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There are two equinoxes every year – in September and March – when the Sun shines directly on the Equator and the length of day and night is nearly equal.
Opposite Sides & Seasons
In the Southern Hemisphere, it is known as the vernal (spring) equinox and marks the first day of spring.
Sun Crosses Celestial Equator
The September equinox occurs the moment the Sun crosses the celestial Equator – the imaginary line in the sky above Earth’s Equator – from north to south. This happens either on September 22, 23, or 24 every year.
Earth’s axis is tilted at an angle of about 23.4° in relation to the ecliptic, the imaginary plane created by Earth’s path around the Sun.
On any other day of the year, either the southern hemisphere or the Northern Hemisphere tilts a little towards the Sun. But on the two equinoxes, the tilt of Earth’s axis is perpendicular to the Sun’s rays, like the illustrations show.
Why Is it Called “Equinox”?
On the equinox, night and day are nearly exactly the same length – 12 hours – all over the world. This is the reason it’s called an “equinox,” derived from Latin, meaning “equal night.” However, even if this is widely accepted, it isn’t entirely true. In reality, equinoxes don’t have exactly 12 hours of daylight.
Traditions and Folklore
In the Northern Hemisphere, the September equinox marks the start of fall (autumn). Many cultures and religions celebrate holidays and festivals around the equinox.
A famous ancient equinox celebration was the Mayan sacrificial ritual by the main pyramid at Chichen Itza, Mexico. The pyramid, known as El Castillo, has 4 staircases running from the top to the bottom of the pyramid’s faces, notorious for the bloody human sacrifices that used to take place here. The staircases are built at a carefully calculated angle which makes it look like an enormous snake of sunlight slithers down the stairs on the day of the equinox.
H/T Time and Date