Terminator (solar)

The terminator is defined as the locus of points on a planet or moon where the line through the center of its parent star is tangent.

An observer on the terminator of such an orbiting body with an atmosphere would experience twilight due to light scattering by particles in the gaseous layer.

[4] This time-difference puts the ionosphere into a unique intermediate state along the terminator, called the "grey line".

[6] It is the lunar equivalent of the division between night and day on the Earth spheroid, although the Moon's much lower rate of rotation[7] means it takes longer for it to pass across the surface.

The illusion results from misinterpreting the arrangement of objects in the sky according to intuition based on planar geometry.

As the particles within an atmosphere are at a higher elevation, the light source can remain visible even after it has set at ground level.

It also enables specific experiments that require minimum interference from the Sun, as the designers can opt to install the relevant sensors on the dark side of the satellite.

Earth's terminator as seen from space
Video of the Earth from the ISS as it approaches the terminator.
An oblique view of the large lunar crater Keeler at the terminator (from Apollo 13 )
The east side of Timocharis crater while at the terminator (from Apollo 15 )
Mosaic of Apollo 16 photos along the terminator showing Darney , Lubiniezky , and Bullialdus craters