Page:The Moon (Pickering).djvu/107

Rh the boundaries of the shifting white layer of hoarfrost which is formed periodically about these craters.

When frost is found lying in the interstices of a broken surface it does not become visible upon the Moon, as we have already seen, until a day or two after sunrise, and it disappears a day or two before sunset. This is the normal condition, according to our observations, except in those comparatively small areas where the snow surface is continuous, and where it is therefore visible at all times from sunrise to sunset. Certain still smaller areas also are found, however, where the snow disappears about or soon after midday. This disappearance cannot be explained by means of black shadows in crevices, and apparently can be due only to evaporation. It implies necessarily an active monthly deposition of hoarfrost to make up for that which disappears.

In the photographs, Plate G, Figure 3, shows the true outline of M and the true outline of A, l, m and n combined. Neither l nor n is as bright as m, so that there is a slight tendency to make the combined outline triangular. A temporary brightening of l, as the Sun rises higher upon it, causes the slight irregularity in the form of the combination crater in Figure 4. In Figure 5, divisions l and n have faded so that they are of the same darkness as the surrounding mare and are therefore invisible. The evaporation of a thin layer of frost under the Sun's rays would readily account for this darkening. The spreading of the white haze north, south and east upon the outer crater walls of Messier as the Stm rises higher upon them, as shown in Figure 5, and its subsequent disappearance in Figtires 7 and 8, illustrate the same phenomenon upon a larger scale. Thus Messier becomes larger in Figure 5 than A.

In Figure 6 the melting about Messier has already begun; m, too, has become somewhat fainter, while the variable spots in the bottoms of the craters are making themselves visible. The difference in brightness between the snow-craters and the two smaller craters north and south of them, shown in Figures 5 and 6, is very striking. In Figures 3 and 4, where the frost has not yet appeared, no such contrast is found.

In Figure 7 the frost in m has completely evaporated, rendering it invisible. At the eastern end of both craters it is also disappearing, partly, perhaps, through evaporation and partly because the Sun is getting low and shining down into the craters tangentially to the surface on the eastern side. It must be remembered that on the Moon the Sun sets in what we call the east. As the frost disappears, both craters shorten, but Messier still appears broader in a north-and-south direction than it is in fact, because we still