The Terminator Screensaver
The Terminator screensaver draws the real boundary between day and night on Earth, computed from the solar declination and hour angle for the current date. It runs in the browser with nothing to install; press ESC to exit.
How the The Terminator screensaver works
The saver renders with three.js on your GPU. The library is vendored into this site rather than fetched from a CDN, and loads only when a 3D screensaver actually runs.
It reads nothing external — the sun's position is computed in the browser from the date.
The scene resizes with the window and releases its WebGL context when the saver stops, so shuffle mode and Multi-view can cycle 3D savers without exhausting the browser's context limit.
If WebGL is unavailable the saver says so on screen rather than showing an empty black box.
A worked example
The sphere is shaded in a fragment shader by its own surface normal against the sun direction, so the terminator is exact rather than a drawn arc. It is deliberately soft: civil twilight extends about six degrees past the geometric line, so a real terminator is a band. Because the Earth's axis is tilted 23.44°, the line leans with the seasons, and near a solstice an entire polar cap sits in permanent light or permanent dark.
Settings & tips
- Accent and Background recolour the scene; Speed changes the pace of the motion and the camera.
- Add it to a Multi-view grid, though 3D savers are heavier than 2D ones and a few per screen is plenty.
- Press ESC at any time to leave fullscreen.
Frequently asked questions
- Why is the boundary not a straight line?
- Because the axis is tilted. If the Earth stood upright the terminator would run pole to pole through every meridian in turn. The 23.44° tilt swings it, which is what creates seasons and, at the extremes, midnight sun and polar night.
- Are the city lights real?
- No. The dark side is scattered with lights weighted toward the mid latitudes, where most people live, but they are generated rather than taken from a night-lights satellite image. The terminator itself is computed properly.
- Does this need a special browser or plugin?
- No. It uses WebGL, which every current desktop and mobile browser supports. Nothing is installed and no plugin is required.
- Does it cost anything or need an account?
- No. Every screensaver here is free, runs in the browser and needs no sign-up.