You may have already noticed that the nights are drawing in and there is a definite need to start digging out the warmer clothing.

And today officially marks the start of autumn, with the days getting colder and shorter as we approach winter.

What is an equinox?

An equinox occurs when the sun passes directly over the Earth's equator, resulting in a day and night that equal in length.

There are two equinoxes every year - one around March 22 which starts spring and one around September 22 to mark autumn.

The word equinox is Latin for "equal night".

Why does it happen?

The equinox occurs because of the tilt of the Earth, which is what causes the seasons.

The Earth's tilt is 23.5 degrees relative to the plane of its orbit and means that, although one revolution of the planet takes 24 hours - it's different depending on the time of year.

During the summer time, the Northern hemisphere is tilted towards the sun. So we get longer days as more light falls on this part of the surface.

In the winter time it's the Southern hemisphere that gets the majority of the light.