Seasons and sky
Equinox
Direct answer
Equinox: An Equinox is the instant when the Sun crosses the celestial equator, marking the March or September boundary between astronomical seasons.
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At a glance
- Found in
- Seasons & Earth Almanac
- Related
- Astronomical Season, Meteorological Season, Solstice, Sunrise
An Equinox occurs when the Sun crosses the celestial equator.
A Sun-Earth orientation for seasonal, atmospheric, tidal, and terrestrial terms.
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The story behind Equinox
Equinox observations have long supported calendars, seasonal reckoning, architecture, and navigation.
Why Equinox matters
Equinoxes anchor seasonal calendars and divide the Sun's apparent annual path into northward and southward crossings.
Equinox in practice: an example
The March equinox begins spring in the Northern Hemisphere and autumn in the Southern Hemisphere.
Where readings of Equinox go wrong
The usual misreading is to expect exactly twelve hours of observed daylight; refraction and the Sun's apparent disk make day length slightly different.
Sources and editorial method
Research references used for this entry include:
- U.S. Naval Observatory: Length of Day and Night at the Equinoxes - Official explanation of equinox geometry and why observed daylight is not exactly twelve hours.
- U.S. Naval Observatory: Astronomical Almanac Glossary - Reference definitions for astronomical coordinates, events, calendars, and time scales.
- Source
- The Great Almanac Dictionary
- Category
- Seasons & Earth Almanac
- Last updated
- 2026-09-26
See the next equinox time
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Common questions
What is an Equinox?
An Equinox is the instant when the Sun crosses the celestial equator, marking the March or September boundary between astronomical seasons.
What do people get wrong about an Equinox?
The usual misreading is to expect exactly twelve hours of observed daylight; refraction and the Sun's apparent disk make day length slightly different.
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