How long is a day? Daylight from the equator to the Arctic

Daylight Guesser names a place and a date and asks how long the sun is up, from sunrise to sunset. Only two things decide the answer: how far the place is from the equator, and how close the date is to a solstice. Here are the numbers for a few cities on the longest and shortest days of the northern year.

Why the equator gets a little more than 12 hours

You might expect exactly 12 hours of day on the equator, every day. Quito actually gets about 12 hours 7 minutes, all year round. Two small effects add the extra minutes. Sunrise counts from the moment the top edge of the sun appears, not its centre. And the atmosphere bends sunlight over the horizon, so you see the sun about half a degree higher than it really is. Together they make every day a few minutes longer than the geometry alone would give.

The same effects mean day and night are not quite equal at the equinoxes. On 20 March Paris already has 12 hours 6 minutes of daylight, and Ushuaia 12 hours 17 minutes. In the northern hemisphere, the day of truly equal day and night comes a few days before the March equinox and a few days after the September one.

The midnight sun starts south of the Arctic Circle

The Arctic Circle, at 66.6° north, is drawn where the sun's centre would just touch the horizon at midnight on the June solstice. Thanks to the same bending of light, the sun stays above the horizon all day on 21 June from about 65.8° north. Polar night, a day with no sunrise at all, needs more: about 67.5° north on 21 December. Reykjavík, at 64.2°, just misses out: 21 hours of sun in June, and a bright twilight in place of night.

Days change fastest around the equinoxes

Near the solstices the length of the day barely moves from one day to the next. Around the equinoxes it changes quickly: in late March London gains about 4 minutes of daylight a day, Helsinki about 5 and Reykjavík about 6. On the equator it hardly changes at all.

The south runs the other way

South of the equator the seasons flip. Sydney's longest day is in December, with 14 hours 24 minutes of sun, and its shortest in June, with under 10 hours. Ushuaia, near the tip of South America at 54.8° south, gets more than 17 hours on 21 December.

How to guess

Start with latitude: near the equator the answer is about 12 hours whatever the date, so you are done. Further from it, ask which hemisphere the place is in and how far the date is from the nearest solstice. Close to an equinox, everywhere is near 12 hours. Close to a solstice, the gap between the longest and the shortest day grows quickly with latitude: about 4 hours at 30°, 7 at 45° and 13 at 60°.