Meteorological autumn began on 1 September, and with the equinox, so begins astronomical autumn. As the leaves begin to change and we start to feel a definite 'bite' in the air, it's hard to believe the searing September heatwave was less than a fortnight ago.
But when exactly is the autumn equinox? What does it mean? How is it different from the solstice? And will we be seeing more aurora this weekend?
As the nights draw in, why not make the most of the extended darkness by brushing up on your constellations with our astronomy for beginners' guide? Or, try out some lunar photography with Pete Lawrence's expert guide on how to take great pictures of the Moon – all you need is a smartphone to get started. And for a full roundup of this year's meteor showers, we’ve got all the key dates listed in our meteor shower calendar.
When is the autumn equinox 2023?
The autumn equinox 2023 in the northern hemisphere is on Saturday 23 September, at 6:50am GMT (7:50am BST in the UK).
For the two hemispheres, the equinoxes are opposite each other. The autumn equinox in the northern hemisphere marks the first day of astronomical autumn. In the southern hemisphere, it is known as the spring equinox, and marks the first day of astronomical spring.
When we refer to the autumn equinox in this article, it is from the perspective of the northern hemisphere, and we are referring to the September equinox.
What is the autumn equinox?
What happens during the autumn equinox? Put simply, the equinoxes mark a particular position in Earth's orbit around the Sun. Kind of like a half-way-hurrah between the solstices.
The autumn equinox is when the Sun crosses the celestial equator (an extension of Earth’s equator when projected into the sky). It goes from the northern hemisphere to the southern hemisphere, hence the opposite spring equinox down under.
What causes the autumn equinox?
The autumn equinox occurs when the tilt of the Earth's axis is positioned so that the northern and southern hemispheres receive nearly equal amounts of light. The result is roughly equal lengths of day and night.
At the equinoxes, the Sun is positioned exactly above the equator, which is why both hemispheres receive (almost) equal amounts of sunlight. After the autumn equinox, we'll feel the nights really starting to draw in, as the UK will receive more darkness and less daylight.
But for our friends at Svalbard, it's a somewhat more gloomy affair. A mere 500 miles from the North Pole, when the Sun sets around the time of the autumn equinox it marks the start of six months of darkness.
But they won't be in complete darkness for all this time; there is a 'buffer' period of twilight at either end of the six months. Total darkness at the North Pole begins roughly in the middle of November, lasting all through the Christmas holiday season, until the end of January.
The term 'equinox' originates from the Latin words aequi, meaning 'equal,' and nox, meaning 'night,' which when combined, translates to 'equal night'.
What’s the difference between an equinox and a solstice?
The difference between an equinox and a solstice is all to do with where Earth is located in its orbit around the Sun. Here on Earth, this shows up as different lengths of days and nights.
At the autumn equinox (and spring equinox), the days and nights are roughly equal in length. The solstices, however, mark the longest (summer solstice) and shortest days (winter solstice) of the year.
The Sun is directly above Earth’s equator at the equinoxes, but at the solstices, one hemisphere is at its maximum tilt towards the Sun. For us here in the UK, the summer solstice is when the northern hemisphere is at its maximum tilt towards the Sun and the Sun is highest in the sky, giving us the longest day.
Flip that around, and at the winter solstice, the northern hemisphere is at its maximum tilt away from the Sun, with the Sun appearing lowest in the sky, giving us the shortest day.
Aurora watch: Can I see the northern lights?
One of the best times to spot the elusive Northern Lights is around the dates of the equinox; they were spotted in Scotland a few days ago, and may even be visible over parts of the UK and Ireland this weekend.
Although still very much of a mystery, the aurorae and geomagnetic storms do tend to occur more frequently around the equinoxes. And this is (probably) down to the interplay of magnetic fields.
Both the Earth and the Sun have north and south poles, and these poles have magnetic fields (just like a magnet has opposing sides). It's widely thought that during the equinoxes, the north-south component of the Sun is roughly opposite that of Earth's magnetic field.
This alignment allows solar wind, which is composed of charged particles from the Sun, to flow with less resistance toward Earth's magnetic poles, where it interacts with our magnetic field. The particles interact with the atmosphere and create the aurora by emitting light.
Observed for millennia: Other names for the autumn equinox
You might have noticed that the autumn equinox is also sometimes also called the First Point of Libra. However, this name is somewhat confusing today, and it's all thanks to the effects of precession (see also No, Saturn isn’t moving into Pisces. It’s not even close).
In astronomy, precession refers to the slow, cyclic wobbling of the Earth's rotational axis over time. This causes changes in the orientation of the celestial poles and the apparent positions of stars when viewed from Earth.
When the Greek astronomer Hipparchus first recorded his observations of the equinoxes back in 130 BC, the point of the spring equinox lay in the constellation Aries, and so was termed 'The First Point of Aries'.
Diametrically opposite this on the celestial sphere is the First Point of Libra. For this reason, the Sun at the September equinox is sometimes described as entering the First Point of Libra.
But, like most things in life, the only thing constant is change; the First Point of Libra now lies in the constellation Virgo, while the First Point of Aries now lies in the neighbouring constellation Pisces.
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