Today, Wednesday, September 23, at 2:05 a.m. Italian time, astronomical summer officially gave way to autumn. Unlike the everyday idea of a season “changing” over the course of a day, astronomical seasons begin at a precise instant: the moment of an equinox or solstice. In 2026, the September equinox occurred at 00:05 UTC, corresponding to 2:05 a.m. in Italy.
The September equinox marks the moment when the Sun crosses the celestial equator, the imaginary line in the sky directly above Earth’s equator, moving from the northern celestial hemisphere into the southern one. It is the same instant everywhere on Earth, although the local date and time depend on the time zone.
The word “equinox” comes from the Latin aequinoctium, meaning “equal night,” a reference to the near-equal lengths of daylight and darkness around the equinox. But the day is not actually divided into exactly 12 hours of daylight and 12 hours of night. Atmospheric refraction and the apparent size of the Sun mean that daylight generally lasts a few minutes longer than darkness.
At the exact moment of the equinox, the Sun is directly above Earth’s equator. At solar noon on the equator, the Sun can therefore be directly overhead, or at the zenith, meaning that vertical objects cast essentially no shadow. At the same time, the terminator — the boundary separating the illuminated and dark sides of Earth — passes through both geographic poles.
Geometrically, this means that the two hemispheres receive sunlight in a nearly symmetrical way. In an idealized model, every point on Earth spends roughly half of its daily rotation on the illuminated side and half on the dark side. In reality, the duration of perceived daylight differs from 12 hours because of atmospheric refraction and the way sunrise and sunset are defined.
The September equinox does not always occur on September 21, as is sometimes taught in school. In most years, it falls on September 22 or 23, although September 21 and September 24 are possible on rare occasions. During the 21st century, a September 21 equinox will occur in 2092 and 2096.
The reason lies in the fact that the length of the tropical year does not correspond exactly to the 365 days of the civil calendar. The timing of the equinox therefore shifts by several hours from one year to the next. Leap years, which add an extra day to the calendar, then shift the calendar date of the equinox back again.
The pattern can already be seen in the next few years. In 2026, the September equinox occurred on September 23 at 2:05 a.m. in Italy. In 2027, it will again fall on September 23, but at 8:01 a.m. In 2028, a leap year, the equinox will move back to September 22, occurring at 1:45 p.m. in Italy.
For people living in the Northern Hemisphere, the September equinox arrives as daylight continues to decrease. The process began with the June summer solstice and will continue until the December winter solstice, when the Northern Hemisphere experiences its shortest day of the year. In the Southern Hemisphere, the pattern is reversed: daylight is increasing, and the September equinox marks the astronomical beginning of spring.
So autumn did not simply begin “sometime” on September 23. Astronomically, it began at one precise moment: 2:05 a.m. in Italy. From that instant onward, in the Northern Hemisphere, the Sun’s apparent path continued southward and daylight began its seasonal decline toward winter.
