<p>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 &quot;<a href="https://en.wikipedia.org/wiki/Lunar_limb" rel="nofollow noreferrer">lunar limb</a>&quot;</p>
<blockquote>
<p>The irregularity of the lunar limb is the cause of Baily's beads.</p>
</blockquote>
<p>And you can find reference to &quot;<a href="https://www.researchgate.net/figure/Paths-of-the-stellar-image-around-Plutos-limb-during-the-occultation-These-paths-in_fig3_231009985" rel="nofollow noreferrer">Pluto's limb</a>&quot;. A star is occulted when it  reaches the limb of the occulting body.</p>

CC BY-SA 4.0
2022-06-17T07:14:10.037Z
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.
planetmaker
CC BY-SA 4.0
2022-06-18T14:19:42.950Z
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.
PatrickComposerPhysicist
CC BY-SA 4.0
2022-06-17T05:46:35.070Z
2022-06-17T05:46:35.070Z
James K
<p>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.)</p>

CC BY-SA 4.0
2022-06-17T08:39:08.777Z
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.
notovny
CC BY-SA 4.0
2022-09-13T12:30:08.750Z
Maybe not a name, but with coordinates the line you describe can be designated [selenographic longitude](https://en.wikipedia.org/wiki/Selenographic_coordinate_system) 90°E and 90°W, or meridians ±90°.
Jeppe Stig Nielsen
<nomenclature>
Name for the boundary between near and far sides relative to an observer
PatrickComposerPhysicist
# Name for the boundary between near and far sides relative to an observer

(Asked by: username_0 on 2022-06-17)

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.)

**Tags:** nomenclature

> **Comments:**
> **username_1**: 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.  
> **username_2**: Maybe not a name, but with coordinates the line you describe can be designated [selenographic longitude](https://en.wikipedia.org/wiki/Selenographic_coordinate_system) 90°E and 90°W, or meridians ±90°.  

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## 1 answers

### Answer by username_3 on 2022-06-17. Score: 4

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](https://en.wikipedia.org/wiki/Lunar_limb)"

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

And you can find reference to "[Pluto's limb](https://www.researchgate.net/figure/Paths-of-the-stellar-image-around-Plutos-limb-during-the-occultation-These-paths-in_fig3_231009985)". A star is occulted when it reaches the limb of the occulting body.

> **Comments:**
> **username_4**: 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.  
> **username_0**: 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.  

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