Page:The Moon (Pickering).djvu/115

Rh we shall find that it will appear to us decidedly smaller than the part of the wall which it previously covered. It is an analogous effect which makes the full moon when rising or setting appear larger than when it is well up in the sky. On the horizon we can compare it with trees and houses and see how large it really is; overhead we have no linear scale of comparison.

It is certain that this is not the only reason, however, nor even the chief one, that makes the Moon appear larger when near the horizon. The same optical illusion applies when at sea, and it applies also to the constellations—for example, to Orion. When rising they appear decidedly larger than when near the meridian, and yet no comparison of their size with that of terrestrial objects is usually possible. There is evidently another circumstance affecting our estimates of angular diameter. The explanation of this was first given by Clausius about thirty years ago, as has recently been pointed out by Professor Searle, but it has not as yet got into the text-books. The circumstance chiefly affecting our estimates of size depends on the angular altitude of the object under consideration.

When we pass under an archway or under the limb of a tree we know that we are nearer to the object than we are when we see it under a lower altitude; at the same time it appears just as large to the average person angularly as it does when we are several feet farther away. We are, in fact, used, all our lives as we walk about, to see objects rapidly shifting their angular positions, yet not appearing as we pass them any larger than they do when we are slightly more distant from them. We thus always unconsciously make some compensation in our minds for the real changes in angular size that actually occur.

If now the limb of the tree that we passed under, instead of really growing angularly smaller at the low altitude than it was when overhead, should remain of the same angular size in all positions, we should say that it looked larger at the low altitude. This is exactly what happens in the case of the heavenly bodies. Unlike all terrestrial objects, they are practically of the same real angular dimensions when on the horizon that they are in the zenith. We involuntarily apply to them the same compensation that we are accustomed to apply to terrestrial objects, and are then naturally surprised to see that they appear larger at the lower altitude.

The superstitions relating to the Moon are many, the majority of them concerning the weather. Besides these, one of the commonest, particularly among sailors, is the