Apparent Size of the Moon

Abstract

MY original intention was to put together several veræ causæ, which might be found, concurrently, to contribute to the universal impression that the moon's disc is larger or smaller, according as it is nearer to the horizon, or to the meridian. I shall content myself, however, with calling attention to what I am now persuaded is the nature of that impression. “Sweet are the uses of adversity.” An attack of hemiopsia is always serious, and may be dangerous (see NATURE, Feb. 24th, 1870; p. 444). I think I owe to it the discovery (for such it was to me), that the variable standard of angular magnitude which infects our visual judgment, can be detected in a small room as certainly as in view of the celestial vault. The distressing affection which succeeds the hemiopsia, as soon as it forms a broken arch around the central hole of the retina, is an instructive spectre in regard to the question I am considering. Being referred to two equally distant sites on the wall of the room, one horizontal and the other considerably elevated, the spectre seems larger in the former than in the latter. I soon proved that this was no accident case. I extemporised a very rough experiment on this wise—I placed a disc 14¼ inches in diameter on the wall 7 feet from the ground, and selected a horizon so that the base of the disc and the horizon were equidistant from a fixed point of observation. I found the disc was about 30° above the horizon. I now took six persons successively, and made each person take an observation from that point, first looking at the disc, and then transferring it in mind to the horizon, where I carefully marked the estimated size. The maximum was 13$\frac{5}{8}$, the minimum 10$\frac{5}{8}$, and the mean 12½ inches. This result is, of course, equivalent to saying that had an equal-sized disc been placed on the horizon, its diameter would, taking the average, have appeared to be 1¾ inch greater than when elevated 30° above it. I think it is worth while making this experiment with greater accuracy, and with a greater number of persons. I have no doubt Mr. Abbott's view of the case would be fully borne out. But I do not understand why the augmentation on the horizon is so much greater in the case of the moon and the sun; nor yet why the rising sun does not present so striking an augmentation as the rising moon. The augmentation of the latter may be partly an effect of external conditions; but the fact of augmentation, in what I have called visual judgment, is a question for the physiologist. I should much like to know what, for instance, such an authority as Helmholtz has to say on the matter.