Many of us grew up learning that autumn begins on the 21st of September but astronomically, that is not always the case. On the Maltese Islands, autumn officially began today, 23rd September, at 2:05am, marking the precise moment of the autumnal equinox.

So why does the date change?

The equinox is not a fixed calendar date. Instead, it is a precise astronomical moment when the centre of the Sun crosses the celestial equator, moving southwards. Since Earth takes approximately 365 days and six hours to complete its orbit around the Sun, the timing of the equinox shifts slightly from year to year. Leap years help keep our calendar aligned with Earth's orbit, meaning the September equinox generally falls on either the 22nd or the 23rd September.

And there's another interesting detail: despite being called an equinox, today does not actually have exactly 12 hours of daylight and 12 hours of darkness.

That is where the Equilux comes in. While the equinox marks the astronomical crossing of the celestial equator, the Equilux is when daytime and nighttime are approximately equal in length in practice. In Malta, this year's Equilux falls on the 26th September, three days after the equinox. After that, nights will become longer than days.

The difference is largely caused by atmospheric refraction and the Sun's apparent size.

So why were we taught that autumn begins on the 21st September? Dates such as 21st March, 21st June, 21st September and 21st December are simply convenient approximations used to teach the seasons. The reality, however, is a little more fascinating.

Did you know about this interesting fact?