The word “equinox” is derived from Latin and literally translates to “equal night”.
On the equinox, day and night are roughly 12 hours long – although not exactly 12 hours long! For example, on 22 September in London, the day length will be 12 hours and 11 minutes and in Shetland 12 hours and 14 minutes. So where does this extra length of day come from?
At both sunrise and sunset, light from the Sun is refracted through the Earth’s atmosphere. Although we can see the Sun, in reality its position is actually below the horizon. This means we get a few extra minutes of daylight because of this light-bending trick of our atmosphere.
Additionally the equinox is measured at the centre of the Sun’s disc, not at its leading or trailing edge which are used as the measure of a sunrise or sunset.
It is not until a few days after the autumn equinox that we get to the point where days and nights are 12 hours long. This is known as the equilux and will occur on 25 September.