# Scientific possibility of stationary moons for a writer of fiction

I tried to do some research to find an answer for this question before posting this but didn't find anything that I could sink my teeth into.

I was wondering:

1. if it would be possible for a planet to have a stationary orbit Moon, so only one side of the planet would see the moon ever
2. what that would do to the oceans and tides of the continent that is on the opposite side of the planet.

Conversely how would oceans and tides be different without the moon?

For my question here I'd like to ask about only the planetary science aspects. For the speculative and hypothetical aspects I can ask a separate question in Worldbuilding SE.

• You might google around for "tidally locked", meaning the moon revolves at the same rate as the planet rotates, and thus hangs over one location.
– user21
Dec 25, 2019 at 20:29
• This question might be on-topic in world building SE. Dec 25, 2019 at 20:43
• I've made some adjustments to your question so that it better fits this site. As pointed out in comments above, the simplest way for this to happen and stay that way for an extended period of time is to have a planet that deviates at least slightly from a sphere. I think this question is okay for this site now and will receive some answers, and you can ask a related but not identical question in Worldbuilding SE as well. Have a look around at existing questions and answers there first before you ask though.
– uhoh
Dec 26, 2019 at 13:23

The possibility you mention is feasible if the moon is in the equivalent of a geostationary orbit, where the orbital period of the moon is exactly the rotation period of the planet and it orbits in the equatorial plane of the planet.

Such a moon would always keep the same face to the planet if it were close enough to the planet (which in turn depends on the spin period of the planet) such that its spin is tidally locked to the same orbital period.

The presence of significant tides may be required to keep this configuration stable, without a tidal elongation of both objects it is likely that it would drift away from synchronous rotation. This in turn probably needs the moon to have a significant size/mass compared with the planet.

The tides on such a planet would essentially be "fixed". i.e. the tidal forces would be constant (with the exception of the possibly quite significant tidal forces due to the star it was orbiting). That would nean no water sloshing around due to tides. The "sea level" at point nearest to, and opposite, the moon would be higher than if there were no moon at all.

Something like this has been asked in another Astronomy SE question (Can a natural satellite exist in a geostationary orbit?) and, as pointed out there, Pluto and Charon are an example of the system you are looking for.

• I've modified my comment above and removed the "locking" bit, indeed it's not tidal locking, but in order to remain geosynchronous for an extended period of time there'd still have to be a deviation in the planet's azimuthal gravity. Earth's weak $J_{22}$ and higher multipole are apparently sufficient to produce some stable equilibrium points 1, 2 where GEO satellites will sometimes drift towards and remain near when they lose station-keeping propulsion.
– uhoh
Dec 26, 2019 at 13:33
• The azimuthal variation in gravity necessary to keep a moon near one of the equilibrium points could be a permanent feature of the planet's inhomogeneity, or it could be induced by tidal forces from the satellite. So tidal locking can not be excluded. Without such an equilibrium point, the chances that a satellite would just happen to have an exactly geostationary period for any length of time are very small.
– uhoh
Dec 26, 2019 at 13:38
• @uhoh Moons and tiny satellites are quite different things. A tidal quadrupole and azimuthal variation of the gravitational field is of course guaranteed. Dec 26, 2019 at 14:45
• I think that saying "Moons and tiny satellites are quite different things" is unnecessary and distracting here. I've added those links simply to show that small objects won't naturally stay synchronous without a preexisting azimuthal variation. A very small moon around a rocky planet would be no different than a cubesat in that it couldn't induce an azimuthal variation, but in some cases it could settle into one, whereas a large moon could induce a variation via tidal forces.
– uhoh
Dec 26, 2019 at 14:53
• @uhoh It is necessary for the precisely the reason you say. A "moon" can be a significant fraction of the mass of the primary. They each induce quadrupole moments in each other. If they are not big enough to do so, then the question - which is asking about what happens to the tides - is totally trivial. I Dec 26, 2019 at 15:16

I think it's good to start with the last question: How would the tides be affected without the Moon? The answer is simple: There would still be tides twice a day caused by the Sun, but they would be smaller. There are lots of sources online that talk about how tides from the Sun and Moon combine; without the Moon, we would just have the tides from the Sun.

In principle, there is no reason the Earth could not have a rotation period of 28 plus days, so that it kept one face towards the Moon at all times. In fact, in the unlikely event that the Earth/Moon system avoids being engulfed when the Sun becomes a red giant, the Earth eventually will have one face towards the Moon at all times, because the tides are gradually slowing the Earth's rotation. (But that would only happen after tens of billions of years.)

But imagine that was the case now, and it took just over 28 days for the Earth to rotate once. Then there would be a permanent tidal bulge under the Moon, and another one on the side away from the Moon. Over time, the rock of the planet would deform so that was in hydrostatic equilibrium just like the water, and so the whole planet would just be very slightly egg-shaped, with the longest dimension along the line from the Earth to the Moon. (It would still look perfectly spherical - it's not like you can see the tides from space.)

But, again, there would still be tides from the Sun, and they would still be twice a day, but each day would be 660 hours.

I have been using the actual Earth-Moon system as an example, and it's an especially good example, because our moon is pretty big compared to our planet. The tidal bulge would tend to keep Earth's rotation in sync with the Moon's orbit, even as perturbations from the gravity of the Sun and other planets would tend to cause the Moon's orbit to change slightly. This is what people mean when they refer to tidal locking.

If you want to imagine a natural moon that is big in the sky like ours, but orbits once in a 24-hour day, then you may have to make some compromises. If you wanted to get serious about making such a scenario realistic, do some research online to find scientists who study how the Moon was formed. Many of them have run computer simulations. If you contacted one of them directly, they might have a more detailed sense of what some plausible configurations are.