What Is the Day/Night Map?
The day/night world map shows you the current illumination of Earth in real time. The blue region is experiencing daytime — the sun is above the horizon — while the dark region is in night. The transition band between them is twilight, where the sun is below the horizon but its light still scatters through the atmosphere, giving us the orange and pink hues of sunrise and sunset.
The boundary between day and night is called the daylight terminator. From space, the terminator is a perfect great circle — a straight line drawn around a sphere. But on this flat map (which uses an equirectangular projection), it appears as a sinusoidal curve. That curve shifts westward by about 15 degrees of longitude every hour as Earth rotates at 1,674 km/h at the equator.
The small yellow dot on the map marks the sub-solar point — the single location on Earth where the sun is directly overhead (at solar noon, zenith angle = 0°) at this exact moment. The sub-solar point moves westward in real time as Earth rotates. Its latitude slowly varies through the year: it reaches 23.5°N at the June solstice (northern summer), crosses the equator at the equinoxes in March and September, and dips to 23.5°S at the December solstice (southern summer).
Why Does the Terminator Curve?
The apparent curvature of the terminator on a flat map is purely a projection effect. The Earth is a sphere, and the terminator is a circle drawn perpendicular to the sun-Earth axis. When you flatten a sphere onto a rectangle, circles that are not aligned with the equator or a meridian become curves. Near the equinoxes, the terminator runs almost straight from pole to pole (nearly vertical). Near the solstices, the tilt of Earth's axis means the sub-solar point is at ±23.5° latitude, which produces the most pronounced S-curve shape you see on the map.
The Grey Line — Radio Enthusiasts' Best Friend
Amateur radio operators call the terminator the "grey line." At the boundary between day and night, the ionosphere — the layer of the upper atmosphere that reflects shortwave radio signals — changes state. Dawn and dusk at the terminator create unusually strong long-distance radio propagation: signals can travel thousands of kilometres along the terminator path with minimal attenuation. Grey-line DXing (long-distance radio contact) is a prized activity in the amateur radio community, and knowing the exact terminator position is essential for timing these contacts.
Twilight: Three Types
The transition from full daytime to full night is not a single line — it is a graduated band known as twilight, divided into three phases:
- Civil twilight (0° to 6° below horizon) — bright enough for most outdoor activities without artificial light. Street lights typically turn on at the end of civil twilight.
- Nautical twilight (6° to 12° below horizon) — the horizon is still visible at sea, enabling celestial navigation. Most stars are visible.
- Astronomical twilight (12° to 18° below horizon) — the sky is nearly fully dark. Only the faintest stars are still affected by scattered sunlight. True astronomical darkness begins at 18°.
The map shows a combined twilight band of approximately ±6° solar altitude. For detailed sunrise, sunset, and golden hour times for your specific location, use our Sunrise & Sunset Calculator.
Practical Uses of the Day/Night Map
This map is useful for far more than academic curiosity. Here are common real-world uses:
- Global team scheduling — instantly see which colleagues are in daytime hours and which are in the middle of the night. Use alongside our Meeting Planner for precise scheduling.
- Travel planning — see what time of day it is at your destination right now, and understand the sun situation before you fly.
- Photography & filmmaking — the terminator marks where golden hour and blue hour are occurring globally right now.
- Amateur radio (ham radio) — track the grey line for optimal long-distance propagation windows.
- Astronomy — know where astronomical darkness currently exists on Earth; plan telescope sessions for you or collaborators worldwide.
- Satellite operations & solar energy — solar panels and satellites in sun-synchronous orbits track the terminator closely.
How the Calculation Works
Our map uses the SunCalc library to compute the sun's current position. First, we find the sub-solar point by scanning longitudes on the equator for where the sun's altitude is maximum (solar noon). Then we scan latitudes at that longitude to find the exact declination. With the sub-solar point known, we apply the great-circle angular distance formula to every pixel of the map: any point where the angular distance from the sub-solar point is less than 90° is in daylight; greater than 90° is in night. The calculation runs entirely in your browser — no data is sent to a server, and the map works offline once loaded.
For individual city times, browse our City Time pages or use the World Clock to see times for cities you care about. For converting a specific time between zones, try the Timezone Converter.