Mock Moons

Abstract

I NEVER noticed that mock moons and mock suns are not always at the same altitude as the moon or sun, but I would point out that when objects are high up, it is very difficult to decide on their relative altitudes. If mock moons are at the same altitude as the moon, then of course they are not on a great circle, but on a small one, and in consequence, except when they are low down, a straight line passing through the mock moons will pass above the moon, and when they are high up, at a considerable distance above it. In such a case, if the observer does not look straight at the moon, he may easily suppose that one of the mock moons is higher up than the other. Is your correspondent (vol. xxvii. p. 606) sure that they were not at the same altitude on the occasion he refers to? If, instead of facing the moon and looking straight at it, he looked more at the right-hand mock moon, the illusion would be produced of the left-hand one appearing higher up. The same illusion is caused when the horizontal lines of buildings or of a window cut the line passing vertically through the moon obliquely; so that great care is required in making these observations.