Events · September 21, 2026
The 23 September Equinox: the Time of Year When Night Gains Fastest
Fondateur de SkyChart
Astronomical autumn begins on 23 September at 00:05 UT, 01:05 in London, 02:05 in Paris: the precise instant when the centre of the Sun crosses the celestial equator heading south. An instant, not a day. And though the equinox has a reputation for balance, what it really marks, for anyone who watches the sky, is the opposite: the moment when everything tips over fastest. This week, every day hands 3.6 minutes to the night at the latitude of Paris. No other week of the year does better.
The length of daylight at Paris across 2026. At the solstices the curve runs flat: its slope is zero, one day resembles the next. At the equinoxes it is at its steepest, 3.6 minutes lost per 24 hours in September. The dotted 12-hour line is crossed not on the 23rd but on the 25th: the day of the equinox lasts 12 h 08.
SkyChart illustration
Why now, and not in December
Intuition puts the great tipping point in midwinter. The curve says the opposite: around the solstices, day length treads water, a few seconds of difference from one day to the next, which is what the word means, sol stitium, the Sun stands still. The Sun's declination follows a near-sinusoid through the year, and a sinusoid crosses zero at maximum speed: at the equinox the Sun sheds 0.39 degrees of declination per day, its record. Every tenth of a degree is a later sunrise and an earlier sunset.
How steep the slide is then depends on your latitude, brutally so:
| City | Daylight lost, per 24 h |
|---|---|
| Helsinki | 5.4 min |
| Lille | 3.8 min |
| Paris | 3.6 min |
| Marseille | 2.9 min |
| Athens | 2.4 min |
That is the numerical answer to a feeling everyone shares in September: yes, night really is falling "all of a sudden", and more so in Edinburgh than in Marseille.
Three wrong things people repeat about the equinox
- "Day and night each last 12 hours." Not on the 23rd: at Paris the day lasts 12 h 08. Atmospheric refraction lifts the Sun by half a degree at the horizon, and daylight is counted from the first to the last ray of the disc's edge, not its centre. True equality, the equilux, falls on 25 September.
- "The Sun rises in the east." Across the year it rises anywhere between north-east and south-east. But near the equinox, and only then, it rises due east and sets due west, azimuth 270 degrees. Use it: note what the sunset touches on your horizon, and that landmark will remain your true west all year. It is the best day for calibrating your street, your window, your observing spot.
- "It is the first day night wins." Off by two days, see above. One thing, however, does flip exactly on the 23rd: at the North Pole, the Sun sets for six months.
What the equinox gives observers
Concrete things. Useful darkness stretches by nearly an hour over September alone: deep-sky objects now rise into an already-black sky, which widens the Andromeda Galaxy's window. Auroras, statistically, favour the equinox weeks; the orientation of Earth's magnetic field then couples better with the solar wind. Nothing is guaranteed, but if an alert fires in late September, take it seriously. And this particular 23 September stacks two more appointments: Venus at peak brightness in the dusk, and, three days later, the full Moon closest to the equinox, the one that carries a name: the Harvest Moon. It gets our next article.
What these figures do not tell you
The 3.6 minutes per day is an average in the immediate vicinity of the equinox; the slope relaxes from mid-October. Day lengths are computed with standard refraction, 34 arcminutes at the horizon: in very cold or very hot weather the atmosphere bends light differently, and real rise and set times move by a minute or two. For your exact times, position by position, SkyChart's Light screen gives sunrise, sunset, the twilights and the Sun's azimuth for any date, including the day you read this.
This article was translated from the French edition by AI, then reviewed before publication. The figures are not translated: times, slopes and azimuths are computed by SkyChart and are identical in both editions.