NavList:

It turns out that the 18° magic number for the altitude of the Sun below the horizon as the definition of astronomical twilight has been standard "since antiquity". Also of interest, the times of twilight seem to have been of greater concern to meteorologists (such as they were a century ago) than to astronomers or navigators. It appears that astronomical twilight was defined very early, civil twilight more recently (c.1800 but not settled even in 1916), and nautical twilight some time after that --when? The definitive analysis of twilight and the history of the concept, up until then, was written by H. H. Kimball in 1916 and published in the "Monthly Weather Review". I am attaching a copy of the paper. Kimball also discussed the varying arbitrary definitions of sunrise and sunset that were in use in the nineteenth century. He notes that older German sources defined sunrise and sunset as the instant when the geocentric sun has an altitude of zero degrees (H_c=0° in navigator's terms). There's lots of interesting material in the article. For example, he notes that the brightness of twilight when the Sun is about 8° 40' below the horizon is comparable to a sky with a Full Moon (at the zenith, but at any moderately high altitude would be equivalent). This, incidentally, confirms something that many navigators know but it's not often stated plainly in the textbooks: a Full Moon night is comparable to nautical twilight.

By the way, the crepuscular arch described in Kimball's paper is the shadow of the Earth on the atmosphere, which apparently figured in some early definitions of civil twilight. There was a recent article on the Sky & Telescope web site describing it.

-FER

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