The Campaign Builder's Guild

Quote from: SamuraiChicken@Llum: Sounds interesting. I'll keep an eye out for it.

@Xeviat: Thanks for reminding me about other moons orbiting the planet. I'll be sure to add them into the folklore. I think the number of 'significant' moons (ones that formed with the planet) should be around 4 or 5, with the rest being captured satellites (and there will be plenty of those).

As for seasons and months, like the idea of having 1 year (in this system) equal to an orbit around the sun (just like on Earth), and 1 month is equal to an orbit around the planet (again, similar to our lunar calendar on Earth). I really want to go with the idea of two eclipses every month, as it would be a great way to keep track of time. I can imagine PCs in this setting keeping track of which day of the month it is so they can prepare for the eclipse.

I love your idea on amphibious life forms. I never thought about it when thinking about tides, and it makes perfect sense for this world.

As for plate tectonics, I completely forgot about the magneto sphere. That would be very important for life on the moon, as well as for compasses to be able to work. Currently, I prefer the moon to have its own magneto sphere, but the idea of the moon being within the planet's magneto sphere is equally an interesting idea (and good to keep in mind for an alternative).

@Ghostman: Thanks for reminding me about the cooling effect during the Total Darkness period. I don't know exactly how long an eclipse would last, but as soon as I do I can then figure out how cold it would actually get. I like the idea of making this period reflect a miniature winter, where the temperature decreases and stays down until the sunlight returns. In winter, this period would be especially harsh. Perhaps people go underground during this time (or underwater?) where it would be a little warmer.

@Freeeakin' Awesome Horse: Thanks for the offer! Yes, I think I will need some help on this project, so go check your inbox.

Thanks to everyone who replied to this thread - you guys have been awesome. As a reward, I'm giving anyone who posted on this thread (and whoever will post) the following badge:

There may actually be an issue with PMing me now that I remember... doesn't work for people for some reason. You could start another thread, email me at bluehigh38@hotmail.com , contact me at Mezerous on AIM, or come to the CBG chat with questions and we'll sort some stuff out. In  the meantime I'll bug some mods about the PM issue.

So that explains why the PM didn't work. Ok, I'll contact you via email.

In other news, the badges I posted didn't seem to work... I guess I'm going to have to fix that.
EDIT: badges should be working now.

Do look into Titan. It's probably your best bet for a model moon. Since you want your moon to be Earth-sized, you'll probably want to scale up the mass of the planet along with the mass of the moon.

I absolutely love the idea of monthly eclipses. Lunar eclipses could look eerie if you can see the shadow appear on the planet (it might make it look like an eye). Solar eclipses would probably be a universally disliked day, a day off in most cultures. I'm thinking of a collective day of mourning. You could also have nocturnal monsters go absolutely crazy and run amok during the eclipse (and similarly, the full moons could be quite bright depending on how large the planet looks, and diurnal creatures could remain active most of the night).

So I finally got around to researching Titan, and I found some interesting things. I like the idea of using Titan as the base model for this setting's moon, though some things will have to be changed.

Titan's diameter is 5,150 km. It is bigger than Mercury (4,879 km across) as well as Earth's Moon (3,474 km). In fact, it is the second largest moon, right behind Ganymede (5,300 km). Also keep in mind that Mars is 6,800 km across.

Titan orbits Saturn at a distance of about 1.2 million kilometers, which is about 3.2 times the distance the moon orbits Earth (our moon is 384,400 km away from Earth). This is rather important to know, since this indicates how far away a moon can be while still having an atmosphere (the atmosphere would be stolen by the gas giant if the moon is too close).

Titan makes a full orbit around Saturn in 15.94 Earth days. This was really exciting to learn. Titan is hurtling through space much faster than our own moon orbits Earth. In fact, all the gas giant moons that form alongside the planet (as opposed to captured moons) orbit their host planet at incredible speeds. While this is pretty fun, it does throw a wrench into my plans: at this speed, an eclipse wouldn't last very long. I was in favor of a slower eclipse because the longer an eclipse lasts, the greater the impact it has on the moon.

Titan has an Inclination of 0.34854 degrees (to Saturn's equator). Titan's inclination is good '" it orbits Saturn on pretty much the same plane as Saturn's equator (and its rings). This is perfect for the eclipses I imagined. However, this means that if the campaign setting's moon is orbiting a Saturn-like planet, the rings would be viewed edge-on (but since it's not perfect, the rings would look like they slowly wobble up and down throughout the month). I don't think this looks nearly as good as Earth's perspective of Saturn's rings, so I'm debating whether or not the setting's planet should have rings.

