A measure of the amount of atmosphere through which light from a celestial object must pass to reach an observer, with airmass = 1 at the zenith and increasing toward the horizon. Used to calculate atmospheric extinction effects.
Compound of 'air' from Old French 'air' and 'mass' from Latin 'massa', coined in the early 20th century as astronomers developed quantitative methods to account for atmospheric effects in photometry and spectroscopy.
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