Twilight -- from Eric Weisstein's World of Astronomy

Observational Astronomy

Rising and Setting

Twilight

The transition period from day to night (or night to day) when the sky is not completely dark, but the Sun has not yet set (or risen). It is technically defined as the period of time beginning (or ending) when the center of the (refracted) Sun is lower than a given elevation. Civil twilight corresponds to the Sun being 6° below the horizon, nautical twilight to 12°, and astronomical twilight to 18°.

Astronomical Twilight, Civil Twilight, Nautical Twilight

Seidelmann, P. K. (Ed.). Explanatory Supplement to the Astronomical Almanac. Mill Valley, CA: University Science Books, pp. 33 and 483, 1992.

United States Government Printing Office. "Civil Twilight," "Nautical Twilight," and "Astronomical Twilight." The Astronomical Almanac for the Year 2000. Washington, DC: U. S. Government Printing Office, pp. A12 and A22-A45, 2000.