What is the 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.

Spring in the South

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. In the southern hemisphere, it's known as the Spring (vernal) equinox.

Sun Crosses Celestial Equator

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 Axial Tilt

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. 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 the Earth's axis is perpendicular to the Sun's rays, like the illustration shows.

Why "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 or observe holidays and festivals around the September equinox.

Topics: Astronomy, Sun, Earth, Seasons, September, Equinox