Day, night and the polar night
Pick a latitude and a season and the calculator gives the length of day. At a solstice one pole has 24 hours of daylight while the other has 24 hours of night.
Day and night happen because the Sun circles like a spotlight over a flat disc, lighting different areas in turn.
A spotlight over a disc cannot give a polar day at one pole and a polar night at the other at the same time. A rotating sphere with an axis tilted 23.4° does it with one geometry — and that is exactly what we observe.
Day length depends on two things: your latitude and the Sun's declination — the latitude the Sun stands over at noon on a given day. At the equator the day lasts about 12 hours all year. The farther north or south you go, the more day length swings between summer and winter.
Beyond the polar circles the extremes appear: the midnight sun (the Sun never sets) and the polar night (it never rises). Crucially, when the north has a polar day, the south has a polar night — at the same moment. A flat model with one nearby Sun cannot produce two opposite poles at once.
Set a latitude and choose a season. You will see the day length at your location and at the equator and both poles.
Your latitude and season
- North Pole
- 24 h · midnight sun
- Your latitude (52° N)
- 16 h 30 min
- Equator
- 12 h 00 min
- South Pole
- 0 h · polar night
The model uses an approximate solar declination and ignores refraction and the Sun's disc size (which slightly lengthen the real polar day). Axial tilt is taken as 23.44°.
- At the equator the day lasts about 12 hours regardless of season.
- At a solstice one pole has 24 h of daylight and the other 24 h of night — simultaneously.
- A tilted, rotating sphere explains this with one model; a “spotlight” over a disc does not.
Day, night and seasons — questions
Earth spins on its axis roughly once a day. The half of the sphere facing the Sun is always lit; the spin carries a given place alternately into light and shadow. No wandering “spotlight” is needed.
Beyond the polar circle (about 66.5° latitude) the Sun does not set for a full day in summer (midnight sun) and does not rise for a full day in winter (polar night). This follows from Earth's 23.4° axial tilt relative to its orbital plane.
When the Northern Hemisphere tilts toward the Sun (summer), the Southern tilts away (winter), and vice versa half a year later. The same mechanism bathes one pole in light and the other in darkness at a solstice.
A single nearby Sun circling over a disc would have to light one pole continuously while entirely skipping the other — geometrically impossible. A sphere with a tilted axis gives both effects at once.