Equal day and night is not on the equinox
On the equinox the day is several minutes longer than the night. When day and night are really equal, city by city.
By latitude
Day length at the equinoxes and solstices
| Latitude | 20 March | 21 June | 23 September | 21 December | Change a day at the equinox |
|---|---|---|---|---|---|
| 0° | 12 h 07 | 12 h 07 | 12 h 06 | 12 h 08 | none |
| 10° N | 12 h 07 | 12 h 43 | 12 h 06 | 11 h 33 | 0.6 min |
| 20° N | 12 h 07 | 13 h 21 | 12 h 06 | 10 h 55 | 1.1 min |
| 30° N | 12 h 07 | 14 h 05 | 12 h 07 | 10 h 13 | 1.8 min |
| 40° N | 12 h 08 | 15 h 01 | 12 h 07 | 9 h 20 | 2.7 min |
| 45° N | 12 h 09 | 15 h 37 | 12 h 08 | 8 h 46 | 3.2 min |
| 50° N | 12 h 10 | 16 h 22 | 12 h 08 | 8 h 04 | 3.8 min |
| 55° N | 12 h 11 | 17 h 23 | 12 h 09 | 7 h 10 | 4.5 min |
| 60° N | 12 h 13 | 18 h 52 | 12 h 11 | 5 h 52 | 5.5 min |
| 65° N | 12 h 15 | 22 h 02 | 12 h 12 | 3 h 35 | 6.8 min |
At 60° north, the latitude of Oslo, Helsinki and St Petersburg, the longest day is 18 hours 52 minutes and the shortest 5 hours 52 minutes. Five degrees further north the June day is 22 hours long and the sun barely sets; at the Arctic Circle it stops setting at all.
How fast
Fastest at the equinoxes, slowest at the solstices
Day length follows a smooth wave over the year. It changes fastest around the equinoxes, when the sun’s declination moves 0.4° a day, and hardly at all around the solstices, when the declination turns. The last column of the table shows the speed at the equinox: under a minute a day in the tropics, nearly four minutes at 50°, nearly seven at 65°. In London the March days lengthen by about two hours over the month.
Around the solstices the day length stands almost still for two or three weeks, which is why sunrise and sunset then move mainly because of the drift of solar noon: see why the earliest sunset is not on the shortest day.
The geometry
Where the numbers come from
The sun moves along a circle parallel to the celestial equator, at the declination of the day. The horizon cuts that circle, and the share of it above the horizon is the day. Ignoring the size of the sun and refraction, the half-day, measured as an angle around the pole, follows from one formula: its cosine equals minus the tangent of the latitude times the tangent of the declination. Divide that angle by 15° to get hours.
- On the equator the tangent of the latitude is zero, so the half-day is always 90°: six hours either side of noon, all year.
- At the equinoxes the declination is zero, so the day is twelve hours at every latitude.
- When the product of the two tangents passes 1 the formula has no answer: the sun does not set or does not rise, the midnight sun and the polar night.
The size of the sun’s disc and refraction then add a few minutes to every day, which is why the table shows 12 hours 7 minutes on the equator. The article on the equinox explains that part.
One city
A year of daylight in London
| Month | Day length | Month | Day length |
|---|---|---|---|
| January | 8 h 21 min | July | 16 h 11 min |
| February | 10 h 01 min | August | 14 h 37 min |
| March | 11 h 50 min | September | 12 h 40 min |
| April | 13 h 52 min | October | 10 h 43 min |
| May | 15 h 37 min | November | 8 h 53 min |
| June | 16 h 37 min | December | 7 h 52 min |
Between mid-February and mid-April London gains almost four hours of daylight, and it loses them again just as fast from mid-August to mid-October. Every city page gives the same figures for its own city, with the longest and shortest day of the year.
Keep reading
Related articles
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How long the sun stays up, or down, in Tromsø, Svalbard, Utqiaġvik, Murmansk and Rovaniemi.
Civil, nautical and astronomical twilight: how long they last
What each stage of twilight looks like, how long it lasts by latitude and season, and where the night never gets dark.