There’s some extent each September when the Universe provides us a delicate nudge and says: “Proper, time to get the woolly jumpers out.” That time is the autumn equinox.
This yr, the September equinox arrives within the early hours of Wednesday 23 September 2026. Whilst you won’t discover something significantly dramatic taking place at 1:05am, it marks an necessary second within the astronomical calendar.
So, what truly is an equinox, why does it occur, and why ought to we care?
So, first issues first: what’s an equinox?
The phrase equinox comes from Latin and roughly interprets as “equal night time”.
There are two equinoxes yearly: one in March and one in September. They occur when the centre of the Solar crosses the airplane of Earth’s equator.
The September equinox marks the start of astronomical autumn within the Northern Hemisphere. However earlier than anybody begins counting hours of daylight with a stopwatch, there’s a slight catch: The equinox doesn’t truly imply we get exactly 12 hours of daylight and 12 midnights.
The precise size of daylight will depend on issues akin to how we outline dawn and sundown. So, the date when daylight and darkness are closest to being equal, often called the equilux, doesn’t essentially fall on the precise date of the equinox, it’s a few days after.
Astronomy generally could make issues sound so easy after which instantly provides a footnote.
A full disk view of the Earth. Picture Credit score: NOAA/NASA
So why are the times getting shorter?
That is the place Earth’s somewhat necessary 23.5-degree tilt comes into play. Our planet isn’t sitting completely upright because it travels across the Solar, its axis is tilted. As Earth travels round its orbit, that tilt means totally different components of the planet obtain totally different quantities of daylight at totally different occasions of the yr.
Throughout our summer season, the Northern Hemisphere is tilted in the direction of the Solar. We get longer days, extra direct daylight and, theoretically, extra alternatives to complain about how heat the workplace is.
After the September equinox, the Northern Hemisphere is tilted away from the Solar as Earth continues its journey round its orbit, thus we get shorter days and longer nights.
Is the equinox a really particular second?
Once we say “the equinox is on 23 September”, it sounds as if your entire day is one way or the other the equinox. That’s not fairly the case. The equinox is a exact second in time and this yr, it happens at round 1:05am BST on Wednesday 23 September.
At that second, the centre of the Solar crosses the celestial equator, transferring from north to south in our sky. After which… that’s it, the Earth carries on doing what it has been doing for roughly 4.5 billion years.
Look the place the Solar rises
So right here is one thing you’ll be able to truly observe: the place the Solar rises and units. Across the equinoxes, the Solar rises roughly due east and units roughly due west. That provides us a stunning little alternative to attach the large mechanics of our planet with one thing we are able to see with our personal eyes.
You don’t want a telescope, or a level in astrophysics. All you must do is discover the place the Solar comes up and the place it goes down.
(However please keep in mind: by no means look immediately on the Solar, even throughout dawn or sundown, as it might probably injury your eyes.)
Dawn Picture Credit score: NASA
Over the approaching months, these positions will steadily shift alongside the horizon. By the point we attain the winter solstice, the Solar can be rising and setting a lot additional south. So the sky is mainly giving us an enormous, slow-moving calendar!
The altering place of the Solar helped individuals all through historical past perceive the seasons and organise life round them. When ought to crops be planted? When ought to they be harvested? When would the times start to shorten? When would winter arrive?
The sky was, fairly actually, a calendar. Though we now have extremely exact astronomical calculations, satellites and computer systems to inform us precisely what’s taking place, there’s nonetheless one thing splendidly easy about standing exterior and noticing the Solar’s place for your self. I feel that’s one of many nicest issues concerning the equinox and also you don’t have to grasp every part about orbital mechanics to understand it. You possibly can merely step exterior, discover that the evenings are getting darker, discover that the mornings really feel totally different after which, in the event you get curious, begin asking questions.
So, glad equinox!