- The phenomenon of the sun's daily disappearance below the western horizon as a result of the earth's rotation.
- Contraction for "time of sunset," which is defined by the U.S. Naval Observatory.
United States Naval Observatory and her Majesty's Nautical Almanac Office Annual. The Astronomical Almanac. U.S. Government Printing Office, Washington, D.C. and HMSO, London, England.
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