What is the solar terminator line shown on the map?

The solar terminator (or twilight zone) is the moving boundary separating the illuminated daylight side of Earth from the dark night side. Because Earth is spherical and tilted at approximately 23.44° relative to its orbital plane, the terminator forms a curved sinusoidal wave across a 2D equirectangular map projection.

Why does the shape of the day and night curve change throughout the year?

The shape shifts continuously due to Earth's axial tilt during its annual orbit around the Sun. During the June solstice, the Northern Hemisphere tilts toward the Sun, resulting in 24 hours of continuous daylight at the North Pole (Midnight Sun). During the December solstice, the Southern Hemisphere experiences 24-hour daylight while the Arctic enters Polar Night. At the equinoxes in March and September, the terminator appears as nearly vertical lines across the map.

How does the 24-hour time scrubber work?

Dragging the 24-hour scrubber dynamically recalculates the subsolar point (the geographic coordinate where the Sun is directly overhead at solar noon) for any hour of the selected date. This lets you preview past and future solar illumination across continents client-side.

What map projection is used?

This tool uses the standard equirectangular (Plate Carrée) projection, which maps meridians and parallels to straight horizontal and vertical grid lines. This provides an unobstructed global canvas for visualizing solar terminator geometry and global city coordinates.

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