The long-awaited September equinox marks the end of summer and brings with it the start of astronomical autumn.

Also known as the autumnal equinox, this celestial event happens every September.

What is an equinox?

Since the Earth is tilted on its own axis, the Sun only illuminates the northern or southern hemisphere at any given time – depending on where the Earth is during its orbit about the Sun.

The only two exceptions to this rule are during the spring and autumnal equinoxes, when the Sun manages to illuminate both hemispheres equally. This happens towards the end of March and September, respectively.

What do we mean by ‘astronomical autumn’?

There are two types of seasons: astronomical and meteorological, and it’s only a minor difference that sets these two apart.

People are generally best accustomed to astronomical seasons, which start at slightly varying dates throughout the year, depending on when equinoxes and solstices take place.

The cycles between seasons usually last between 89 and 93 days.

The variability of these seasons makes it difficult for experts in agriculture, commerce, and other fields to calculate and compare seasonal statistics. So, this is where meteorological seasons come in.

These seasons are more consistent, as they are split into groups of three months each.

For example, while traditional astronomical autumn usually starts between the 21st and 23rd of September (with this year’s autumnal equinox having taken place this morning), meteorological autumn starts on the 1st of September every year, lasting for 91 days until the 30th of November.

Did you know about this?