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CluFa
2017-09-02, 10:34 PM
Hi,

I'm running a homebrew Pathfinder game, and on my world (Territh), well, the solar system of Territh, I've established that it's a super Earth with three visible moons, Mother, Daughter, and Potato. Mother and Daughter are like our Moon, only one is smaller than the other. Mother's orbit is further out than Daughter's, and Potato is a captured asteroid in stable orbit. Lately I've been thinking about their relationship to the planet, and since our own recent eclipse, I've been wondering how often conjunctions of the three orbiting bodies, and eclipses would occur. Now being the GM, I can use them as plot convenience fairies (you have do the thing before Mother overshadows Daughter, and together they blot out the Sun, or you'll have all of the bad happen. Potato is an innocent bystander, having only been here for 500 years), but I'd like to have a little rhyme, and reason behind the fantasy. I think it adds a level of verisimilitude to the game. I haven't fleshed out the rest of the solar system yet, I will probably wait until I can talk my players into a Starfinder game before I get that deep in the weeds.

CluFa

2017-09-03, 11:42 AM
Probably the simplest way is to arbitrarily set how long the cycle of each moon is (Earth's moon orbits Earth about every 29.5 days). Then set another arbitrary point in the past when they were aligned. Then do math. If you want to be more techical, keep in mind that this will vary in different parts of the world.

For eclipses, check out Eclipses For Beginners - http://www.mreclipse.com/Special/LEprimer.html

Roughly...
1. Eclipses only happen during full moons
2. They occur roughly 2-4 times a year in Earth
- maybe a cycle of 2-3-4-3-2?
3. Roughly 1/3 are not noticeable, 1/3 are partial and 1/3 are total

So, let's say Mother orbits every 40 days, Daughter every 25 and potato every 15.

Mother will align with Daughter every 200 days
Mother will align with Potato every 120 days
Daughter will align with Potato every 75 days
All 3 will align every 600 days.

If you have similar months and a similar year to Earth, that puts the triple alignment roughly every 20 months.

If the orbit of the moon's are parallel with eachother, I believe they will experience eclipses at the same time. If not, they may have independent eclipse cycles.

Hopefully that gets you going in the right direction. I'm on mobile so my apologies for any typos.

rferries
2017-09-03, 11:55 AM
This might get more replies if you get a mod to move it to World-Building.

A quick google check says we get 2 total eclipses every 3 years... but that doesn't help if you have 2 other celestial bodies to worry about, each with their own orbits. It'll be like herding cats to get the three moons to align, so I'd rule it happens once every 100/1000 years or something similarly portentous. Wish I could be more help but without an astronomy major to chime in on here that's probably the best you can do. :D

Knitifine
2017-09-03, 11:58 AM
Why is the third moon called potato?

steelsmiter
2017-09-03, 12:05 PM
Why is the third moon called potato?

Because it's too small for its gravity to produce a spheroid body.

Knitifine
2017-09-03, 12:08 PM
Because it's too small for its gravity to produce a spheroid body.Not sure if you're related to the OP in any way, but I'm less confused about that possibility of the moon being shaped like a potato, and more confused about potatos being as universal in a world as concepts like 'Mother' and 'Daughter'. Is this fantasy Ireland, the planet? :smallbiggrin:

CluFa
2017-09-03, 03:29 PM
Why is the third moon called potato?

The third moon is a captured asteroid, and non spherical, shaped kind of like a potato. Potatoes are native to this region, and that's the local name for the third Moon.

In most of the rest of the world, I imagine it's called "Rock," or "Lump," or "Holy sh*t, how is that thing hanging there!?"

CluFa :smallbiggrin:

CluFa
2017-09-03, 03:30 PM
Not sure if you're related to the OP in any way, but I'm less confused about that possibility of the moon being shaped like a potato, and more confused about potatos being as universal in a world as concepts like 'Mother' and 'Daughter'. Is this fantasy Ireland, the planet? :smallbiggrin:

It's a big planet, and for now I'm working in one region, and this region has potatoes. :smallbiggrin:

CluFa

CluFa
2017-09-03, 03:34 PM
Because it's too small for its gravity to produce a spheroid body.

Pretty much, big enough to be seen from the planet, but not having enough of the right elements to produce enough gravity to form a sphere.

CluFa :smallsmile:

CluFa
2017-09-03, 03:43 PM
Probably the simplest way is to arbitrarily set how long the cycle of each moon is (Earth's moon orbits Earth about every 29.5 days). Then set another arbitrary point in the past when they were aligned. Then do math. If you want to be more technical, keep in mind that this will vary in different parts of the world.

For eclipses, check out Eclipses For Beginners - http://www.mreclipse.com/Special/LEprimer.html

Roughly...
1. Eclipses only happen during full moons
2. They occur roughly 2-4 times a year in Earth
- maybe a cycle of 2-3-4-3-2?
3. Roughly 1/3 are not noticeable, 1/3 are partial and 1/3 are total

So, let's say Mother orbits every 40 days, Daughter every 25 and potato every 15.

Mother will align with Daughter every 200 days
Mother will align with Potato every 120 days
Daughter will align with Potato every 75 days
All 3 will align every 600 days.

If you have similar months and a similar year to Earth, that puts the triple alignment roughly every 20 months.

If the orbit of the moon's are parallel with each other, I believe they will experience eclipses at the same time. If not, they may have independent eclipse cycles.

Hopefully that gets you going in the right direction. I'm on mobile so my apologies for any typos.

Thank you so much, this is very helpful. As far as the orbits go, yes, they would be parallel, Potato originally had more of an eccentric orbit, but is falling into line do to the influence of the gravity from the other two moons, and the planet. it still "tumbles" in its orbit, but more slowly than in the days it was first captured, it's more of a slow roll these days.

Again, thank you so much!

CluFa :smallsmile:

CluFa
2017-09-03, 04:00 PM
This might get more replies if you get a mod to move it to World-Building.

A quick google check says we get 2 total eclipses every 3 years... but that doesn't help if you have 2 other celestial bodies to worry about, each with their own orbits. It'll be like herding cats to get the three moons to align, so I'd rule it happens once every 100/1000 years or something similarly portentous. Wish I could be more help but without an astronomy major to chime in on here that's probably the best you can do. :D

I appreciate it, thank you. As was mentioned about the eclipse we just had, an eclipse falling on a particular region is a pretty rare event, and won't be repeated in that exact place for a long time. And with the three orbiting bodies there would be minor eclipses of Daughter, and Potato a bit more often than a full total eclipse.

CluFa :smallsmile: