Lunar terminator

From Wikipedia, the free encyclopedia
Jump to: navigation, search
Oblique photo 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)

The Lunar terminator is the division between the illuminated and dark parts of the Moon.[1]


It is the lunar equivalent of the division between night and day on the Earth's sphere, although the Moon's much lower rate of rotation[2] 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.

Lunar terminator illusion[edit]

The lunar terminator (or tilt) illusion is an optical illusion arising from the erroneous expectation of an observer on Earth that the direction of light illuminating the moon (i.e. a line perpendicular to the terminator) should correspond with the position of the sun in the sky or a sun which has set, when it does not appear to do so. The cause of the illusion is simply the observer is 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.[3][4]

In popular culture[edit]

The lunar terminator features as a plot element in the animated series "Futurama", season 1, episode 2.

See also[edit]


  1. ^ List of basic lunar features
  2. ^ The lunar day cycle is 29.53 Earth days in length (see [1]), so the terminator moves across the lunar surface at 15.4 kilometers per hour.
  3. ^ Jones, Christopher B (January 2014). "Lunar Terminator Illusion". Ellipsis: unfinished thought…. Retrieved 21 May 2016. 
  4. ^ Myers-Beaghton, Andrea K; Myers, Alan L, The Moon Tilt Illusion (PDF) 

External links[edit]