Page:The Moon (Pickering).djvu/112

78 of M. Antoiniadi.the well-known astronomer, which is placed next to it, and which shows what he really saw, and not what he fancied. The crab and the girl reading were both evidently drawn from photographs (see Frontispiece), and not from the Moon itself, as they both show far more detail than can possibly be seen with the naked eye. If in the latter we imagine Tycho as the setting Sun, a rather pretty picture could be devised without straying very far from the outlines as presented to us by our satellite.

The size of the Moon as seen in the sky is very deceptive, and according to different persons differs from that of a cart-wheel to a silver dollar, depending on the distance at which these objects are assumed to be placed. Most people think it appears to be about a foot in diameter, from which Professor Young argues that to most people the distance of the surface of the sky is about 110 feet. Artists usually represent the Moon of much too large a size in their paintings to agree with the perspective of the rest of the picture. They also occasionally represent it in evening scenes with the horns of the crescent turned downward instead of upward. A little thought will show us that the horns must always point away from the Sim. The true angular size of the Moon is about half a degree; it can therefore always be concealed behind a lead pencil held at arm's length.

The Sun and Moon when rising or setting appear to most persons of from two to three times the diameter that they have when near the meridian. The cause of this phenomenon has been a source of speculation from the earliest times. Before optical science' was thoroughly developed, some thought that the image was really magnified by the vapours near the horizon. Not only is this incorrect, but in point of fact the Sun is slightly and the Moon measurably smaller when near the horizon, because they are farther off than when overhead.

The true explanation is twofold. Human estimates of angular dimensions are dependent not merely on the angular dimensions themselves, but also on several extraneous circumstances. The case is analogous to our estimates of weight, which are dependent primarily on the real weight of the object, but secondarily upon its bulk. Thus a pound of lead feels much heavier than a pound of feathers.

One circumstance affecting our estimates of angular dimension is the linear dimension of the object itself. It was pointed out by Alhazen about 900 years ago that if we hold the hand at arm's length and notice what space it apparently covers on a distant wall, and then move the hand well to one side so that it is in front of some very near object,