September equinox

Illumination of Earth by Sun on the day of an equinox
UT date and time of
equinoxes and solstices on Earth[1]
event equinox solstice equinox solstice
month March June September December
day timeday timeday timeday time
2010 2017:322111:282303:092123:38
2011 2023:212117:162309:042205:30
2012 2005:142023:092214:492111:12
2013 2011:022105:042220:442117:11
2014 2016:572110:512302:292123:03
2015 2022:452116:382308:212204:48
2016 2004:302022:342214:212110:44
2017 2010:282104:242220:022116:28
2018 2016:152110:072301:542122:23
2019 2021:582115:542307:502204:19
2020 2003:502021:442213:312110:02

The September equinox (or Southward equinox) is the moment when the Sun appears to cross the celestial equator, heading southward. Due to differences between the calendar year and the tropical year, the September equinox can occur at any time from the 21st to the 24th day of September.

At the equinox, at the equator, the Sun rises directly in the east and sets directly in the west. Before the Southward equinox, the Sun rises and sets more to the north, and afterwards, it rises and sets more to the south.

The September may be taken to mark the end of summer and the beginning of autumn in the Northern Hemisphere but marks end of winter and the start of spring in the Southern Hemisphere.[2]


See also: tropical year

The September equinox is one point in time commonly used to determine the length of the tropical year.

Date and time of the September equinox[3] that occurred from the year 2001 to 2011 are listed as follows:

Date Time
2001-09-22 23:04
2002-09-23 04:55
2003-09-23 10:47
2004-09-22 16:30
2005-09-22 22:23
2006-09-23 04:03
2007-09-23 09:51
2008-09-22 15:44
2009-09-22 21:18
2010-09-23 03:09
2011-09-22 13:22


The point where the Sun crosses the celestial equator southwards is called the first point of Libra. However, due to the precession of the equinoxes, this point is no longer in the constellation Libra, but rather in Virgo.

The September equinox passed from Libra into Virgo in year −729, will pass into Leo in year 2439.

Apparent movement of the Sun in relation to the horizon

At the equinox, the Sun rises directly in the east and sets directly in the west. However, because of refraction it will usually appear slightly above the horizon at the moment when its "true" middle is rising or setting. For viewers at the north or south poles, it moves virtually horizontally on or above the horizon, not obviously rising or setting apart from the movement in "declination" (and hence altitude) of a little under a half (0.39) degree per day.

For observers in either hemisphere not at the poles, the further one goes in time away from the September equinox in the 3 months before that equinox, the more to the north the Sun has been rising and setting, and for the 3 months afterwards it rises and sets more and more to the south.

Human culture


The September equinox marked the first day of the French Republican Calendar.


West Asia
East Asia


See also

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