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Re: Definition of the twilight intervals
From: Paul Hirose
Date: 2015 Aug 17, 12:14 -0700
From: Paul Hirose
Date: 2015 Aug 17, 12:14 -0700
The Explanatory Supplement to the Astronomical Almanac (1992) says, "9.341 Twilight. This is caused by the scattering of sunlight from the upper levels of the Earth's atmosphere. It begins at sunset (ends at sunrise) and is conventionally taken to end (or begin) when the center of the Sun reaches an altitude of -18°." The next paragraph has the altitude limits for civil and nautical twilight. So the twilight periods overlap, according to the Explanatory Supplement. However, I don't have the latest edition, which was published recently after years of preparation. Someone please verify the definition is the same.
