- The autumn or ‘southside’ equinox will occur today (September 22) at 8.21pm BST.
- The equinox marks the point when the sun appears to shine directly over the equator
- From today there will be cold days with cold winds as well as falling leaves
If you live in the Northern Hemisphere, it’s almost time to take your coat out of storage, as autumn officially begins tonight.
The autumnal equinox will occur today (September 22) at 20:21 BST – officially marking the arrival of autumn and cooler temperatures for the Northern Hemisphere.
This event occurs every year in September and marks the point when the Sun appears to shine directly over the equator, and day and night are of roughly equal length across the globe.
Along with two solstices, the year’s two equinoxes mark the change of seasons as the Earth revolves around the Sun.
After the autumnal equinox, each day in the Northern Hemisphere there is less daylight than darkness.
Every year there are two equinoxes – in September and March – when the Sun shines directly over the equator and the length of day and night are approximately equal. During the equinoxes, we experience fairly average temperatures and equal length of day and night.
2021. solstice and equinox for
Spring Equinox – March 20, 9.37 a.m. (GMT)
Summer Solstice – June 21, at 4.32 am (BST)
autumn equinox – September 22, 8:21 pm (BST)
winter solstice – December 21, at 3.58 pm (GMT)
‘In the Northern Hemisphere, the September equinox marks the beginning of a period that brings us later sunrises and earlier sunsets,’ NASA explained.
‘We’ll feel even colder days with chilly winds and dry, fallen leaves.’
The Earth is tilted on its axis, which means that as our planet orbits the Sun, the Sun moves the northern or southern hemisphere depending on where Earth is in its orbit.
However, at two points in the year the Sun will illuminate the northern and southern hemispheres equally. These are known as equinoxes.
Anna Ross, an astronomer at the Royal Observatory Greenwich, explains: ‘Earth takes a year (or 365-one-quarter days) to orbit the Sun once and it is slightly tilted on its axis.
‘So for half of the year, the Northern Hemisphere is slightly tilted toward the Sun, which means we have longer daylight hours and warmer temperatures.
NASA graphic showing the difference between the solstice and the equinox. The solstice occurs in June and December; Equinoxes occur in March and September
At an equinox, Earth’s terminator – the dividing line between day and night – becomes perpendicular and connects the north and south poles. Pictured, the terminator line is perpendicular with the September equinox
Are day and night equal at the equinox?
Day and night are not exactly equal at the equinox, but they are close to being equal.
As the old Farmer’s Almanac points out, daylight begins from the moment any part of the Sun is above the horizon, and it does not end until the last part of the Sun has set.
If the Sun shrinks to a stellar point and we live in a world without air, the spring and autumn equinoxes would actually be ‘equal nights’.
Astronomer Dr Ed Bloomer from the Royal Observatory said: ‘The equinoxes are generally considered to mark when the periods of day and night are exactly the same length.
‘Unfortunately, this is not really true, but as a general rule we will soon start to notice that as we move towards the summer months, the days get longer and the nights shorter.’
‘For the second half of the year, we’re tilted slightly further from the Sun, so we have longer nights and cooler weather. It is the inclination that gives us our seasons.
‘There are two points in the year where the Earth’s tilt with respect to the Sun is at its zenith.’
These two extreme tilts occur each year – one in March called the spring equinox, and one in September (the beginning of autumn for the Northern Hemisphere) called the southward equinox.
While the southbound equinox marks the point at which the Northern Hemisphere enters autumn, the Southern Hemisphere enters spring.
For the Northern Hemisphere, the southward equinox begins the countdown to the winter solstice—the shortest day of the year and the beginning of winter—on December 21.
The solstice—which is essentially the opposite of the equinoxes—occurs when a planet’s geographic pole is most tilted toward the Sun in the northern or southern hemisphere.
Ross said, ‘There are two points in the year where the Earth’s tilt with respect to the Sun is at its zenith. ‘These days are known as Sankranti.
‘On these dates, depending on which hemisphere you live in, you will either experience mid-summer with the longest day of the year or meridian with the longest night.
‘Between these two extremes, we find our spring and autumn months, with the exact midpoints being the equinoxes.’
The winter solstice is the day of the year when the northern hemisphere receives the least amount of sunlight and the southern hemisphere has the most
Today marks the beginning of ‘celestial autumn’, which is one of two definitions of when the season begins and ends.
Astronomical autumn is different from meteorological autumn, which always begins on 1 September and ends on 30 November.
Meteorological seasons are based on the annual temperature cycle and are more rigidly defined by the months of the year.
The Met Office explains: ‘These seasons are split to coincide with our Gregorian calendar, making it easy to compare seasonal and monthly data for meteorological observations and forecasts.’
In terms of stargazing, there is nothing in the sky that is specifically associated with the equinoxes (other than the 12.11 hours of the Sun).
‘However, the Moon has just passed the full moon phase, so it will light up beautifully…