September equinox

The September equinox is a specific moment in time. It occurs when the Sun appears to cross the celestial equator. The celestial equator is an imaginary line in the sky. During this event, the Sun moves southward. Because the calendar year differs from the tropical year, the date varies. It usually falls between September 21 and September 24.

This equinox marks a major shift in the seasons. In the Northern Hemisphere, astronomical summer ends. This transition begins the season of autumn. In the Southern Hemisphere, the seasons are reversed. The September equinox marks the end of astronomical winter. This event signals the start of astronomical spring, or the vernal equinox.

The movement of the Sun changes for observers on Earth. At the moment of the equinox, the Sun rises due east. It also sets due west. Before this date, the Sun rises and sets more northerly. After the equinox, the Sun rises and sets more southerly. This begins a six-month period of southerly movement. This cycle started earlier at the June solstice.

Observers at the poles see a different view. At the North or South Poles, the Sun moves horizontally. It stays near or above the horizon. The Sun's altitude changes by less than 0.39 degrees per day. This change is known as declination. Refraction also affects how we see the Sun. Because of refraction, the Sun usually appears slightly above the horizon. This happens even when its true middle is rising or setting.

Astronomers use this point to study the tropical year. The tropical year is the time it takes for the Sun to return to the same position. The point where the Sun crosses the equator southward is called the First Point of Libra. However, the stars appear to shift over long periods. This process is called the precession of the equinoxes. Because of precession, the point is no longer in the constellation Libra. It is now located in the constellation Virgo.

This stellar shift happens very slowly. The solar point moved from Libra into Virgo in -729. This is also known as 730 BCE. The point will continue to move through the stars. It is predicted to enter the constellation Leo in the year 2439. These long cycles show how the positions of stars and the Sun change over many centuries.

Many cultures have used the equinox for calendars and festivals. The French Republican Calendar used the equinox as its first day. This system was used from 1793 to 1805. In West Asia, the Iranian calendar begins the month of Mehr at the equinox. This is celebrated during the Jashne Mehregan festival. In Japan, Autumnal Equinox Day is a public holiday. In Korea, the Chuseok harvest festival occurs around this time.

Other traditions also connect to this solar event. The Jewish holidays of Sukkot and Rosh Hashanah often fall near the equinox. In the United Kingdom, harvest festivals were traditionally held near the full moon closest to the equinox. In the Americas, the Cahokia Woodhenge site is used for observances. Visitors stand where the Mississippian people once watched the sunrise. These diverse traditions all follow the movement of the Sun.