Need help from an Astronomy Nerd lol

Homebrew and House Rules

Im working on a new world.
Earth size and style.
13 months of 28 days, each 24 hours
Lunar rotation is 3 days per phase, standard phases similar to our Luna
2 moons

The first moon is the size of Luna, and at the 34* latitude (roughly where the country is located) the moon rises at @8pm, while the sun is still up, and sets about 8am, while the sun is already up on both equinoxes.Like the sun, this will see an extreme of 16 hours at solstice.

The second revolves around the world twice as fast, though it rotates on its axis at the same speed. Its also roughly 1/3rd the size of Luna.

So, can one of the astronomy geeks work up a "lunar calendar" for me? Tides, eclipses and all that would be appreciated too. We can discuss a fee, if necessary, though Im hoping to hook a grad student that needs a project, to be honest.

Sovereign Court

I am an astronomy grad student, but not in need of a project, I have my own dissertation for that :-)

Still, I might be able to do some basic help. A few questions first, what you described is a bit unclear.

1. You said the moon has "standard phases" as our moon. I assume you mean 8 phases (new, crescent, quarter, gibbous, full, gibbous, quarter, crescent), although that's fairly arbitrary. That gives you a 24 day rotation, a bit quicker than Earth's moon.

2a. I'm not quite sure what you're trying to get across about the moon's rising and setting times. The moon's orbit won't evenly line up with the sun's, so it won't always be at the same phase at equinox from one year to the next, and shouldn't have a consistent rising time. The moon will always rise and set opposite the sun (rise ~6 pm set ~6:am) at new moon, and the opposite at new moon, regardless of time of year.

2b. Do you want to know when it will rise and fall at other times? If so, I think you'll need more information, such as what the inclination of the moon's orbit relative to sun-Earth plane, etc, but I'd have to think about this some more.

3. The second moon revolves twice as fast. I assume what you mean is that it has a period of 12 days, and spins on its axis once every 12 days (so, like Earth's moon, it always shows the same face to the Earth). Doing the math, that gives an orbit that is roughly 9% the distance away from the Earth that our moon is (roughly 30,000 km)

4. Is the 1/3 size the volume? Radius? Mass? Apparent size? Assuming you meant 1/3 the radius, it would still appear much bigger than the second moon since it's much closer.

Note that tidal forces scale even faster with radius than gravitational force, so that second moon will cause pretty intense forces, even with a mass 1/3 or 1/27 (if it's 1/3 radius) of our moon's. Back of the envelope, you'd have a force somewhere between 300 and 30 times (depending on whether you meant radius or mass earlier) greater than the moons.

The moon is within the Earth's Roche limit, so it's possible to have a moon at that distance. But I would recommend drastically lowering the mass.

Sovereign Court

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Also, just as a general note, you should think about what you're actually trying to accomplish from a world building point of view. Do you actually care exactly when high and low tides are? Do you want a certain schedule, and then plan to backtrack to get that? Is it actually going to affect your story/fun in any way?

Ignoring the astronomy, think about the fact that it's a game. Will your players actually care if you get the physics right, or will they just say, "ok there are two moons, cool, let's go kill the dragon and steal it's stuff?"

The tide moves at the speed of the plot, but can just as easily he rolled for randomly.

He might just be genuinely curious?


Can I just answer with one word: Magic.

I'll explain in a few more. If magic is rife in your world, you don't have to explain things in terms of lunar cycles, progressions etc. Look at George R. R. Martin's Westeros, with random lengthed seasons that are never explained. It doesn't detract from the setting - if anything, the unexplained magical mystery adds to it. Resist the need to explain everything with modern science, because in a world where magic is real, science may be wrong.

RPG Superstar Season 9 Top 16

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^ I was actually kind of confused by the long seasons in Westeros. I thought they were just being metaphorical at first, referring to times of peace and plenty as summer, and times of war and loss as winter.

I'd just keep cycles earth-like even if you have two moons or whatever, most people aren't going to think too hard on it. And if your players do, then have them do the research for how it should work, problem solved.

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I don't think what you're describing works with Astronomy, for it to have a rising and setting time that is perfectly in synch with the day/night cycle no matter the season the moon can't orbit the planet, it has to just float above it at a fixed point on the night side. And since I'm betting you want "moon always at night" and "sun always at day" with no overlap that's what you're probably going to stick with.

Ergo it's just magic that makes it work and just math that determines when.

Therefore: Tides are mostly on a 12-hour cycle, they get high at noon and midnight, they get low at dawn and dusk. The second moon affects the tides 1/3rd as much but on a 6-hour cycle, either fighting the effects of the extremes or helping them get more extreme.

The second moon DOES orbit. Every night it rises just after the moon and rockets past it (passing at midnight) and then does the exact same thing with the sun. Note this is the highest of high tide.

Since "a wizard did it," eclipses and phases are determined by whatever you think is appropriate. I recommend using two real-world calendars for lunar eclipses and having the 13th month of the year be magically special that it never, ever has an eclipse. It can be a plot point when one happens against all odds and the sages and astronomers of the world are freaking right out.

And if the party wants to make a chart, it becomes their project.

Ive tried to post 3 times and had the damned machine eat all 3 posts.

Thanks for the ideas and info. There were good questions I want to answer to further develop this project. But until my computer stops buggering all to %^^&* Im calling it a night. Thanks

Ciaran Barnes wrote:
The tide moves at the speed of the plot, but can just as easily he rolled for randomly.

Once you're in the game, you can randomly roll to figure out when players arrive in relation to the tides, but when designing the game world and setting, these things can be relevant (or may not, depends what you intend to do with the info)

To the OPer: if there are some intentions with the lunar calendar that are not explained in the OP, it might be easier to state what you want (or what your game needs) and work from there, rather than the other way around.

As Ciaran said above, tell us to what speed your plot needs to run, and we'll figure out that of the moons, tides and lunar calendar.

I just remembered a fantasy world can have the Sun orbit the Planet, because A Wizard Did It. Then you can have Aumonos (name for the sun I just made up) ever-chasing and ever-opposite Lunaras (name for the bigger slower moon I just made up which will have to glow with its own inner light, but that just makes Moonstone cooler and all glowy) and Rannutos (a name for the other moon I just made up) rocketing past both of them and being the only source of actual eclipses.

There is also a butt joke somewhere in here but I'm having trouble revealing it fully.

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