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World map with terminator (April)

The terminator is the line between the illuminated, day side and dark, night side of a planetary body (also known as the "grey line" or the "twilight zone"). It is defined as the locus of points on a moon or planet where the line through the Sun is tangent. The line separates the portions of the earth experiencing daylight from the portion of the planet experiencing darkness. While one half of the Earth is illuminated at any point in time, the location of the terminator line varies by time of day due to the rotation of the earth on its axis as well as the revolution of the earth around the sun. The seasons impact the location of the terminator line most dramatically.

In the frame of reference of a rotating planet, the terminator sweeps across the surface of the planet bringing sunrise on one side of the planet and sunset on the opposite side.

The orientation of the terminator between the northern and southern hemispheres depends on the season. On the spring and fall equinoxes (around March and September 21), there is no tilt of the earth with respect to the sun so the terminator line is parallel with the axis of the earth and with the lines of longitude.

A composite image (simulated earth) showing the terminator dividing night from day, running across Europe and Africa. Observers on the surface of the earth along this terminator will see a sunset.

Examination of the terminator can yield information about the surface of the body; for example, a fuzzy terminator indicates the presence of an atmosphere.

Amateur radio operators take advantage of conditions at the terminator (so-called "grey line" conditions) to make long distance contacts. Under good conditions radio waves can travel along the terminator to antipodal points.

The terminator line is at its greatest angle with respect to the axis on the winter and summer solstices (around December and June 21), when it is approximately 23.5 degrees off the axis.

Photograph of part of the terminator crossing the surface of the Earth, as seen from the ISS. The terminator is diffuse and shows the gradual transition to darkness that is experienced as twilight on the surface.

Excluding travel near the poles, the Concorde and the Tupolev Tu-144 were the only passenger airplanes able to overtake the terminator. On certain early evening transatlantic flights departing from Heathrow or Paris, it was possible to take off at night and catch up with the sunset — from the cockpit you could see the sun rise in the west.

• Current terminator
• aa.usno.navy.mil - Website calculating synthetic images (B&W or color) representing the terminator for a given time (date & hour)