Seasons and sky
September Equinox
Direct answer
September Equinox: The September Equinox is the instant the Sun's apparent geocentric center crosses the celestial equator southward.
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At a glance
- Found in
- Seasons & Earth Almanac
- Related
- March Equinox, June Solstice, Astronomical Season, Equinox
The September Equinox is the Sun's southward crossing of the celestial equator.
A Sun-Earth orientation for seasonal, atmospheric, tidal, and terrestrial terms.
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The story behind September Equinox
Harvest and new-year traditions have often gathered around the September seasonal turning, although their dates and meanings differ by culture.
Why September Equinox matters
September Equinox begins astronomical autumn in the Northern Hemisphere and spring in the Southern Hemisphere.
How to read September Equinox
September Equinox speaks to seasonal chronology, solar declination, and comparisons of daylight between hemispheres.
September Equinox in practice: an example
After this equinox, the subsolar point moves into southern latitudes until the December solstice.
Where readings of September Equinox go wrong
A common error is to call it the autumn equinox without naming the hemisphere.
Sources and editorial method
Research references used for this entry include:
- NASA Science: Seeing Equinoxes and Solstices from Space - NASA explanation of axial tilt, the terminator, seasonal illumination, equinoxes, and solstices.
- U.S. Naval Observatory: Length of Day and Night at the Equinoxes - Official explanation of equinox geometry, refraction, and the difference between equinox and equilux.
- Source
- The Great Almanac Dictionary
- Category
- Seasons & Earth Almanac
- Last updated
- 2026-09-26
Follow the southward solar crossing
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Common questions
What is the September Equinox?
The September Equinox is the instant the Sun's apparent geocentric center crosses the celestial equator southward.
What do people get wrong about the September Equinox?
A common error is to call it the autumn equinox without naming the hemisphere.
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