This time-difference puts the ionosphere into a unique intermediate state along the terminator, called the “grey line”. Amateur radio operators take advantage of conditions along the terminator to perform long distance communications. Called "gray-line" or "grey-line" propagation, this signal path is a type of skywave propagation. Under good conditions, radio waves can travel along the terminator to antipodal points. The lunar terminator is the division between the illuminated and dark hemispheres of the Moon. 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 means it takes longer for it to pass across the surface. Due to the angle at which sunlight strikes this portion of the Moon, shadows cast by craters and other geological features are elongated, thereby making such features more apparent to the observer. This phenomenon is similar to the lengthening of shadows on Earth when the Sun is low in the sky. For this reason, much lunar photographic study centers on the illuminated area near the lunar terminator, and the resulting shadows, provide accurate descriptions of the terrain. The lunar terminator (or tilt) illusion is an optical illusion arising from the erroneous expectation of an observer on Earth that the direction of sunlight illuminating the Moon (i.e. a line perpendicular to the terminator) should correspond with the position of the Sun, but does not appear to do so. The cause of the illusion is simply the observer not taking into account that the observed slope of a light ray will change across the sky because of the lack of visual clues to establish 3D perspective. Examination of a terminator can yield information about the surface of a planetary body; for example, the presence of an atmosphere can create a fuzzier terminator. 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. These particles scatter the light, reflecting some of it to the ground. Hence, the sky can remain illuminated even after the sun has set. Images showing a planetary terminator can be used to map topography: the position of the tip of a mountain behind the terminator line is measured when the Sun still or already illuminates it while the base of the mountain remains in shadow. Low Earth orbit satellites take advantage of the fact that certain polar orbits set near the terminator do not suffer from eclipse, therefore their solar cells are continuously lit by sunlight. Such orbits are called dawn-dusk orbits, a type of Sun-synchronous orbit.