Pasachoff, Jay M. Hopkins Observatory, Williams College, Williamstown, Massachusetts.
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One of the two places in the sky where the ecliptic crosses the celestial equator; or one of the two times of the year when the Sun crosses these points. The ecliptic is the great circle across the sky that marks the mean path of the Sun; the celestial equator is the great circle that is an extension into the sky of the Earth's mean Equator. These two great circles meet at two points, one of which is the vernal equinox and the other the autumnal equinox. The Sun passes the vernal equinox each year about March 20, and the autumnal equinox about September 22. The vernal equinox can occur as early as March 19 and as late as March 21; for most of the twenty-first century it will be on March 20. The autumnal equinox can occur as early as September 21 and as late as September 24; for most of the twenty-first century it will be on September 22. The dates and times drift with the difference between the actual solar years and 365 days, and are corrected by leap years. This results in a 4-year variation superimposed on a negative 11-min-per-year slope (illus. a and b). Since 2000 was an ordinary leap year in the Gregorian calendar, unlike 1900, the dates will continue to decline through 2100 (illus. c). See also: Astronomical coordinate systems; Calendar; Time
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