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.

The Earth during the equinox. (Not to scale)

Fall in the north, spring in the south

Equinoxes and solstices are opposite on either side of the equator, and the September equinox is also known as the “autumnal (fall) equinox” in the northern hemisphere and as the“spring (vernal) equinox” in the southern hemisphere.

The Sun crosses 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.

September Equinox in Halifax, Nova Scotia, Canada is on
Wednesday, March 23, 2015 at 5:22 am ADT /08:22 UTC.

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, in reality equinoxes don’t have exactly 12 hours of daylight.

What happens on the equinox?

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, the Earth’s axis tilts a little away from or towards the Sun. But on the two equinoxes, the tilt of the Earth’s is neither away from nor towards the Sun. In fact, it is perpendicular to the Sun’s rays, like the illustration shows.

Traditions and Folklore

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

©2015 timeanddate.com