Does the boundary between the near (visible) and far sides of an object have a name, analogous to the terminator which divides day from night? (For a spherical object, this would be a great circle in the plane normal to the observer's line of sight, just as the terminator is a great circle normal to the direction of illumination.)

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    $\begingroup$ It's not a great circle unless the light source (in the case of the terminator), or the observer (in the case of the surface horizon) is at an infinite distance. $\endgroup$
    – notovny
    Commented Jun 17, 2022 at 8:39
  • $\begingroup$ Maybe not a name, but with coordinates the line you describe can be designated selenographic longitude 90°E and 90°W, or meridians ±90°. $\endgroup$ Commented Sep 13, 2022 at 12:30

1 Answer 1


For the moon (on which the precise position of this line is of interest in predicting the moment of eclipse, etc) this is called the "lunar limb"

The irregularity of the lunar limb is the cause of Baily's beads.

And you can find reference to "Pluto's limb". A star is occulted when it reaches the limb of the occulting body.

  • $\begingroup$ I think it's safe to say that 'limb' is the word generally - even when it might not be used too often. Also for the Sun one speaks of the phenomenon of 'limb darkening' which describes the apparent lower temperature of the outer parts of the 'disk' as seen by an observer. $\endgroup$ Commented Jun 17, 2022 at 7:14
  • $\begingroup$ OK, that makes sense. I had always somehow thought of the "limb" as being less definite, more like the indefinite outer region of the disk than the exact edge. But I can see how it makes sense to use it that way. The context where this came up is in explaining lunar phases to students, where the angle between the limb and the terminator is relevant. $\endgroup$ Commented Jun 18, 2022 at 14:19

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