There is a phenomenon called the ho rizontal moon, which is this, that it ap pears larger in the horizon than in the meridian ; whereas, from its being fur ther from us in the former case than in the latter, it subtends a less angle when in the horizon. It is perhsps not easy to give a satisfsctory answer to this decep tion. Gasaendus thought, that as the moon was less bright in the horizon than in the meridian, we looked at it, in the former situation, with a greater pupil of the eye, and therefore it appeared larger. But this is not agreeable to the princi ples of optics, since the magnitude of the image upon the retina of the eye dors not depend upon the size of the pupil. Des Cartes thought that the moon apiieared largest in the horizon, because, when com paring its distance with the intermedi ate objects, it appeared then furthest off; and as we iudre its distance !treater ill.
that situation, we, of course, think it larger, supposing that it subtends the same angle. Dr. Berkeley accounts for it thus : faintness suggests the idea of greater distance ; the moon appearing faintest in the horizon, suggests the idea of greater distance ; and, supposing the angle the same, that must suggest the idea of a greater tangible object. He does not suppose the visible extension to be greater, but that the, idea of a greater tangible extension is suggested by the alteration of the visible extension. He says, 1. That which suggests the idea of greater magnitude, must be something perceived ; for that which is not per ceived can produce no effect. 2. It must be something which is variable, because the moon does not always appear of the same magnitude in the horizon. 3. It cannot lie in the intermediate objects, they remaining the same ; also, when these objects are excluded from sight, it makes no alteration. 4. It cannot be the visible magnitude, because that is least in the horizon. The cause, therefore, must lie in the visible appearance, which pro ceeds from the greater paucity of rays coming to the eye producing faintness.
Mr. Rowning supposes, that the moon ap pears furthest from us in the horizon, be cause the portion of the sky which we see appears not an entire hemisphere, but only a portion of one; and hence we judge the moon to be further from us in the horizon, and therefore larger. Dr. Smith, in his optics, gives the same reason. The same circumstances take place in the sun. Also, if we take two stars near each other in the horizon, and two other stare near the zenith at the same angular distance, the two former will appear at a much greater distance from each other than the two latter. On this account, people are, in general, much deceived in estimating the alti tudes of the heavenly bodies above the horizon, judging them to be much great er than they are. The lower part of a rainbow also appears much wider than the upper part ; and this may be consi dered as an argument, that the phenome non cannot depend entirely upon the greater degree of faintness of the object when in the horizon, because the lower part of the bow frequently appears brighter than the upper part, at the same time that it appears broader. Also, faint ness can have no effect upon the angu lar distance of the stars and as the dif ference of the apparent distance of the two stars, whose angular distance is the same in the horizon and the zenith, seems to be fully sufficient to account for the apparent variation of the moon's diame ter in these situations, it may be doubt ful whether the faintness of the object enters into any part of the cause.
The mean distance of the moon from the earth is about two hundred and thirty nine thousand miles ; and her semi-dia meter is nearly three-elevenths of the radius of the earth, or about one thou sand and eighty-one miles. And as the magnitudes of the spherical bodies are as the cubes of their radii, the magnitude of the moon : magnitude of the earth : : 33 : 113 : : 1 : 49 nearly. See Vince's Astronomy.