I've decided that the planet I'm writing will have two moons in a horseshoe orbit, closely modeled off of Saturn's Janus and Epimetheus. For the purposes of this question, I'll be calling them Moon and Luna, and the planet Earth 2.

Necessary info:

  • Earth 2 and Moon are both a bit larger than Earth and our moon.
  • Luna is half the size of Moon and composed mostly of ice.
  • Moon and Luna are tidally locked, and orbit farther away from Earth 2 than Earth's moon orbits us.
  • From Earth 2, Moon appears roughly the same size in the night sky as Earth's moon from Earth.
  • Neither satellite is habitable.

My question:

  • Assuming this configuration is stable, would both moons have the same phases, or different ones? I'm curious how this might affect the development of a calendar system by an early humanlike species.
  • 1
    $\begingroup$ Welcome to worldbuilding, please ask one worldbuilding question per post. You can find more info in the help center $\endgroup$
    – L.Dutch
    Apr 12, 2021 at 16:22
  • $\begingroup$ Well, you will need to wait until a mod reopens. Good luck! $\endgroup$
    – user84509
    Apr 12, 2021 at 16:24
  • $\begingroup$ @Santiago, I appreciate your enthusiasm and willingness to help new members. Try to keep the comments compact by not hitting enter at each period, in this way you spare the OP from being flooded by notifications and also reduce the chances of the entire comment set to be moved to chat. $\endgroup$
    – L.Dutch
    Apr 12, 2021 at 16:27
  • $\begingroup$ Will do that, sorry It’s a little hard because a bug on my phone $\endgroup$
    – user84509
    Apr 12, 2021 at 16:29
  • $\begingroup$ We have a number of questions about horseshoe orbits on this Stack. We know of asteroids with horseshoe orbits, so they're possible. However, there's no objective way to answer your second question, which is too broad (see help center, the book test) and a reason to close the question. Can you reduce this to a single, specific, objective question? $\endgroup$
    – JBH
    May 13, 2021 at 4:28

2 Answers 2


Tidally locked moons in a horseshoe orbit sound like trouble. If the moons are moving to different positions around the planet, relative to one another, how do they keep rotating at the right speed to keep one face to the planet? I also have my doubts about the stability of the orbit, but that is assumed in the question.

Given these things, the phases of the moons should not be at all mysterious. When you look at our moon in the Earth's sky, see it as a ball, with the lit half facing toward the sun. Any moon near the same position in the sky is in the same direction relative to you and the sun, so it will have the same phase. Any moon in another position in the sky will have whatever phase the first moon would have if it were there.

  • $\begingroup$ As a side effect, the civilizations under these moons will make great progress in the science of illumination and optics, but their celestial mechanics will be set back hundreds or thousands of years, because the heavens will not make any sense at all. Great fodder for myths of the Great Queen Moon, and her persistent Stalker who keeps on approaching and then being chased off all the time. $\endgroup$
    – PcMan
    May 13, 2021 at 16:28

Yea, Mike is right about the tidally locked thing. Their facings would shift chaotically based on a ration of the masses of the moons and the period of orbit. This kind of tidal shifting is actually what causes Io to stay molten (It's much worse in Jupiter's crowded neighborhood). It might be enough to keep the ice moon from cooling.

They would definitely not have the same phases. Luna would shift back and forth between further away and closer to the planet. When it's closer than Moon, it would move faster, when it's further away, it would move more slowly. They would shift between being near-ish to each other to being on the opposite side to being near-ish again.

I recommend using http://orbitsimulator.com/ to simulate the three body system if you want to learn, for instance, what the period would be between when they're left/right and when they're right/left.


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