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I would like to give my imaginary earth-analogue planet multiple (natural) moons.

Is the orbital period of such moons at all related to their size (or apparent size)?

In other words, am I free to have a large close-by moon, or a small far-away moon, or anything in between, without it having an effect on orbital period?

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  • $\begingroup$ See en.wikipedia.org/wiki/Orbital_speed . In most common cases, orbital period is a function of the moon's velocity, not size. $\endgroup$
    – user535733
    Oct 14, 2018 at 19:51
  • $\begingroup$ There are actually a lot of questions like this on worldbuilding SE, just use the search bar (at the top) and look for "moon size" etc. $\endgroup$
    – MParm
    Oct 14, 2018 at 20:44

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Yes and no.

There are two factors that determine an object's visible size from the planet: its physical size and its orbital distance.

Physical size is essentially independent; it can be virtually anything you like. The main thing that matters in orbital dynamics is mass, and as long as it stays low compared to the planet (I've heard 1/50th or 1/100th of the planet or less) it will have negligible effect on the orbit. Depending on the body's composition and structure, it can have varying densities, so the same mass can be more or less compact as you desire. Compare for instance Mars's two moons, which are (relatively) lightweight carbonaceous asteroids, with Earth's own moon, which is composed of heavier rock and is about twice as dense.

The other factor is orbital distance, and this plays directly into the orbital period. The further out the orbiting body is, the longer it will take to complete its orbit. You can see the equations governing this behavior here, but the gist is that the satellite's orbital period is proportionate to the one-and-a-halfth power of the orbital distance. Meanwhile the apparent diameter falls off linearly with distance.

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    $\begingroup$ Nice answer. I'll just nitpick about moon sizes. I believe Charon's mass, compared to Pluto's, is in the range of 1/8th. $\endgroup$
    – The Square-Cube Law
    Oct 15, 2018 at 1:44
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    $\begingroup$ @Renan Yes, but Charon's size is such that it's hard to model in terms of "Charon revolves around Pluto" and all the math is way more complicated. (To start with, Pluto is tidally locked to Charon, so the idea of its orbital period is... complicated, to say the least.) So long as the moon remains smaller, you don't have that problem. $\endgroup$
    – Cadence
    Oct 15, 2018 at 1:55

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