AUTUMN EQUINOX - Sun moves across equator to the southern hemisphere
2019's Autumnal Equinox is at 8.50am(BST) on Monday 23rd September. This is the point when the sun (as viewed from Earth) crosses the equator. Length of day for virtually all locations on Earth will be slightly longer than 12 hours, length of night slightly shorter than 12 hours.
Away from the meteorological world (where we work in full months, 1st September being the start of the new season) many people regard this as the start of Autumn.
It is easy to think that from Monday, although a slow process at first, the nights start to become longer than the days. It's not that simple!
The way we view light....
Sunrise and sunset defined
It is not the centre of the sun that defines the sunrise and sunset but the moment the upper edge (the top) of the sun's disk becomes visible above the horizon and then fully disappears below the horizon. The time it takes for the sun to fully rise and set, which is several minutes, is added to the day and subtracted from the night, and therefore the equinox day lasts a little longer than 12 hours.
Light bends and so time shifts
Because sunlight is refracted by Earth's atmosphere, it takes a little 'time' to catch up. This refraction - or bending of the light - causes the Sun’s upper edge to be visible from Earth several minutes before the edge actually reaches the horizon. So we see the light (ie have more daylight) before we see the sun.
EQUILUX - Day and night of equal length
The point at which the length of day and night are closest to exactly 12 hours comes a few days after the Autumnal Equinox. This year's Equilux (meaning 'equal light') for northern parts of the UK falls on the 25th September.