Terminator (solar)

The terminator, also known as the twilight zone, is a moving line that separates the illuminated day side and the dark night side of a planetary body (or any body not illuminated uniformly). It is defined as the locus of points on a planetary body where the solar incidence angle is 90 degrees. This means it's where the sun appears to be exactly on the horizon.

On Earth, and other bodies with an atmosphere, scattering of light by the atmosphere means that the region near the terminator experiences twilight, hence the alternate name "twilight zone." The apparent width of the terminator is influenced by atmospheric scattering and refraction. In an idealized vacuum environment with no atmosphere, the terminator would be a sharp, well-defined line.

The terminator moves across the surface of a rotating body as that body rotates, giving rise to daily cycles of light and darkness. Its speed is greatest at the equator and decreases towards the poles, eventually reaching zero at the poles during their respective solstices (where there is continuous daylight or darkness).

The shape of the terminator is not always a straight line. It is affected by the topography of the planet's surface. Mountains and other elevated features will be illuminated before the surrounding lowlands, causing the terminator to appear jagged or uneven. Shadows cast by these features distort the terminator's otherwise circular path.

Observations of the terminator can reveal information about a planet's atmosphere and surface features. For example, the presence of dust or haze in the atmosphere can broaden the terminator. The irregularity of the terminator can also indicate the presence of mountains, craters, or other surface irregularities.