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One of the worlds I’m building has three moons in 1:2:4 resonance, with the full moons syncing up once a cycle, meaning all three are full at different times but at the same time when the moon with the longest orbit completes a single orbit.

What I’ve been thinking about lately though is where they would appear in the sky based on their orbit and phase, especially after finding the following image:

enter image description here

Now, as far as I’m aware, the moon appears in roughly the same place to an observer because both the moon and earth are in constant motion. However, with multiple lunar bodies, I get the feeling this movement will have a more pronounced effect on observation.

Essentially what I’m trying to figure out is where these moons would appear in relation to one another in the night sky and how much, if at all, they would appear to shift to an observer.

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    $\begingroup$ I'm going to try and simulate this later, so what is the length of the orbits? Both of the Planet and it's moons $\endgroup$
    – bendl
    Commented Jan 7, 2018 at 15:19
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    $\begingroup$ @bendl: Planet's orbit is 568 Earth days, and the lunar orbits are 33, 66, and 132 days. There may be some decimals I'm neglecting, but these are the important numbers at least. $\endgroup$
    – Pleiades
    Commented Jan 8, 2018 at 4:12
  • $\begingroup$ Are the moons on exactly the same orbital plane as each other? What are their sizes and distance from the planet, or, if you have this information, what is their angular size from the planet? $\endgroup$
    – bendl
    Commented Jan 8, 2018 at 22:19

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The 3 moons will roughly be on the same orbital plane, and once they are in the opposition to give a full moon, they will appear aligned. They will occupy the positions marked in red in the figure.

enter image description here

If their angular size in the sky is larger than the angular distance between them, the observer will see a fancy eclipse. If not, the sky will show a peculiar show of 3 full moons set on a straight vertical line.

The inclination of their orbital plane with respect to the orbital plane of the planet will determine the elevation in the sky.

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  • $\begingroup$ "...in the opposition to give a full moon"?? Did you mean position? $\endgroup$
    – EveryBitHelps
    Commented May 1, 2018 at 13:35
  • $\begingroup$ @EveryBitHelps, In positional astronomy, two astronomical objects are said to be in opposition when they are on opposite sides of the celestial sphere, as observed from a given body (usually the Earth) $\endgroup$
    – L.Dutch ♦
    Commented May 1, 2018 at 14:34
  • $\begingroup$ Ah. Ok. Thanks. Wouldn't the offset position of moon2 mean that it can never be aligned with both the other moons at the same time? One or the other, yes. Both at same time, no. So you would have moon2 full and the other 2 at some other position OR you could have the full moon1 and moon3 aligned but not have moon2 in this alignment? Sorry not explaining properly, hope you understand what I'm trying to express. $\endgroup$
    – EveryBitHelps
    Commented May 1, 2018 at 14:49

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