The autumn equinox occurs when daylight and darkness are an approximate equal duration – in fact, equinox is Latin for “equal night”.
Astronomically, the equinox marks the end of summer and beginning of autumn. But there are actually several different ways of deciding when autumn begins – for meteorologists autumn started 1 September.
The Earth has an axial tilt – it does not sit straight in the solar system but instead leans on its axis at a 23.4 degree angle. As a result, the northern and southern hemisphere either point towards or away from the Sun depending on the time of year.
When the northern hemisphere points towards the Sun it is summer here in the UK and when it points away it is winter.
But there are two times in the Earth’s orbit when neither hemisphere directly points towards or away from the Sun, resulting in a nearly equal amount of daylight and darkness. These are called equinoxes, and the spring vernal equinox takes place in March.
It’s just an infinitesimally small point, just a moment that occurs.
And this year the autumn or September equinox happens at precisely 8.21pm on the 23rd of September. Astronomically, once we pass that point we’re in autumn in the northern hemisphere and heading towards the December or winter solstice.
During this time, the autumn sky also appears, which lowers the visibility of the summer constellations as it moves lower into the west.
This paves the way for rich, mythological constellations to appear such as Pegasus, Perseus, Andromeda, Cassiopeia and Cepheus. These beautiful and identifiable constellations are all related to the same mythological story, which has been told many ways.
The modern incarnation of the story is Clash of the Titans.
This season also enables us to see some one off events such as the total lunar eclipse on 28 September at around 3.47am, which will see the moon turn red as it passes through the shadow of the earth.
On 17 October in the dawn skies, Mars and Jupiter will come very close together, creating a beautiful conjunction and finally, on 21 and 22 October, the annual Orionids meteor shower will take place.
If you’re out and about gazing at the stars, there’s a good chance you’ll be able to see some of the meteors associated with that shower. So go out and make the most of it.