On September 22, 2026, almost all calendars and weather apps will display the same promise: twelve hours of daylight, twelve hours of night, a perfect balance between light and darkness. That is not true. All around the globe, except exactly at the equator at the precise moment of tipping, the day lasts longer than the night on that day, and the gap is measured in minutes, sometimes in tens of minutes depending on where you are.
The error lies in the word itself. The term “equinox” comes from the Latin aequus meaning equal and nox meaning night. A misleading etymology, since the equality it promises only occurs in a theoretical scenario: the Sun would be a tiny bright point, and Earth would be devoid of atmosphere. Neither is true.
Key takeaways
- The calendrical equinox never corresponds to true day–night equality
- Two optical tricks systematically extend every day of the equinox
- The equi-lux, the real day of equality, occurs several days after the equinox
A Sun That Cheats by Its Size and by Its Light
The first and simplest cause is straightforward: the Sun is not a point in the sky, it is a disk. Because the Sun is perceived on Earth not as a single point but as a sphere, the day becomes longer than the night because the upper limb of the Sun can be seen even when its center remains below the horizon. In concrete terms, astronomers declare sunrise as soon as the top edge of the solar disk appears on the horizon, not when its geometric center reaches it. The same logic applies at sunset: the day ends only when the last fragment of the disk has completely disappeared.
The second cause is subtler: the atmosphere plays the role of the troublemaker. Earth’s atmosphere refracts sunlight: even if the Sun’s limb is just below the horizon, its rays can still reach the surface. We see the Sun before it has geometrically crossed the horizon, and we continue to see it after it has crossed in the opposite direction. This phenomenon is not unique to the equinox: it lengthens every day of the year, somewhat like a magnifying lens that slightly straightens the image of an object near the water.
Taken together, these two optical distortions are not negligible. The apparent radius of the Sun is about 16 arc minutes, and the atmospheric refraction is about 34 arc minutes. The combination means the Sun’s upper limb can be seen while its center lies 50 arc minutes below the true horizon. Fifty arc minutes is nearly a full degree, or twice the Sun’s own apparent diameter. A tiny offset on the scale of the sky, but enough to skew all calculations of daily duration.
Minutes accumulate as you move northward
Here is the figure that surprises most: even at the equator, where one might expect perfect equality, the gap exists. Day is longer by 13 minutes and 2 seconds than the night at the equator on an equinox. Thirteen minutes, solely due to the Sun’s disk and refraction, in a region where the Sun rises and sets nearly vertically.
The further you move from the equator, the more the Sun’s angle at the horizon becomes shallow, and the more time refraction has to act as a magnifier. This difference in duration grows as you head toward the poles: in London it is already 21 minutes and 36 seconds, in Narvik (Norway) it reaches 44 minutes. Forty-four minutes is almost three-quarters of an hour of extra daylight where one would expect perfect balance. In New York, for example, a local almanac listed sunrise at 6:43 and sunset at 6:54, yielding a day length of not 12 hours, but 12 hours and 11 minutes on the equinox day.
This is not an isolated scientific anecdote: the American Naval Observatory, the world reference for these calculations, confirms the mechanism in its own protocol. Sunrise and sunset times in almanacs are calculated with a standard atmospheric refraction of 34 arc minutes and a half-diameter of 16 arc minutes for the disk, so that at the stated time the Sun’s geometric center is actually 50 arc minutes below a regular and clear horizon. This protocol is not country-specific: it is universal, which explains why the phenomenon occurs on all continents.
The Equi-lux, the True Day of Equality, Arrives Later
If the equinox does not deliver the promised equality, another day takes care of it: astronomers and meteorologists call it the equi-lux. It is the date, depending on latitude, when day and night truly last twelve hours each. A reference chart used by amateur astronomy places this transition, in September, around the 25th or 26th for most temperate northern latitudes, i.e., several days after the calendar equinox of September 22 or 23.
This shift grows with latitude, just as the equinox-day minute gap does. The higher you go toward the north, the more the real equality is pushed back in time: at 60° north, it occurs several days after the calculation for 40° north. Proof that this is not merely rounding, but an optical and geometric phenomenon that accumulates.
The next time a calendar proclaims the equinox as the day of perfect equality, one detail to keep in mind: the true equality, the one you can verify with a moment’s sight, will arrive a few days later, and will be even later the farther you live from the equator. In Paris, in Lille, or in Brest, the difference between equinox and equi-lux never exceeds a few minutes, but it is enough to remind us that the sky, unlike calendars, does not keep to neat round numbers.
Sources: calculatorian.com | fr.scienceaq.com
