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The value of the zenith angle to calculate sunrise and sunset times is commonly defined as 90°50', and is "obtained by adding the average apparent radius of the Sun (16') to the average amount of atmospheric refraction at the horizon (34')", according to http://aa.usno.navy.mil/faq/docs/RST_defs.php. This value is also used for the Sunrise/Sunset Algorithm published in the Almanac for Computers, 1990, see https://web.archive.org/web/20161202180207/http://williams.best.vwh.net/sunrise_sunset_algorithm.htm.