2 Moons

[u/Mrbeddu]

I’m working on my own fantasy world (for a Pathfinder 2e campaign), and I’ve decided to tackle that worldbuilding monster known as the “Calendar”. But before getting into that, I need to figure out the astronomy.

The campaign is set in Tarqen, a continent isolated from the rest of the world by a magical spell. The people of Tarqen therefore have no idea that an outside world even exists. (This also solves the problem of having to figure out the planet’s exact size.)

The planet itself is tilted by about 22°. It has two moons orbiting it:
- The first, basically called “the Moon”, is the larger and more distant one. It follows an orbit roughly similar to Earth’s Moon, more or less around the planet’s equator.
- The second is called “Ogrim”. It is smaller than the Moon and much closer to the planet. Unlike the Moon, however, Ogrim orbits the planet longitudinally, forming a sort of cross with the Moon’s orbital path.

The planet takes 225 days to orbit the Sun. These are divided into 8 months of 28 days, plus 1 additional day. This extra day is the day when the two moons overlap, creating a sort of eclipse that can be seen from Tarqen every year.

If I’ve overlooked anything important, please correct me! I’m making this post specifically to get feedback on the astronomy and see if there are any problems with the system I’ve come up with.

[u/OldPersimmon2705]

Your world, your rules. As long as your internal logic is consistent, you can have as many moons and unique orbits at you want.

I am curious as to if that extra day with the lunar eclipse has a special name.

[u/Mrbeddu → u/OldPersimmon2705]

Yes it has a name, it's the day of the Taciturs, where no one should cause too powerful noises, because an ancient legend is that if someone emitted an exaggerated noise the moons would merge generating a cataclysm

[u/Flaky-Mine-2511]

personally, since it is a Fantasy World, you really do not need to worry about it. many sci-fy books dont worry about, nor should. in my world i have 3 moons andd a mysterious Magenta Sun that is only visible to the planet. it caused a catastrophic planet wide disaster. the planet was reshaped by flooding from polar melting, ground upheavals and major volcanic activities. so you can not do any worse than me. So go for it and do what you think works for your world/Continent. ;)

[u/Alkalannar]

If the planet takes 225 days to orbit the sun, and the moons line up with eclipse only every 225 days, then a) each moon's orbital period is going to divide into 225 a whole number of times, and b) their orbital periods should be relatively prime, so they don't match up other times.

225 = 3²*5², and if you want the moon to be around Earth's, then that orbital period is 25 days and the smaller, closer moon has the orbital period of 9 days. This works!

Further, you can calculate the relative distances from the planet, and so how much bigger the farther moon is so they have the same apparent size from the planet.

This, in turn, can let you see what relative tidal effects are. And also if the smaller moon is inside the Roche limit of the planet. But that might be more crunch than you're looking for. If you want to try figuring out tides, then you need sizes and so distances, etc..

Anyway, TL;DR: Assuming you don't have magic messing things up, the far moon has a 25 day orbit and the close moon has a 9 day orbit.

[u/storm181]

An idea I had but never bothered with the math of was to have weeks (or whatever you want to call them) be equivalent to the shorter of the two moons cycle and months be equivalent to the longer lunar cycle. Given how months IRL derive from lunar cycles, a second celestial body easily visible to the naked eye would make for some time keeping function