Titan has an axial tilt of zero degrees. Having no tilt means it doesn't have seasons. So unlike Titan, the campaign setting's moon will have enough tilt to generate seasons (since everyone loves seasons).[note=Here's an Idea] The setting's moon would have formed along with the planet, though by doing so there is no reason why the moon would have a tilt, and would likely be tidally locked to the planet. To justify these significant changes, there is the possibility that the setting's moon was hit by a large meteor in the past (before life was formed). The impact was big enough to give the moon an axial tilt and to knock it out of tidal lock. The moon has been like that ever since, and the large crater could be seen even to this day in the form of a massive circular sea.[/note]

Titan has a synchronous rotation around Saturn. This is bad: it has tidal lock. However, it makes sense due to Saturn being very massive, and thus an object has to be very far away to not have a synchronous rotation. After researching other moons, it appears that gas giants have enough mass to make nearly anything orbiting them have tidal lock, with the exception of captured satellites. I'd hate to invoke the "Because the GM says so" rule, but I might end up doing it just so that the moon doesn't have a synchronous rotation (I don't like the idea of each day lasting half a month).

I like your idea with the crater. If you look at massive impact craters you will find in the center that due to how shock is distributed on impact a peak forms. This is an interesting point to latch onto as the center of your sea might have an awesome island mountain.

Quote from: SamuraiChickenTitan has an Inclination of 0.34854 degrees (to Saturn's equator). Titan's inclination is good '" it orbits Saturn on pretty much the same plane as Saturn's equator (and its rings). This is perfect for the eclipses I imagined. However, this means that if the campaign setting's moon is orbiting a Saturn-like planet, the rings would be viewed edge-on (but since it's not perfect, the rings would look like they slowly wobble up and down throughout the month). I don't think this looks nearly as good as Earth's perspective of Saturn's rings, so I'm debating whether or not the setting's planet should have rings.

Having an orbit on the same plane as the mother planet's equator would not guarantee frequent eclipses, unless the mother planet's equator is always at a 0 degrees to the sun. If the planet's axis is tilted and your moon orbits always directly over the equator, then your moon's plane of orbit will be tilted relative to the sun.

Further, is there any reason why the rings would have to be on a plane with the planet's equator? Couldn't you just say that the rings are tilted relative to the moon's orbit? This would cause the rings to have observable phases, where they appear to widen and diminish.

What would the effects of the low gravity be on civilization? Would they build taller? Use lots of gliders to get around? Flight would be easier, and would make a fun campaign piece play with. You could use all of those impractical designs for flying machines and have fun with them.

What tech level are you going for? High tech could be fun; I wonder if you could use a gas giant as a power source, like a wind turbine or something. Likewise, low tech could allow some fun stuff. It would allow you to play with the whole Avatar thing, but better written (Fern Gully much?). Maybe a mixture? Colonial space opera?

I have to say that you seem more interested in the astronomy aspect of this setting, so far, rather than the people inhabiting it. I find it's more compelling get a basic gist of what the setting looks like, then really focus on the civilizations that the players are going to interact with. I'd really like to see some culture stuff bandied around. Granted, that's the stuff I can comment the best on, since my science background is limited to what I dimly recall of high school physics and wikipedia.

Quote from: GhostmanFurther, is there any reason why the rings would have to be on a plane with the planet's equator? Couldn't you just say that the rings are tilted relative to the moon's orbit? This would cause the rings to have observable phases, where they appear to widen and diminish.

If the moon formed along with the planet it will generally be aligned with the rings (exception would be rings created by a massive collision or by a captured body falling beneath the roche limit... both of which however are not stable for long term and would be absorbed by the planet within a few hundred to several hundred thousand years). For this reason it might be a good idea to give the planet a high inclination in relation to the sun (30-85 degrees) and make the moon a captured moon. This would handle the tidal lock problem, the eclipse issue, and the ring issue all in one tidy solution.

So, it would look something like this?

Both the gas giant and it's moon are axially tilted (although at differing angles). The rings of the planet are in plane with it's equator, hence also tilted from the point of view of the moon. The orbits of the planet and it's moon are both contained in the same plane, ensuring that an eclipse occurs on the moon each and every time it makes a full trip around the planet.

Quote from: GhostmanSo, it would look something like this?

Both the gas giant and it's moon are axially tilted (although at differing angles). The rings of the planet are in plane with it's equator, hence also tilted from the point of view of the moon. The orbits of the planet and it's moon are both contained in the same plane, ensuring that an eclipse occurs on the moon each and every time it makes a full trip around the planet.

Yes exactly that.

I am currently working on this idea already for my own thing, but it uses the exact same set up as the one shown in the picture that ghostman made. however in mine the seasons come from the orbit around the gas giant not an axial tilt. in my set up you have:

a lunar eclipse (moon between sun and gas giant) at the summer equinox causing the day to be normal and the night sky is fully filled by the bright side of the gas giant with 2/3rd daylight illumination, lasting X days. Rings fully visible to the southern hemisphere during summer nights.

