As we move towards darker times, it marks the point when day and night are supposed to be nearly the same length. You might assume this happens exactly on the equinox, according to the Encyclopaedia Britannica.
That's why it's called equinox – a term that means equal nights.
"But if you measure the actual length of the day, it doesn't add up," Kaare Aksnes tells Science Norway. He is an astronomer and professor emeritus at the University of Oslo's Institute of Theoretical Astrophysics.
Kaare Aksnes received the King's Medal of Merit in gold in 2006. Aksnes is considered one of the world's leading researchers in orbit calculation for artificial satellites, planetary moons, asteroids, and comets, according to the Great Norwegian Encyclopedia.(Photo: Bjørn Sigurdsøn / NTB)
An astronomical perspective
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From an astronomical perspective, night and day are the same length everywhere on Earth on the equinox.
But as Aksnes points out, that isn't exactly what we experience on the ground.
According to the weather forecasting site Yr, in the Oslo region the sun rises at 07:01 and sets at 19:16. That makes the day about fifteen minutes longer than the night.
Three days later, however, something interesting happens. On September 25 the sun rises at 07:08 and sets at 19:07 – meaning day and night are practically equal.
This day has its own name: equilux, which means equal light.
So why does this happen three days after the equinox?
We can thank the atmosphere
Take a look at the diagram below. It shows what Earth looks like during the autumn and spring equinoxes.
On these days, the sun is completely perpendicular to Earth, on the axis that runs between the north and south poles.
This is what Earth looks like from space during the spring and autumn equinoxes. The sun’s rays strike the planet at a right angle to its axis, which in theory gives us equal day and night everywhere on Earth.(Image: By Image by Przemyslaw 'Blueshade' Idzkiewicz / CC BY-SA 2.0)
In theory, that means night and day are the same length no matter where you are. And this would be true if Earth were just a solid rock without an atmosphere.
But Earth does have an atmosphere.
When sunlight passes through the atmosphere, the light bends. This is called refraction, the same effect that makes a straw in a glass of water look 'broken.'
A pencil, for example, looks broken when the light passes from one medium to another, here from air to water.(Photo: Kuki Ladron de Guevara / Shutterstock / NTB)
"The sun is lifted by a whole solar diameter because of refraction," says Aksnes.
Because of this bending effect, we see the top of the solar disc before it has physically risen above the horizon.
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The reverse happens at sunset. We see the last light of the day after the sun has physically dropped below the horizon.
This diagram shows how light bends in the atmosphere, though it isn’t to scale. The sun at the bottom right is where it actually is, while the sun at the top right is how it appears to us.(Image: Sciencia58 / CC BY-SA 4.0)
That’s why the day ends up slightly longer than the night, even on the autumn equinox.
But the length of days and nights changes throughout the year. Since the summer solstice on June 21, nights have gradually been getting longer.
And a few days after the equinox, night finally grows long enough to match the day, so we actually get a night and a day that are roughly the same length in our atmosphere.
Exactly when this happens depends on where you are. According to Time and date, for example, this happens on September 26 at the 40th parallel, such as in Madrid.
In the Oslo area, it happens on September 25, with sunrise at 07:08 and sunset at 19:07 – making the day and night almost completely equal.
But these estimates are difficult to pin down with absolute precision, says Aksnes.
Water vapour in the atmosphere
He explains that the degree to which light bends depends in part on how much water vapour is present in the atmosphere, as well as on temperature.
This makes it hard to pinpoint the exact times of sunrise and sunset.
"The farther north you go, the more stretched out sunrises and sunsets become. That makes the calculated times increasingly inaccurate," he says.
According to Aksnes, this mismatch between the autumn equinox and the actual balance of day and night has been recognised for centuries
But the precise reason why day length doesn’t match the equinox wasn’t understood until modern times, he explains.
