I am currently worldbuilding for a D&D campaign. And while many campaigns have worlds with one or more moons, I wanted to change it up by making this world the moon of a much larger world that is in orbit around a Sun. I want the days to stay approximately the same length, but I imagine months and years will be drastically different. And since this small world is in orbit around a much larger one, there will be times when my world will be encased in darkness inside the shadow of the larger world.

I assume there are challenges, yay magic for some of them, but I would like it to maintain some realistic qualities. Narrowed to a single (initial) and very interesting question:

Is there a type of orbit that would minimize time inside the larger worlds shadow?

Other deleted pieces of this fine question, moved down here for safekeeping:

Namely, what kind of temperature and pressure extremes can this world expect at various locations. What kind of orbit could it have to maintain some life sustaining capabilities? How would this change seasons or weather? How would the larger world appear to someone on the surface of the small world?

  • 4
    $\begingroup$ Hello Brian, welcome to Worldbuilding. We try to avoid closing the questions of new users, but your question violates a fair number of rules (see help center and help center). Specifically (a) You're allowed to ask one and only one question. (b) Questions must be specific with a reasonable expectation of a single best answer. (c) This is a duplicate of a lot of Qs already on this site. Can you focus your question on a single issue not asked before (before it gets closed)? $\endgroup$
    – JBH
    Commented May 19, 2022 at 6:56
  • $\begingroup$ I chopped out a bunch and put them at the bottom. They are ok, they are just sleeping until they can have questions of their own later on. I kept my favorite of the lot and put it in bold. Welcome to WB stack Brian! $\endgroup$
    – Willk
    Commented May 19, 2022 at 23:24
  • $\begingroup$ Consider the earth. When the moon is in the earth's shadow is precisely a lunar eclipse, which only happens once every few months. Mike's answer illustrates some particular ways you can try to minimize it, but it is in general a relatively infrequent occurrence. And since moons tend to be totally locked to their planets, this is only relevant for inhabitants of the "near side." If you set your campaign on the far side of the moon then it won't feel much like a moon but you will avoid any effects of the shadow. $\endgroup$
    – DanishChef
    Commented May 20, 2022 at 2:44

2 Answers 2


There's no need to be complicated. And you don't need to go very far to see an example as to why:

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The Moon's orbit is tilted at 5 degrees relative to the Earth's. Taking the average distance of the Moon from the center of the Earth (which I'll round off to 390,000 kilometers for simplicity), simple trigonometry means the center of the Moon can be 34,000 kilometers "above" or "below" the center of the Earth. Since the Earth only had a radius of about 6300 km, that means the moon isn't going to be anywhere near Earth's shadow a great deal of the time. Which is intuitively obvious if you know there are, on average only two lunar eclipses a year, some years have three, and in some there are none at all. Which means the vast majority of the time, the Earth's shadow isn't interacting with the moon.

Now, for your situation, scale it up. For a larger planet simply increase the radius of the moon's orbit, and increase the inclination of the moon's orbit in respect to the planet's, and you're done. To cite another real-life example, look at Saturn and its rings, where the outer rings can be in sunlight even when behind the planet, thus any moon orbiting further out will be in sunlight as well.

  1. Make it a Trojan or put it into a horseshoe orbit. Theia is thought to have been a Mars-sized planet that was in the same orbit as the Earth, but 60 degrees off. (Eventually there was a mishap...) Epimetheus and Janus swap places in their orbit around a larger planet, but planets could do the same around a star. This solution isn't quite a moon, but prevents all shadows.

  2. Give it a distant orbit. The Moon moves away one or two more centimeters every year; in 50 billion years it will have gone quite some distance, if no other accidents happen to it first (highly unlikely). It can actually be triple the distance from Earth before it goes out of the Hill sphere. The further the orbit, the harder it is to meet shadow.

  3. Polar orbit. A Moon that crosses the equatorial plane as quickly as possible has the least chance of getting eclipsed. Even our own Moon spends much of its time too far north or south to be at risk.

  4. Resonance. This is the trickiest one to work out, but orbits often have a numerical resonance with one another. If the moon's orbit crosses the ecliptic a certain exact integer number of times per year, so that there is always the same pattern of crossings, then they can be misaligned so the moon is closest to 180 degrees opposite the sun on the ecliptic, it is never actually on the ecliptic, but north or south of it. So no eclipse.


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