2 mid points where the day night cycle is normal, the gas giant covers half the night and daytime sky. Rings visible at dawn at one mid point, visible at dusk on the other.

Solar eclipse (gas giant between moon and sun) at the winter equinox. Daytime sky is filled with the dark side of the gas giant, night time is normal. Daytime is 1/3rd normal light, lasting X days. Rings fully visible to northern hemisphere during day.  

More logs for the fire....

@Endless_Helix: I haven't dealt with the civilizations so much, mostly because I wanted to deal with the science portion first (making sure my science was correct). I do intend to work on the civilization aspect, though I hardly have anything written out.

While the science discussed in this thread can be largely used in many different campaigns, I've been thinking of making a pre-space age civilization for my setting. I think it would work best for the mix of sci-fi and fantasy elements that I'm going for.

If you have any ideas on how life/civilization can be affected or inspired by this astronomical layout, then feel free to post your ideas here. A few have already been posted, such as more amphibious life forms due to more extreme tides, eclipses making nocturnal and diurnal creatures go crazy, and even the significance of the sun, planet, and other moons in the setting's folklore.

@Ghostman: Thanks for the picture! Pictures always help describe these sorts of things, and I thank you for taking the time to work on it. This layout solves a lot of our problems.

Quote from: Nomadic(exception would be rings created by a massive collision or by a captured body falling beneath the roche limit... both of which however are not stable for long term and would be absorbed by the planet within a few hundred to several hundred thousand years).

were[/i] a recent phenomenon? What if the ancients never saw a planetary ring, and the ancients recorded the events and observations of a captured satellite being ripped apart and forming the ring, which can be seen to this day? It's an interesting idea, but it might result in the ancients being bombarded with stray meteors during the ring formation (or would they be relatively safe?). I'd like to get some second opinions before I officially write it into the setting's history.

@Kapt'Lath: That is pretty similar to what I had in mind, though I imagined the moon to have a faster orbit, resulting in the eclipses on a monthly basis. You do bring up a good point: if the orbit is slow enough, the eclipses could work like seasons. I also like your descriptions of how the planet appears in the sky (I forgot to work out how it would look when in the northern hemisphere as opposed to the southern hemisphere).

It's been a while since my last post, but I've been talking with Freakin' Awesome Horse and he has been a huge help with this project.

The following calculations were based off of this model: The campaign setting's planet is equal to Saturn, the moon's size is equal to Earth's, and the distance between the planet and the moon is the same as the distance between Saturn and Titan. The setting's moon rotates on it's axis once every 24 hours, and makes a single orbit around the planet in 30 days.

Calculation #1: If you were standing on the setting's moon, how big would the planet appear in the sky? For this calculation, I'm going to compare the planet's size to our moon, since the moon is something we can all relate to.

Our moon is 384,400 km away from the Earth, and Titan is 1,200,00 km away from Saturn. This means that Titan is roughly 3.2 times further away from Saturn than our moon is from Earth (384,400 x 3.2 = 1,230,080).

Our moon's diameter is 3,474 km, and Saturn's diameter is 120,000 km. This means that Saturn is about 35 times bigger than our moon is (3,474 x 35 = 121590).

So if Saturn was the same distance away from the Earth as our moon is, it would look 35 times bigger than our moon. However, if we place Saturn 1,200,00 km away from Earth, it would look 3.2 times smaller than it would at 384,400 km away. So 35 divided by 3.2 = 11.

This means that if Saturn was 1,200,00 km away from Earth, it would look 11 times bigger than out moon. So how bug is 11 times larger? It would be about the size of your hand with outstretched fingers at arm's length.

Calculation #2: The following quotes were taken from emails:

Quote from: Samurai ChickenThanks for the calculations update. Ultimately, I'd like the eclipse to last at least a day, or a little over (but probably no more than 3 days). With your recent calculations, a day-long eclipse seems the most realistic goal to aim for. Adjusting the orbit period is probably the easiest way to do so, and I'm still deciding on how long it should be. 30 days worked because it was close enough to a month on earth. The 60-day orbit period would be my next favorite candidate. I feel that anything longer than that would make it harder to involve eclipses in the campaign setting - I'd like the eclipse to happen often enough to be a very significant and common even to the cultures and creatures living on the moon.

Again, not a problem. 60 days will get you an "accurate" day based on the most recent method. However the original calculations, based on a ratio of lunar eclipse time to total orbital period for the Earth's moon could also work. I think I can safely recommend the new method over the old one simply for that fact that it'll allow more hand-waving in terms of exactly how long you want the eclipse to last. There are certainly a lot of things that could complicate duration and calculations that we've taken ideal cases for if only to avoid tedious and messy calculations, such as the eccentricity of the orbit and variable orbital velocity because of it.[/quote]

So there you have it: a 60 day orbital period will result in each eclipse lasting about a day.