Day/Night Terminator Calculator
Where on Earth is it day and where is it night right now? This calculator computes the subsolar point (where the Sun is directly overhead) and the terminator line (the boundary between day and night) for any given UTC date and time. The terminator traces a great circle around Earth, and its position depends on the Sun's declination and the time of day.
How We Calculate This
The subsolar point latitude equals the solar declination, calculated using the NOAA solar position algorithm. The longitude is derived from UTC time and the equation of time. Terminator coordinates are calculated where the solar zenith angle equals 90°.
Frequently Asked Questions
What is the subsolar point?
The subsolar point is the location on Earth where the Sun appears directly overhead (at the zenith). Its latitude equals the Sun's declination, and its longitude depends on the time of day.
What is the terminator?
The terminator is the line that separates the illuminated (day) side of Earth from the dark (night) side. It's where the Sun appears on the horizon - sunrise on one side, sunset on the other.
Why does the terminator curve?
The terminator follows a great circle around Earth, but on a flat map projection (like equirectangular), it appears as a curved line. Only at the equinoxes does it appear as a straight vertical line.
What is the equation of time?
The equation of time is the difference between apparent solar time (sundial time) and mean solar time (clock time). It varies throughout the year due to Earth's elliptical orbit and axial tilt.
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