Why doesn’t fall (always) start on September 21?

The beginning of autumn begins at the exact moment of the autumnal equinox. 295192339/mbruxelle – stock.adobe.com

Contrary to popular belief, the seasons do not start every 21st of the month. For the fall, it is even rather a rare thing. This year autumn falls on Friday, September 23, 2022. Explanation.

Have you wondered why thefall starts this year on September 23rd? The idea that the spring and autumn equinoxes fall each year on March 21 and September 21 is a simplification that belies the astronomical parameters that mark the revolution of our planet around the Sun.

Every year, the beginning of autumn begins at the exact moment ofautumnal equinox, when the line that marks the limit between day and night on the surface of the planet passes through the two poles. The duration of the day is therefore equal to the duration of the night, everywhere on Earth. And accordingly, during the equinox, the sun rises exactly in the east and sets exactly in the west.

Why aren’t the orbits of the planets circular?

But why doesn’t this equinox fall on the same day every year? First parameter, the year can last 365 days, or 366 when it is a leap year. This variation therefore leads to a shift in the dates of the seasons.

The second parameter is that the Earth does not describe a perfectly circular orbit around its star. If this were the case, the seasons would be of equal duration, and their beginning could be more regularly based on the Gregorian calendar, which was precisely designed to prevent these dates from shifting in time.

But now, the Earth’s orbit is a little bit eccentric: it is not a circle, but a very slightly elongated ellipse, with an Earth-Sun distance which varies between 147 and 152 million kilometers.

Diagram of the Earth’s orbit, the elliptical shape of which has been greatly exaggerated to show the varying lengths of the seasons. Credits: Gothika (original image), Belg4mit (corrections), Simon Villeneuve (translation), Whidou (correction of various SVG bugs) CC BY-SA 3.0 XVMe25bb2b4-801f-11e6-8dff-7de7d03cae34-805×479.jpg

However, the laws of orbital mechanics cause the planet to revolve faster around the Sun when it is closest to it. An effect that leads to a shift in the duration of the seasons and therefore in their start date.

Because of all these parameters, the spring equinox mainly falls on March 20 or 21 (and exceptionally on March 19), while the autumn equinox generally falls on September 22 or 23 (but can also fall, although more rarely, on September 21 or 24). And the next time the autumnal equinox takes place on September 21 will be in… 2092!

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