Day and Night Map

How it works

The subsolar point (where the sun is straight overhead) is computed in your browser from the sun’s declination and Greenwich sidereal time using the algorithms in Jean Meeus’s Astronomical Algorithms. The terminator is then found directly: for each degree of longitude, the latitude at which the sun sits on the horizon, allowing 0.833 degrees for refraction and the solar disc, and that line is closed over the pole in darkness to make the night polygon. A second, fainter polygon marks civil twilight, where the sun is less than 6 degrees below the horizon.

The map is a Web Mercator projection, so the curved terminator looks S-shaped and the polar regions are stretched; the geometry is still exact for each point. Nothing is fetched except the map tiles.

Questions

What is the terminator?

The line separating the sunlit half of the Earth from the dark half: sunrise along one side of it and sunset along the other. It sweeps west at about 1,000 miles an hour at the Equator.

Why is the night side curved?

Half of a sphere is lit at any moment, and a great circle drawn on a flat Mercator map becomes a wave. In June the curve leaves the Arctic in daylight; in December the Antarctic.

Where is the sun directly overhead?

Somewhere between the Tropic of Cancer and the Tropic of Capricorn, at the longitude where it is solar noon. The yellow dot shows the exact point and the readout gives its coordinates.