Page:The Moon (Pickering).djvu/40

20 or -460° F. What the temperature of the day side may be under a vertical Sun is very uncertain. Sir John Herschel and Lord Rosse thought it might exceed that of boiling water. Ericsson concluded that it was far below zero, while Professor Langley considers it very uncertain, but probably not far from the freezing point.

The most satisfactory test hitherto made seems to be that of Professor Very, who compared the amount of heat received from the Moon by a bolometer with that received from an equal angular area of sunlit melting snow. The heat was next in each case allowed to pass through a piece of clear glass before reaching the bolometer. The glass allows nearly all the reflected heat to pass, but absorbs that radiated by the body itself. The total radiation in the two cases was about the same, but while the reflected heat was much greater from the snow than from the Moon, it was found that the radiated heat was much greater from the Moon than from the snow. This means that while the snow is the better reflector, as, indeed, we can see by inspection, the Moon is the hotter body. The observation is so direct and simple that it seems impossible to deny the accuracy of the conclusion, but of course it gives us no clue as to what the actual temperature is.

From his researches on the lunar spectrum Professor Langley is convinced that the Moon's temperature must be considerably below that of boiling water, so that all we can say for the present is that when the Sun is in the zenith the temperature on the Moon must lie somewhere between the freezing and boiling points of water, and not very near either of them.

It would be interesting to repeat Professor Very's observation, comparing the radiation from the surface of the Moon with that from a surface of rock illuminated by the Sun, and at temperatures ranging from the melting point of snow to the highest attained by rocks on the Earth's surface when exposed to a nearly vertical Sun. It would seem that in this manner we might somewhat increase the accuracy of our present knowledge of this interesting subject.