Terminator (solar) - The Earth's Terminator

The Earth's Terminator

On Earth, the terminator is a circular line with a diameter that is approximately that of the Earth. The terminator passes through any point on the Earth's surface twice a day, once at sunrise and once at sunset, apart from polar regions where this only occurs when the point is not experiencing midnight sun or polar night. 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 (with exceptions during eclipses), 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 terminator line also varies by time of year: on Earth the angle of the line is almost parallel to lines of longitude during the equinoxes, and at its maximum angle of approximately 23.5 degrees during the solstices.

At the equator, under flat conditions (no obstructions such as mountains; or at a height above any such obstructions), the terminator line moves at approximately 1600 kilometers per hour (1000 miles per hour). This speed can appear to be increased when near obstructions—such as the height of a mountain, for example—as the shadow of the obstruction will be broadcast over the ground in advance of the terminator line along a flat landscape. The speed of the terminator line decreases as one approaches the poles, where it can reach a speed of zero (full-day sunlight or darkness).

Supersonic aircraft like jet fighters or Concorde and Tupolev Tu-144 supersonic transports are the only aircraft able to overtake the maximum speed of the terminator. However, slower vehicles can overtake the terminator at higher latitudes and it is possible to walk faster than the terminator at the poles, near to the equinoxes. The visual effect is that of seeing the Sun rise in the west.

Amateur radio operators take advantage of conditions at the terminator (so-called "grey line" conditions) to perform long distance communications. Under good conditions, radio waves can travel along the terminator to antipodal points. This is primarily because the D layer, which absorbs high frequency signals, disappears rapidly on the dark side of the terminator line. This process is known as skywave propagation.