For astronomers, autumn starts tonight, but for meteorologists, autumn has already begun

Tonight at 9:21 p.m., autumn begins for astronomers. However, meteorologists have been in it for weeks.

For meteorologists, autumn has already begun. They ushered in the autumn season on September 1. But for astronomers, autumn starts today. Or to be more precise: tonight, at 9.21 pm (Dutch time).

Sun

Astronomers determine the beginning of autumn from the sun. For them, autumn begins when the sun is exactly above the equator and day and night are of approximately equal length worldwide. In the Northern Hemisphere, this so-called autumnal equinox heralds autumn. For astronomers in the Southern Hemisphere, spring is about to start at the same time.

Spring

As the earth orbits the sun, the sun is perpendicular to the equator twice: in September and in March. In March, the event marks the beginning of spring for astronomers in the Northern Hemisphere. In the southern hemisphere, the same equinox ushers in autumn.

The approach of the meteorologists

Meteorologists take a different approach. Their choice to welcome autumn on September 1 can be traced back to almost 250-year-old agreements. In 1780, one of the first international weather organizations – Societas Meteorologica Palatina – decided that three consecutive calendar months should be considered one season. And since then, autumn begins on September 1, only to be followed exactly three months later (on December 1) by winter, after which spring is welcomed on March 1 and summer on June 1.

Meteorologists therefore do not have to think about when autumn starts in 2087, for example: just on September 1. Astronomers, however, have to do some calculations. Astronomical autumn is not always on the same date. This is due to the fact that it does not take the earth exactly 365 days to complete one orbit around the sun. Instead, it takes our planet 365 days and 6 hours to do that. It means that this year’s equinox is 6 hours later than last year. However, once every four years there is a leap day and the equinox suddenly takes place 18 hours earlier than in the previous year. And so the autumnal equinox on our calendar shifts slightly from year to year and slightly back once every four years. Usually this takes place on the 22nd and 23rd of September, but occasionally also on the 21st or 24th.