Day and night map: where the sun is right now
One curve across a world map tells you who is awake, who is asleep, and who is about to be — and you can scrub it forward to see tomorrow.
A day/night map is a world map with one extra thing drawn on it: the line separating the half of the earth in sunlight from the half in darkness.
That line is called the terminator, and it is not decoration. It is the single most compact answer to "is it a reasonable hour to contact this person?" — because you can see it rather than compute it.
What you are looking at
| On the map | What it means |
|---|---|
| Bright side | Local sun is above the horizon — daytime |
| Shaded side | Local sun is below the horizon — night |
| The curve between | Sunrise on one edge, sunset on the other |
| The curve's shape | How far the sun is from the equator today |
The line moves west at roughly 1,600 km/h at the equator, which is just the earth turning. Over 24 hours it sweeps the whole map once.
Why the line is curved and not vertical
This is the part that surprises people. If the earth's axis were perpendicular to its orbit, the terminator would be a straight vertical line and every place on earth would get exactly twelve hours of daylight every day.
The axis is tilted about 23.4°. So the terminator tilts too, and the amount it tilts changes through the year:
- At the solstices (June and December) the tilt is at its maximum. One pole is in permanent daylight, the other in permanent darkness, and the curve sweeps well past 66° latitude.
- At the equinoxes (March and September) the tilt is near zero and the line is close to vertical. Day and night are roughly equal everywhere.
That is why a day/night map in June looks nothing like the same map in December, and why the Arctic is either entirely lit or entirely dark depending on the month.
Reading it for scheduling
The practical use is that darkness is a proxy for "do not call."
If you keep four cities on a map, you can see at a glance which of them are in daylight together — and that shared daylight is a decent first approximation of a shared working window. It is not exact: people wake before sunrise in winter and work after sunset. But it answers the question "is this the middle of their night?" instantly, which is the question that actually matters.
For a precise shared window, you want an overlap calculation rather than a sunlight one — that has its own page.
Scrubbing it forward
A static day/night map tells you about now. A map with a time slider tells you about the meeting you are trying to schedule.
Dragging time forward moves the terminator west and moves every city's local clock with it, so you can watch a proposed slot arrive in each place and see who is in daylight when it does. It converts a scheduling question into a visual one.
How the line is actually computed
It is real astronomy, not an approximation drawn by hand. The calculation needs two things about the sun on the date in question:
- Declination — how far north or south of the equator the sun is, which comes from the earth's axial tilt and its position in orbit.
- The equation of time — the discrepancy between clock time and true solar time, caused by the earth's elliptical orbit and its tilt. It runs to about ±16 minutes over a year.
From those you get the subsolar point: the single spot on earth where the sun is directly overhead at this instant. Every point 90° away from it is on the terminator.
The full derivation is here, including why the curve is not quite a circle on a flat map.
Related: Where is it daytime right now? · Time zone overlap
Timezone: World Clock Widget
A world clock, converter and live day/night map for iPhone — 325 cities, offline, no account.
Figures on this page are computed from the IANA time zone database. Last generated 2026-08-18.