Say I have a moon orbiting a planet. Due to tidal locking, if we model the moon as a sphere, there exists an axis which passes through the center of the moon which is always pointed toward the center of mass of the orbited planet. This axis uniquely defines an orthogonal plane passing through the center of the moon, which intersects with the surface of the moon to uniquely define a particular meridian of the moon. How could I concisely refer to this meridian? As far as I have read, meridians of celestial bodies are typically defined relative to arbitrary "prime meridians" defined by well-known sites on the body's surface. I don't know if there's a general term for what I am looking for.

This is important for a minor plot point in a story I'm writing, wherein a mining company on a moon finds that their automated mining bots are having a harder time removing material at sites along this meridian.

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    $\begingroup$ A pole is a point. A point, despite its name, doesn't point in any direction. $\endgroup$ – L.Dutch - Reinstate Monica Jun 14 '18 at 5:30
  • $\begingroup$ @L.Dutch I have changed my question according to your comment; thank you. $\endgroup$ – RedRobin Jun 14 '18 at 17:52

Prime meridian is the general term, and although tidally-locked moons' prime meridians are often defined in terms of the sub-planetary point (directly "beneath" the planet as it hangs in the moon's sky), there doesn't seem to be a special term for this arrangement.

According to this Wolfram Research article on planetary coordinates:

For each planet, a system of longitude lines is set up by first defining a prime meridian to act as the origin of longitude.... The prime meridian is an arbitrarily chosen meridian, typically passing through a prominent feature of the planet, such as a crater.

In the case of the Galilean moons, Io, Europa, Ganymede, and Callisto.... The prime meridian passes through the mean center of the moon's disk as it faces Jupiter.

It might be worth noting that the sub-planetary point actually librates during the period of its orbit; just as the face of the moon visible from the planet changes slightly, so too does the apparent position of the planet from the moon. The meridian uses an average value.

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The solar terminator is the line between the daylight side and night side of a planet. However, this isn't perfect as your moon will experience both reflected light from the planet and direct light from the star. So...


This is a curious word that sometimes means jungles, beaches, and mai tais. It's also used to identify two latituded on earth: the tropics of cancer and capricorn. That tradition could be carried to your lunar meridian.


Honoring German-Flemish cartographer Gerardus Mercator, from which we also have Mercator Projections (the way we look at spherical worlds on 2D paper).


I'm running out of ideas, but I like the sound of this one. It comes from the bisecting plane.


Equator comes from "equal," refering to the top and bottom being conceptually identical. Diquator comes from two halves. You could also use biquator, but I like diquator better.


And I've officially run out of ideas, but you can use Bob for anything, right? I mean, the British use(d) it to refer to a pound note (and their police, somewhat formally). He was a primary character in the movie French Kiss. I mean, what's not to like about the name Bob?

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    $\begingroup$ Bob is also the designated driver when a group of friends goes out for drinking... $\endgroup$ – L.Dutch - Reinstate Monica Jun 14 '18 at 7:10

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