If you attempt to watch a star setting in the middle of the night, you may well be in for a disappointment. Unless the western horizon is a high one, as the star approaches it, it will gradually become dimmer and (depending upon a number of factors including, of course, the initial brightness of the star) may well disappear completely. Even in a location where the skies are little affected by modern light pollution, this will still be the case. The cause is atmospheric extinction—the absorption and dispersion of the starlight as it passes through the earth's atmosphere on the last leg of its long journey to the human observer on earth. When a star appears close to the horizon, its light has to pass through many times as much atmosphere as when it is high in the sky. The practical effect is that surprisingly few stars—even bright ones—are actually visible at the moment of rising or setting, even in remote places on the darkest of nights.

The effect of extinction is that a star will only be visible above a certain altitude, known as the extinction angle. That angle will depend upon the brightness of the star and upon atmospheric conditions. At high latitudes, where the celestial bodies rise and set at relatively shallow angles, extinction adds another confounding factor to any attempt to date a monument on the basis of postulated stellar alignments.

See also:

Astronomical Dating.

Altitude; Star Rising and Setting Positions.

References and further reading

Aveni, Anthony F. Skywatchers, 105-107. Austin: University of Texas Press, 2001.

Ruggles, Clive, ed. Archaeoastronomy in the 1990s, 157, 163. Loughborough, UK: Group D Publications, 1993.

Schaefer, Bradley E. "Atmospheric Extinction Effects on Stellar Alignments." Archaeoastronomy 10 (supplement to Journal for the History of Astronomy 17 [1986]), S32-S42.

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