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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 equinox may be taken to mark the end of summer and the beginning of autumn (autumnal equinox) in the Northern Hemisphere, while marking the end of winter and the start of spring (vernal equinox) in the Southern Hemisphere.
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 that occurred from the year 2010 to 2018 (UT) are listed as follows:
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.
The September equinox marked the first day of the French Republican Calendar.
- West Asia
- The Southward equinox marks the first day of Mehr or Libra in the Iranian calendar. It is one of the Iranian festivals called Jashne Mihragan, or the festival of sharing or love in Zoroastrianism.
- East Asia
- The Mid-Autumn Festival is celebrated on the 15th day of the 8th lunar month, often near the autumnal equinox day, and is an official holiday in mainland China, Hong Kong, Taiwan and in many countries with a significant Chinese minority. As the lunar calendar is not synchronous with the Gregorian calendar, this date could be anywhere from mid-September to early October.
- In Korea, Chuseok is a major harvest festival and a three-day holiday celebrated around the Autumn Equinox.
- The traditional East Asian calendars divide a year into 24 solar terms (节气, literally "climatic segments"), and the autumnal equinox (Qiūfēn, Chinese and Japanese: 秋分; Korean: 추분; Vietnamese: Thu phân) marks the middle of the autumn season. In this context, the Chinese character 分 means "(equal) division" (within a season).
- In Japan Autumnal Equinox Day (秋分の日 Shūbun no hi) is a public holiday. Higan (お彼岸) is a Buddhist holiday exclusively celebrated by Japanese sects during both the Spring and Autumnal Equinox.
- The traditional harvest festival in the United Kingdom was celebrated on the Sunday of the full moon closest to the September equinox.
- The Southward equinox was "New Year's Day" in the French Republican Calendar, which was in use from 1793 to 1805. The French First Republic was proclaimed and the French monarchy was abolished on September 21, 1792, making the following day (the equinox day that year) the first day of the "Republican Era" in France. The start of every year was to be determined by astronomical calculations following the real Sun and not the mean Sun.
- Neopagans observe the September equinox as a cardinal point on the Wheel of the Year. In the Northern Hemisphere some varieties of paganism adapt Mabon traditions. In the Southern Hemisphere, the vernal equinox corresponds with Ostara.
- The reconstructed Cahokia Woodhenge, a large timber circle located at the Mississippian culture Cahokia archaeological site near Collinsville, Illinois, is the site of annual equinox and solstice sunrise observances. Out of respect for Native American beliefs these events do not feature ceremonies or rituals of any kind.
- United States Naval Observatory (4 January 2018). "Earth's Seasons and Apsides: Equinoxes, Solstices, Perihelion, and Aphelion". Retrieved 18 September 2018.
- "Defining Seasons". www.timeanddate.com. Retrieved 2017-09-21.
- "Earth's Seasons — Naval Oceanography Portal". www.usno.navy.mil. Retrieved 2017-09-21.
- "Visitors Guide to the Woodhenge". Retrieved 2017-12-19.
- Iseminger, William. "Welcome the Fall Equinox at Cahokia Mounds". Illinois Department of Natural Resources. Retrieved 2017-12-20.
- "Winter Solstice Sunrise Observance at Cahokia Mounds". Collinsville Chamber of Commerce. Retrieved 2017-12-20.
- "Cahokia Mounds Mark Spring Equinox : The keepers of Cahokia Mounds will host a spring gathering to celebrate the vernal equinox". Indian Country Today. Indian Country Media Network. Retrieved 2017-12-20.