What is the autumn equinox?

With September well under way, the days are getting shorter and the autumn equinox is right around the corner.

This year, it will take place on Wednesday 23 September at 01:06 BST.

There's nothing to see exactly; rather, the equinox is a moment in time.

It marks the moment when the Earth's axis is perpendicular to the Sun, making day and night nearly equal in length across the world.

In the northern hemisphere, the September equinox marks the start of astronomical autumn.

South of the equator, where the seasons are reversed, it is the first day of astronomical spring.

The autumn equinox varies each year in its exact time and date but almost always takes place on 22 or 23 September in the northern hemisphere.

However, there are two definitions for the start of autumn.

The second is meteorological autumn, which always begins on 1 September and ends on 30 November.

Each of the four meteorological seasons lasts for three calendar months.

This makes it easier to compare and contrast weather data sets year on year since the dates remain the same every year, unlike the variable astronomical seasons.

Astronomical autumn ends with the winter solstice in the middle of December when astronomical winter begins.

The word "equinox" comes from Latin and means equal night.

At equinox, the daylight measurement is taken from the point when the centre of the Sun is at the horizon, and is 12 hours sunrise to sunset and 12 hours sunset to sunrise.

However, it is the top edge of the Sun which is visible first at sunrise and last at sunset.

Also, because of light refraction - light being bent by the atmosphere - there is still light for a time when the Sun is not directly visible. This adds extra time to daylight hours.

In autumn, this means that the time when day and night are actually equal happens after the equinox.

This is known as the equilux, which means equal light.

This year, that happens in the UK on Friday 25 or Saturday 26 September, depending on your latitude.

While the equinoxes occur in March and September and bridge the gap between winter and spring and summer and autumn, the solstices occur in June and December.

In December in the northern hemisphere, the winter solstice marks the moment when the Earth on its axis is most tilted away from the Sun.

Once again, it's a precise moment in time.

From our point of view, on the day of the winter solstice, the Sun takes the lowest and shortest path across the sky giving us the least amount of daylight.

The winter solstice in the UK this year will take place at 20:50 GMT on 21 December. This moment also marks the summer solstice in the southern hemisphere.

After the winter solstice, daylight hours begin to increase again.