Day and night are not equal at the equinox, the real date is the equilux

Par Sam Arkwell · publié le 19 septembre 2026

The short answer: no, day and night will not be equal at the equinox. On September 23, 2026, the day of the astronomical handover, daylight will still last about 12 hours and 8 minutes in New York and Chicago, and 12 hours 10 in London. The true equality has its own name, the equilux, and its own date: day length crosses the 12-hour line during the night of September 25 to 26 across the mid-northern latitudes, making the 26th the closest thing to a perfect split. The extra minutes have two culprits: atmospheric refraction, which shows you the sun while it is still geometrically below the horizon, and the very definition of sunrise, counted from the first ray of the sun’s upper edge rather than its center.

Why the equinox does not split the day in half, where the extra minutes come from, and the exact date of the crossing.

The equinox splits the geometry, not the light

The equinox is an instant of absolute precision: on September 23, 2026 at 00:05 UTC, the center of the Sun crosses the celestial equator, the worldwide, to-the-minute event our equinox story covered. On that day, geometrically, every point on Earth does spend half a rotation on the lit side. Yet the sunrise tables tell another story: about 12h08 of daylight in New York, 12h10 in London. The mismatch is not a rounding error; it lives in the definition of what counts as seeing the sun.

First culprit: atmospheric refraction. The atmosphere bends light downward, so at the horizon we see the sun about half a degree higher than it really is: at the moment its image touches the horizon, the actual sun has already set geometrically. Refraction hands us a sunrise in advance and a sunset in overtime, several bonus minutes at each end of the day. Second culprit: the definition of sunrise and sunset, fixed at the first and last ray of the sun’s upper edge, not its center; the sun’s half-diameter adds its own head start at dawn and its own stoppage time at dusk. Stacked together, the two effects stretch the equinox day by roughly 8 to 10 minutes at mid-northern latitudes: that is where the promised equality goes.

The equilux: the night of September 25 to 26

Since the equinox day runs long and autumn is currently trimming daylight by 2.5 to 4 minutes per day, the fastest rate of the year, the true crossing is just a matter of counting down the calendar. Our own calculations, run with the standard astronomical algorithm, agree across the map: day length drops below 12 hours during the night of September 25 to 26, 2026. New York goes from 12h02 on the 25th to a hair under 12 hours on the 26th, Chicago and London follow within a minute, and Paris crosses the same night. September 26 is the equilux for most of the mid-northern latitudes, the differences between cities living inside a couple of minutes.

The word deserves its definition: equilux, literally equal light, is the day when the span between sunrise and sunset comes closest to exactly 12 hours. Unlike the equinox, a single planetary instant, the equilux is strictly local: latitude sets how fast the days are shrinking, and therefore when the line is crossed. Around 40 to 50 degrees north, it trails the equinox by about three days; closer to the equator it drifts further out, and in the tropics the question dissolves entirely, day length hovering near 12 hours all year without ever crossing cleanly. In the southern hemisphere the whole story runs mirrored: their equilux comes a few days before their spring equinox, on the way up.

What the coming week does to your daylight

The handover schedule now reads cleanly: on the 23rd, astronomical autumn begins inside a day that still overshoots 12 hours; on the 26th, light hands the keys to darkness; and from there every day widens the gap until the December solstice. Concretely, in New York, sunset slides from about 6:58 pm on the 19th to 6:46 pm on the 26th, with dusk following half an hour behind; mornings run the other way, the sun rising visibly later each day, and the week of the equilux is when many commutes tip into twilight at both ends.

There is a consolation in the ledger, and it is a real one: symmetry. What September writes in losses, March will write back in gains, equinox on the 20th, equilux a few days ahead of it that time, and the same refraction bonus paid out on the bright side. The atmosphere is an honest debtor: everything it seems to take in autumn, it returns in spring, with interest. Until then, the true 12-hour day has an appointment on September 26, one of only two a year; the online clock will happily time it for you, to the second.

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