Simple question that might seem odd. Not going to get too specific but I am curious, more or less, as to what it would look like for a planet that had a moon (A habital one) which was in essence tidally-locked to the star of the system. In addition, just how probable/possible is that, really?

It orbits its planet as any satellite does but isn't tidally locked to it. Instead its spin in relation to its own orbit around the planet (and its own orbit of the sun) causes it to always have the same side facing the sun. I assume this would cause for habitual, essentially daily eclipses for the side in eternal day (Almost equating to its own equivalent of a "night") but I can't find what else that might entail.

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    – L.Dutch
    Jan 20, 2019 at 20:08

2 Answers 2


Tidal lock happens when the tidal forces, exerting a momentum on a body, synchronize its rotation around its axis with the revolution around the main body.

Since for a moon-planet-star system the forces exerted by the planet will always be greater than those exerted by the star, there is no way for the moon to be tidally locked with the star and not with the planet.

The only case in which the tidal forces generated by the star are stronger than those generated by the planet implies that the moon is orbiting the star and not the planet.

  • $\begingroup$ So does a moon have to be tidally locked to its planet? Is it not actually feasible a moon's own rotation could not sync like that with a star? $\endgroup$
    – Magilla
    Jan 20, 2019 at 23:22
  • $\begingroup$ @Magilla It would be a short term (in astronomical terms) and temporary coincidence if it was. $\endgroup$
    – Tim B
    Jan 20, 2019 at 23:57

For this to happen, the moon would have to be tide locked to the planet while the planet itself is tide locked to the star.

Notice that surface exposed to the star will vary slightly throughout the year due to axial tilts and also a movement called libration.

  • $\begingroup$ This still wouldn't work, the moon has to orbit the planet in order not to crash into it $\endgroup$
    – Tim B
    Jan 20, 2019 at 23:57
  • $\begingroup$ @TimB I am suggesting a Moon that is tifslly locked to a planet, thus it is in orbit around the planet. $\endgroup$ Jan 21, 2019 at 0:34
  • $\begingroup$ in which case it doesn't always face one side towards the sun $\endgroup$
    – Tim B
    Jan 21, 2019 at 9:33
  • $\begingroup$ @TimB it does if the planet is tidally locked to the star. Do a simulation. $\endgroup$ Jan 21, 2019 at 9:55
  • $\begingroup$ The planet P faces p towards star S. Moon M faces m towards P. As P orbits S p is always pointing towards S, however m is pointing towards P not towards s. No need to run a simulation, just look at our moon. It's tidally locked on our planet and yet experiences days and nights (hence new moon, full moon, etc). Our planet being tidally locked would not change the moons orbit. It has to orbit or it crashes. $\endgroup$
    – Tim B
    Jan 21, 2019 at 17:05

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