains the first complete map of the moon. Names borrowed from geography and classical myth ology are assigned to definite regions and features. A system was introduced by Riccioli in his Alinagestum novurn of designating the more conspicuous smaller features by the names of eminent astronomers and philosophers, while the great dark regions were designated as oceans, with quite fanciful names: Mare imbrium, Oceanus Procellarum, etc. This nomenclature has been retained. With the improvement of the telescope more extended maps based on drawings and measurements were constructed by J. H. Schroter of Lilienthal, and by W. Beer and J. H. Madler in the nineteenth century. The application of photography to astronomy has greatly simplified the recording of lunar formations. Pioneer work was done by J. W. Draper of New York as early as 185o followed by Bond at Harvard, De la Rue in England and Ruther ford in New York. Excellent photographs have been taken at a number of observatories including Lick and Paris. A whole vol ume of Harvard Annals (vol. Li.) is devoted to photographs of the moon showing different portions with varying illumination.
The moon as seen by the naked eye is a beautiful object with bright and dark patches, but as seen in even a small telescope it is magnificent and the various formations stand out as in the photographs. The most interesting time to watch the moon with a telescope is near half moon. The boundary between the bright and dark portions, known as the terminator, then shows a very rugged appearance and even a short stay at the telescope enables one to watch the sun's light catch new mountain peaks.
On account of the nearness of the moon it is possible with large telescopes to see objects less than a mile in diameter. The most striking formations on the moon are the craters, which are of all sizes up to 1 oo miles or more in diameter and are scattered over the surface with great profusion, frequently overlapping. These craters in appearance closely resemble the volcanic craters on the earth, and it is possible that they have a similar origin. They have, however, often so large a diameter compared with height that the analogy may not be so close as at first appears. The typical crater has a surrounding ring rising to anything up to 20, 000ft. above the general level. The floor of the crater may be higher or lower than the outside level. Often there may be a cen tral peak or peaks within the crater. The darker areas which are not so much covered by craters have been considered to be seas of lava which have spread over the moon's surface at a later date than that of the formation of most of the craters. Other striking features on the moon are streaks or rays which radiate from a number of craters to many hundreds of miles.
As the moon is devoid of air and water there can be no meteorological variations of the kind experienced on the earth. Changes on the moon's surface could therefore be produced only by the direct action of the sun's rays. It is at once evident that there must be an enormous range in the temperature of the sur face rocks of the equatorial regions of the moon which are alter natively exposed to 15 days of solar radiation and then to 15 days of darkness. During the last century attempts were made by Lord Rosse, S. P. Langley and others to measure the moon's heat, but the apparatus available was not sufficiently sensitive. Recently, however, bolometers of sufficient sensitivity have been constructed to measure not only the total heat from the moon, but that in different wave-lengths.