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September equinox

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.

Illustration image
The Earth during the September equinox.
The Earth's during the September equinox (ill. not to scale).

Seasons are opposite on either side of the equator, so the equinox in September is also known as the "autumnal (fall) equinox" in the northern hemisphere. However, in the southern hemisphere, it's known as the "spring (vernal) equinox".

What happens during an equinox?

The September equinox occurs the moment the Sun crosses the celestial equator – the imaginary line in the sky above the Earth’s equator – from north to south. This happens either on September 22, 23, or 24 every year.

The Earth's axis is always tilted at an angle of about 23.5° in relation to the ecliptic, the imaginary plane created by the Earth's path around the Sun. However, the tilt's orientation changes throughout the year. On any other day of the year, the Earth's axis tilts a little away from or towards the Sun. But on the two equinoxes, the Earth's axis tilts neither away from nor towards the Sun and is perpendicular to the Sun's rays, like the illustration shows.

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 customs

In the northern hemisphere the September equinox marks the start of fall (autumn). Many cultures and religions celebrate or observe holidays and festivals around the September equinox.

Topics: Astronomy, Sun, Earth, Seasons, September

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Equinox & Solstice

  1. March Exquinox
  2. June Solstice
  3. September Equinox
  4. December solstice

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When is fall equinox?

The fall (autumnal) equinox in the Northern Hemisphere is in September. In the Southern Hemisphere it's in March. more