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I'm a new D&D 5E Dungeon Master and I've found out recently about tidal locking.

For the first RPG setting I'm preparing with a friend, the planet the players will be on has 3 moons. Me and another DM will play two different campaigns on the same world, we decided to play them on different sides of the planet (imagine cutting the "sphere" vertically passing through the north and south poles and perpendicular to the equator).

Now on my campaign the players and NPCs have no idea a third moon exists, since it will only show itself on my friend's side of the planet.

We figured the moon would have to be tidally locked, for flavour it would be always visible day and night from the place they will start on, always in the center of the sky (basically showing the same side of the moon on the same side of the world).

Would that be possible or make any sense in reality from physics' perspective?

I've seen this similar question: Multiple moons but only one of them is tidally locked to its planet? (I'm bad at math/physics so I can't really think too much on the equations but I was wondering if the case I described would be feasible)

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    $\begingroup$ Maybe I didn't used the proper terms, but looking at the example of Pluto and Charon this would be mutual tide locking right? Pluto and Charon seem to have the same properties I described $\endgroup$
    – Zorgatone
    Oct 18, 2022 at 16:46
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    $\begingroup$ Works fine. Pluto has exactly this -- Charon is mutually tide locked (orbits over a fixed location on Pluto's surface) but the other moons "wander in the sky". The larger ones, Nyx and Styx, are probably tide locked and show only one face to Pluto, none the less. $\endgroup$
    – Zeiss Ikon
    Oct 18, 2022 at 17:16
  • $\begingroup$ As answered, this situation is totally possible with real physics. Now: should you care about the realism of a tidally locked moon in a fantasy multiverse where magician and gods can deeply alter reality ? Will it positively change the audience experience (your players and yourself) ? If you are a new DM, consider how much time you invest for what benefit. Worldbuilding is addictive ;) $\endgroup$
    – Uriel
    Oct 18, 2022 at 19:31
  • $\begingroup$ All the text related to D&D and DMs and players is fluff. It's irrelevant to the question $\endgroup$
    – VLAZ
    Oct 19, 2022 at 4:48

2 Answers 2

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The Earth, when seen from the Moon, appears fixed in the sky, because the Moon is tidally locked to Earth.

This means that, if you want the Moon to appear always in the same point in the sky, the planet has to be tidally locked to its moon, which implies that the moon is comparable in mass to the planet itself (sort of a Pluto-Charon pair).

Additionally, the moon will be in the center of the sky only on a given point of the planet surface, not everywhere.

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  • $\begingroup$ This is exactly the answer I would have given, including the addendum. For the majority of the side of the planet from which it is visible, the moon would not appear in the centre of the sky. $\endgroup$
    – jdunlop
    Oct 18, 2022 at 17:14
  • $\begingroup$ Planet and moon do not have to be mutually tidally locked for this setup. The planet could coincidentally be at the right rotation speed to have one face presented to its moon. So the the mass of both can in practice be very different. $\endgroup$
    – Uriel
    Oct 18, 2022 at 19:25
  • $\begingroup$ I would add that the planet would rotate at the orbital speed of the moon it's tidelocked to. The Earth is actually slowing down over time, and after 50 Billion years, would be tidelocked to the Moon. That would be long after the Sun has turned into a white dwarf. $\endgroup$ Oct 18, 2022 at 19:30
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Planet and the largest moon can be tidally locked to each other. They will appear in a fixed place in the sky when looked from one to the other.

Small moons can tidally lock to the planet, but the planet won't be tidally locked to them (viewing from the planet, they will move across the sky).

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