So that’s it for the summer. The season comes to a close for us in the northern hemisphere this week on 22 September as we enter the autumn.
The moment is marked by the autumnal equinox, which is the moment when the sun crosses the celestial equator, moving south, and the days become shorter than the nights. The exact moment this happens is 19:19 BST on 22 September and it is caused by the movement of Earth along its orbit, gradually tilting the northern hemisphere away from the sun.
The name equinox comes from the Latin aequus, meaning equal and nox, meaning night. Hence it is said that the equinox is when day and night are the same length, which would be true if Earth had no atmosphere.
The effect of the air we breathe is to bend the light from the sun, so we actually see our parent star on the horizon before it has actually risen, and then again just after it sets. This gives a few extra minutes of daylight at the equinox.
A few days later, the lengths of the day and the night really are equal. This is known as the equilux, and occurs on either 25 or 26 September depending on your precise location.
