Page:The Moon (Pickering).djvu/35

 CHAPTER III

the Moon's atmosphere is much less dense than our own we can leam without a telescope by mere inspection. If the Moon had a dense atmosphere we should see that large portions of its surface were at times obscured by clouds, as is the case with the Earth, Jupiter and Venus. We should also notice that its surface features, especially near the edge, were indistinct, and with the telescope these peculiarities would become much more conspicuous. Nothing of the sort is ever seen, however. The only possible exception consists of certain small local clouds, in whose existence some selenographers believe.

To say that the Moon's atmosphere is very rare is one thing, but to measure its density is quite another, and that astronomers can even get a clue to this without actually going there, barometer in hand, may at first sight seem very surprising. The method actually employed, however, is very simple. We know that when a ray of light coming from a star passes through the Earth's atmosphere in a nearly horizontal direction it is bent out of its course or refracted through an angle of about half a degree. Similarly, a ray of light coming from a star and passing very near to the Moon's surface should be refracted by its atmosphere, and by measuring the angle through which it is bent we should be able to compute how dense that atmosphere must be.

This is the view which astronomers generally have held, and it is undoubtedly correct when the atmosphere is reasonably dense. When, however, the atmosphere becomes extremely rare, so that the individual molecules of which it is composed are relatively far apart, we are not sure just what will be the effect on the refraction, and it is barely possible that the Moon may in reality have a much denser atmosphere than observation would seem at first sight to indicate. This refraction is extremely slight, for the Moon frequently passes in front of, or occults, small stars, but the displacement produced at such times is so small that we cannot measure it. We cannot therefore say what the exact density of the Moon's atmosphere really is. On the assumption that it 15