Day and Night Map Screensaver
The Day & Night Map screensaver shows a real Earth map with the live day/night terminator — the moving line that separates the sunlit half of the planet from the dark half. It computes where the sun is directly overhead right now from the current date and time, then brightens the day side and dims the night side accordingly. It is free, runs in your browser using a bundled Earth image, and you press ESC to exit.
How the Day & Night Map screensaver works
The map is the bundled assets/img/earth.jpg, letterboxed as a flat 2:1 projection. It computes the subsolar point — the longitude where the sun is overhead, from UTC time, and the solar declination, from the day of the year — so no internet connection is required.
It then walks every pixel of the map. For each one it converts the position to a latitude and longitude, computes the local solar elevation using the standard formula sin(lat)·sin(dec) + cos(lat)·cos(dec)·cos(hour angle), and decides whether that spot is in daylight or darkness.
Pixels where the sun is low or below the horizon are blended toward a dark night tint, with the blend strength ramping up through the twilight zone so the terminator appears as a soft band rather than a hard edge. The depth of the shading is set by the Night darkness control.
Finally it draws a glowing subsolar marker at the point where the sun is directly overhead and stamps the current UTC time. The whole map re-renders every ten seconds so the terminator keeps creeping westward in real time.
A worked example
With the defaults — Night darkness 85, Night tint #06091c, and a Background of #03040a — roughly half the world map is bright daylight and the other half is dimmed to a deep blue-black, separated by a soft twilight band. A warm glow marks the spot where the sun is overhead and the live UTC time is printed in the corner. Lower Night darkness toward 40 to keep the night side more visible, or change the Night tint to recolor the shadowed hemisphere.
Settings & tips
- Use the Night darkness slider (40–95) to set how deeply the night hemisphere is dimmed.
- Change the Night tint to recolor the shaded, after-dark side of the map.
- Leave it running — the map re-renders every ten seconds, so you can watch the terminator drift across the globe over time.
Frequently asked questions
- What does the day and night map screensaver show?
- It shows a real world map with the live terminator, the line dividing the sunlit day side of Earth from the dark night side, based on the current sun position.
- Is the day and night terminator accurate to right now?
- Yes. It computes the subsolar point from the current UTC time and the day of the year, so the day and night line matches the real sun position and updates every ten seconds.
- Does the day and night map need an internet connection?
- No. The Earth map is bundled with the page and the sun position is calculated locally, so it works fully offline.
- Can I make the night side darker or lighter?
- Yes. A Night darkness slider from 40 to 95 controls how deeply the night hemisphere is dimmed.
- Is the day and night map screensaver free?
- Yes. It is completely free with no download, runs in your browser, and you press ESC to exit.