Autumn will begin today, 22 September, precisely at 3.11 pm Italian time. The moment in which autumn begins and summer ends is called the autumn equinox: it is the precise moment in which the Sun is at the zenith of the Earth’s equator, that is, exactly above the head of a hypothetical observer who is at a specific point on the equator line (more or less in the middle of the Pacific Ocean). To say that the equinox is a full day is therefore incorrect, because it is only the precise instant in which an astronomical phenomenon occurs. And the time when that moment happens changes every year.
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Since the autumn equinox can fall from year to year on different days (September 22 or 23, like this year), seasons don’t always change on the same day. However, the day of the autumn equinox has a particular characteristic: it is one of only two days a year – the other is the spring equinox – in which the day has the same duration as the night (even if it is not exactly so due to some interactions of light with the Earth’s atmosphere).
https://www.youtube.com/watch?v=drujIxWmxyc
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How the equinoxes work
The beginning of the seasons is marked by precise astronomical events: summer and winter begin with the solstices – in which the hours of daylight per day are at their maximum (summer) or at their minimum (winter) – while spring and autumn begins on the day of the equinoxes – the times when the length of the day is equal to that of the night. However, due to a phenomenon called “atmospheric refraction”, the light of the Sun is bent by the atmosphere: it is the thing that we see the Sun a few minutes before it actually rises. This means that in reality tomorrow in Italy the day will still be a little longer than the night for a few minutes. The true equality between day and night will be in a few days.
The equinoxes and solstices (and the durations of the day and of the night) are determined by the position of the Earth in its revolving motion around the Sun, that is, the movement that our planet makes by turning around its reference star. The equinox is the time when the Sun is at the intersection of the plane of the celestial equator (the projection of the equator on the celestial sphere) and that of the ecliptic (the apparent path of the Sun in the sky). At the solstice, however, the Sun at noon is at the maximum or minimum height with respect to the horizon.
Autumn on the covers of the New Yorker.
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The variations in the moment in which equinoxes and solstices occur are caused by the different length of the solar year and that of the calendar (this is the reason why there are leap years, which allow us to put the accounts back on track). In Italy, spring falls between 20 and 21 March, summer between 20 and 21 June, autumn between 22 and 23 September, autumn, winter between 21 and 22 December. In addition to the astronomical seasons, then, there are also the meteorological ones: they begin about twenty days earlier than the solstices and equinoxes, and always last three months. They indicate more precisely the periods in which annual climatic variations occur, especially in mid-latitudes with temperate climates.
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