Abstract
Robebt R. Coles, Hayden Planetarium, has an article insititle in Sky and Telescope of June, and aong a number of celestial phenomena that interesting and sometimes puzzling features il included the apparently greater diameter of the moon compared with the diameter when it has attained a higher altitude. Some text-books still repeat the old explanation, discarded many years ago, that the horizon moon is so situated that its size can be easily compared with terrestrial objects, but a,t higher altitudes we are deprived of these for comparison. Anyone can disprove this theory if he observes the moon near the horizon at sea, where no terrestrial objects are available for comparison. Yet the moon looks as large when rising or setting over the sea as it does when viewed on the land. Some . years ago, Drs. E. G. Boring and A. H. Holway, two Harvard psychologists, after a series of experiments, concluded that the illusion is due to a physiological cause. It has been found that,objects viewed straight ahead appear larger than do those of the same size in positions where the eye must be raised to see them. Although this theory is almost certainly the correct one, the basic causes are still somewhat of a mystery. The illusion can be observed in the constellations also, such as the Plough, which appears very much larger when low on the horizon than when high in the sky. Other groups of stars, like the Great Square of Begasus, the Northern Cross, etc., exhibit the same phenomenon. An experiment which can be performed by anyone on some of these groups of stars, or preferably on the moon, will show that the old theory is incorrect. When the moon is near the horizon, gauge it between the thumb and forefinger and notice it shrinking; as the finger and thumb are separated it appears to swell again. This experiment is referred to elsewhere in the same issue of Sky and Telescope, and shows that the illusion's due to a physiological or psychological cause.
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Sky Fantasia. Nature 158, 580 (1946). https://doi.org/10.1038/158580a0
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DOI: https://doi.org/10.1038/158580a0