Sunday, September 22, 2023 to the 2.42pm Italian will officially be theautumn equinoxthat is, the precise one astronomical moment where the circle of illumination (that is, the imaginary line that divides the illuminated half of the globe from the one that remains “in the dark”) passes exactly through the two Poles. At the equinoxes, in all places on Earth the day is marked by 12 hours of light and 12 hours of darkness. Contrary to popular belief, the autumn equinox typically does not fall on September 21, but on September 22 or 23, or more rarely on September 21 or 24. The autumn equinox in 2023, for example, fell on September 23.
What does it mean in astronomy?
The term equinox comes from Latin aequa nox which means “night equal (to day)”: this term identifies a precise astronomical moment that occurs when the Sun It is located exactly along thecelestial equator (i.e. the projection of the Earth’s equator onto the celestial sphere). This means that at the equator the Sun’s rays are perfectly at their zenith during astronomical midday: if at the instant of the equinox we were at the equator, the Sun would be at midday right above our heads and objects would not cast shadows.
To the Tropic of Cancer and of the Capricorninstead, the solar rays will be inclined by 66°33′ while at Polar Circles of 23°27′ (with respect to the tangent of the respective points). This phenomenon occurs twice a year, in September and March.
On the days of equinox day and night are the same lengthof 12 hours each, throughout the planet (while at the equator day and night have the same duration every day of the year). During this event the circle of illumination passes through the Earth’s poles, “cutting the Earth” into two equal portions of light and dark.
The equinox of September that’s what we call autumn equinox because it marks the beginning of this season for us who live in the Northern Hemisphere. But in the Southern Hemisphere it is the opposite: the seasons are reversed and this date corresponds to thespring equinox.
What changes with the autumn equinox?
In the Northern Hemisphere the day will continue to get shorter more and more than the night, reaching its minimum during the northern winter solsticewhile in the southern hemisphere the days will continue to get longer, reaching their maximum length at the southern summer solstice.
In September the days also begin to cool down and the sun begins to shine. first autumn leaves that start the color change, transforming the bright summer green into gold, amber and ruby hues. Autumn is also a good time to observe at the Arctic Circle the aurora borealis.
Why the autumn equinox 2024 does not fall on September 21 but on September 22
In the popular imagination the autumn equinox lasts 24 hours, but as we have anticipated the equinox is a single moment: that, precisely, in which the Sun transits the celestial equator along its apparent annual motion in the sky. To be precise, the autumn equinox will fall on September 22, 2024 at 2:43 PM.
Many people think, perhaps from distant school memories, that the equinox falls on September 21st because this is the beginning date of autumn. In reality the date of the autumnal equinox is not always the same: changes regularly due to the fact that the solar year lasts 365.25 days: there is a small gap compared to the 365 days in which the year is divided in the Gregorian calendar (i.e. the solar calendar used in almost the entire world since 1582). For these reasons it is expected that in the future we will have to move up the equinox of a few days. Typically the autumn equinox falls on September 22 or 23more rarely the 21 or 24. For example, between 2000 and 2030 there is no not even a year where the September equinox falls on the 21st.
The idea that the equinox falls on the 21st probably comes from the fact that it was once taught that all seasons began on the 21st, but the seasons officially begin on the days of solstices and of the equinoxes. The situation is different for the weather seasonswhich begin the 1st and they finish the 28, 29, 30 or 31 (so March 1st – May 31st, June 1st – August 31st, September 1st – November 30th, December 1st – February 28/29th).
