Day Length Calculator

In short Day length is set by your latitude and the sun declination, and how much it swings across the year is almost entirely a question of how far you are from the equator. At the equator the difference between the longest and shortest day is barely a minute. In London it is close to nine hours — 16h 38m on the June solstice against 7h 50m in December. At the Arctic circle it is the full twenty-four. The thing worth knowing is that the equinox is NOT the twelve-hour day, and this is not a rounding detail. The equinox is the instant the sun CENTRE crosses the celestial equator. Sunrise and sunset are defined on the upper LIMB and include atmospheric refraction, and both of those lengthen the day — so the equinox day runs over twelve hours everywhere on earth. Measured for March 2026 that excess is about 6.7 minutes at the equator, 8.4 minutes at 40 degrees and 15.1 minutes at 65 degrees. The day that really is twelve hours long has its own name, the equilux, and it falls a few days before the equinox in spring and a few days after it in autumn, further away the higher your latitude. One more thing people get backwards: day length changes fastest at the EQUINOXES, not at the solstices. Around a solstice the sun declination is barely moving, so day length is nearly flat for a fortnight either side — which is why late June feels like it stays light forever and then collapses in September.