Terminator (solar)

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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.

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.

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.