Half-hours with the Telescope

The sort of scrutiny required for the discovery of changes, or for the determination of their extent, is far too close and laborious to be attractive to the general observer. Yet the kind of observation which avails best for the purpose is perhaps also the most interesting which he can apply to the lunar details. The peculiarities presented by a spot upon the moon are to be observed from hour to hour (or from day to day, according to the size of the spot) as the sun's light gradually sweeps across it, until the spot is fully lighted; then as the moon wanes and the sun's light gradually passes from the spot, the series of observations is to be renewed. A comparison of them is likely -- especially if the observer is a good artist and has executed several faithful delineations of the region under observation, to throw much light upon the real contour of the moon's surface at this point.

In the two lunar views in Plate 7 some of the peculiarities I have described are illustrated. But the patient observer will easily be able to construct for himself a set of interesting views of different regions.

It may be noticed that for observation of the waning moon there is no occasion to wait for those hours in which only the waning moon is visible during the night. Of course for the observation of a particular region under a particular illumination, the observer has no choice as to hour. But for generally interesting observations of the waning moon he can wait till morning and observe by daylight. The moon is, of course, very easily found by the unaided eye (in the day time) when not very near to the sun; and the methods described in Chapter V. will enable the observer to find the moon when she is so near to the sun as to present the narrowest possible sickle of light.

One of the most interesting features of the moon, when she is observed with a good telescope, is the variety of color presented by different parts of her surface. We see regions of the purest white -- regions which one would be apt to speak of as snow-covered, if one could conceive the possibility that snow should have fallen where (now, at least) there is neither air nor water. Then there are the so-called seas, large grey or neutral-tinted regions, differing from the former not merely in color and in tone, but in the photographic quality of the light they reflect towards the earth. Some of the seas exhibit a greenish tint, as the Sea of Serenity and the Sea of Humors. Where there is a central mountain within a circular depression, the surrounding plain is generally of a bluish steel-grey color. There is a region called the Marsh of Sleep, which exhibits a pale red tint, a color seen also near the Hyrcinian mountains, within a circumvallation called Lichtenberg. The brightest portion of the whole lunar disc is Aristarchus, the peaks of which shine often like stars, when the mountain is within the unillumined portion of the moon. The darkest regions are Grimaldi and Endymion and the great plain called Plato by modern astronomers -- but, by Hevelius, the Greater Black Lake.