REFRACTION NEAR THE HORIZON

G. M. Clemence

Peer Reviewed

Abstract:	The subject of astronomical refraction at low altitudes became a matter of practical and theoretical concern with the publication of an article by Charles H. Smiley in the March 1950 number of this journal, which apparently showed that modern theories of refraction are greatly in error. Th e evidence was based on 6252 observations of the sun’s vertical diameter when it was near the horizon; the variation of the sun’s apparent vertical diameter with altitude is a direct measure of the variation of the refraction, and the refraction itself at different altitudes can be calculated from the variations, provided that its value is known for some one altitude. Smiley concluded that at an altitude of 4 degrees, the actual refraction is greater than the theoretical value by more than a minute of arc (10 percent of its whole amount), that the error increases with decreasing altitude, exceeding 2 minutes at 2 degrees, after which the error diminishes, the refraction at the horizon ‘being actually smaller than the theoretical value.
Published in:	NAVIGATION: Journal of the Institute of Navigation, Volume 3, Number 1&2
Pages:	36 - 41
Cite this article:	Clemence, G. M., "REFRACTION NEAR THE HORIZON", NAVIGATION: Journal of The Institute of Navigation, Vol. 3, No. 1&2, 1951, pp. 36-41.
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