Apparent Size of Celestial Objects

Abstract

ABOUT fifteen years ago I was looking at Venus through a 40-inch telescope, Venus then being very near the Moon and of a crescent form, the line across the middle or widest part of the crescent being about one-tenth of the planet's diameter. It occurred to me to be a good opportunity to examine how far there was any reality in the estimate we form of the apparent size of celestial objects. Venus through the telescope, with a magnifying power (speaking from memory) of 135, looked about the size of an old guinea, i.e., of a crescent cut off from that coin. The Moon, to my naked eye, appeared the size of a dessert plate. Having fixed their apparent dimensions in my mind, I adjusted the telescope so that with one eye I could see Venus through the telescope, and with the other the Moon without the telescope, and cause the images to overlap. I was greatly surprised to find that Venus instead of being about one-sixth of the diameter of the Moon was rather more than double its diameter, so that when the adjustment was made to bring the upper edge of the Moon coincident with the upper point of the crescent of Venus, the opposite edge of the Mpon fell short of the middle of the crescent, a very palpable demonstration of the fallacy of guesses at size, when there are no means of comparison.