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Old 06-17-2009, 02:23 PM   #1
Johnny Angel
 
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Default Celestial Bodies

Here in the real world we have one sun and one moon. However, in many fantasy worlds, there are multiple suns and/or multiple moons... or even if there is still only one sun or one moon for a fantasy world, sometimes the set up is different from how things are here in our world.

I tried to google and find some answers about how certain differences in celestial arrangement might impact a world, but the majority of what I found were discussions talking about waves and ocean currents - which would be different indeed, but a little more information would be nice. I know that most such things are somewhat handwaved in fantasy and fictional worlds, but are there any studies that have been done to theorize what the effects of some of these fantastical and fictional set ups might actually realistically have? Also, of some of these set ups, what are some that are actually viable; what are some that aren't?

For me personally, in particular I have a few set ups which I have contemplated. I found some of my old notes from things I had apparently been working on during various times. (It's always interesting to me when I go through old notebooks.)

In one world concept the world was between two suns which were both of equal size and equal mass. Instead of orbiting around a sun like our world does, the two equal sized suns held the world in place, and it just sort of spun in place. This also meant that there was no such thing as night as we understand it; the darkest part of the day was more akin to dawn or dusk whenever a side of the world would be spun away from being directly facing either of the two suns.

In the current world I'm building, I've been fiddling around with a few different concepts, but I'm not entirely sure what I want to do yet. I'd like to educate myself a bit more on what some of the possible actual results would be from altering the celestial arrangement of a world's universe. I'm leaning toward having multiple moons, which by itself seems fairly easy to model. The arrangement of the moons would probably mainly just impact the oceans of the world, cause Lunacy to be a bit more problematic to a PC as a disadvantage, and slightly alter how the people of the world kept track of time. I had imagined one set up in which two twin moons orbited around each other as the pair both orbited around the planet together.
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Old 06-17-2009, 03:50 PM   #2
tantric
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Default Re: Celestial Bodies

Which inspires me to ask: can a terrestrial world naturally have a ring?
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Old 06-17-2009, 03:53 PM   #3
Fred Brackin
 
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Default Re: Celestial Bodies

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Originally Posted by Johnny Angel View Post
In one world concept the world was between two suns which were both of equal size and equal mass. Instead of orbiting around a sun like our world does, the two equal sized suns held the world in place, and it just sort of spun in place. This also meant that there was no such thing as night as we understand it; the darkest part of the day was more akin to dawn or dusk whenever a side of the world would be spun away from being directly facing either of the two suns.
Sorry, probably won't work without turtles and elephants or other sorts of divine intervention.

Maybe if it was absolutely at the center of mass for the two sun system.....but it'd never form there in the first place. Even if it was captured somehow it's not likely to stay intact.

Well, maybe if the suns are pretty distant from each other. They probably need to be to make your temperature situation work out. If you just doubled the Earth;'s amount of solar energy Bad Things would happen.

The result would need to be be more like two dim lights in the sky all the time. Instead of 2 dawns it might be more like perpetual twilight.

The usual habitable binary hypothesis is more like "planet orbits one Sun like usual. The other is at least as far away as Saturn and doesn't effectively contribute daylight.".

It might be possible to have the suns orbiting each other and the planet be in one of the 4th or 5th Trojan spots. That would have the planet in a stable orbit trailing 60 degrees behind one of the suns in its' orbit.

Multiple moons are a lot easier but note that small moons like Mars' are too small and dim to be very visible. Any close moons would probably also be in the planet's shadow cone a lot of the time (like lunar eclipse only more so) and therefore blacked out.
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Old 06-17-2009, 03:55 PM   #4
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Default Re: Celestial Bodies

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Originally Posted by tantric View Post
Which inspires me to ask: can a terrestrial world naturally have a ring?
Sure, no reason why not. A big one like Saturn's would take some managing though.
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Old 06-17-2009, 05:08 PM   #5
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Default Re: Celestial Bodies

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Which inspires me to ask: can a terrestrial world naturally have a ring?
Yes. If a satellite goes closer than the roche limit, it will be torn apart by tidal forces and the result will be a ring. For a while at least.
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Old 06-17-2009, 05:50 PM   #6
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Default Re: Celestial Bodies

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Sorry, probably won't work without turtles and elephants or other sorts of divine intervention.

Maybe if it was absolutely at the center of mass for the two sun system.
Or the L1 Lagrange point between two stars of different mass. But that isn't dynamically stable, is it?
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Old 06-17-2009, 06:09 PM   #7
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Which inspires me to ask: can a terrestrial world naturally have a ring?
Reply hazy: ask again later. Astronomers aren't yet agreed how Saturn comes to have such rings, what created them, and what maintains them.

Earth presumably had a ring between the time of the impact of Theia and the time when the resulting debris coalesced to form the Moon, but the planet was a bit too molten to be habitable at the time, and the span involved is variously estimated as between about a month and about a century.

Some time within the next fifty million years Phobos's decaying orbit will bring it inside Mars' Roche limit, at which time it will disintegrate to form a ring. The ring will be inconspicuous, unstable, and probably transient.

NOTE: important part of what makes Saturn's rings so spectacular is that they are made of innumerable little pieces of ice or of other material coated with ice. And that is only possible out beyond the Frost Line, far enough away from the Sun to be cool enough that ice in space remains solid rather than melting or subliming. No planet with a blackbody temperature above about 150 K (-253°F) can have icy rings. And no planet habitable to mankind can be that cold.

If a habitable planet does have rings, they will be comparatively inconspicuous dust rings like Jupiter's.
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Old 06-17-2009, 09:06 PM   #8
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Default Re: Celestial Bodies

I would think that a terrestrial planet with a ring would have very interesting skies: lots and lots of meteors. In order for a dust ring to be visible, it would have to be pretty dense, compared at least to an icy ring. And if there is that much stuff up there, some of it must be in decaying orbits and showing up as meteors. It would at least be enough to foster interesting religions.

In addition, a ring system that is relatively stable probably requires shepherding moons; I have no idea how many this would require, or the sizes of the moons involved.

For those that are interested, there is a good piece of info on the shape of the shadows cast by the rings here (look for the January 7, 2009 blog entry...it doesn't directly link to it, unfortunately.)
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Old 06-17-2009, 09:25 PM   #9
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Default Re: Celestial Bodies

I think that this is the link SteveH wanted: http://planetfuraha.blogspot.com/200...1_archive.html
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Old 06-17-2009, 10:10 PM   #10
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Quote:
Originally Posted by Fred Brackin View Post
Sorry, probably won't work without turtles and elephants or other sorts of divine intervention.

Maybe if it was absolutely at the center of mass for the two sun system.....but it'd never form there in the first place. Even if it was captured somehow it's not likely to stay intact.

Well, maybe if the suns are pretty distant from each other. They probably need to be to make your temperature situation work out. If you just doubled the Earth;'s amount of solar energy Bad Things would happen.

The result would need to be be more like two dim lights in the sky all the time. Instead of 2 dawns it might be more like perpetual twilight.

The usual habitable binary hypothesis is more like "planet orbits one Sun like usual. The other is at least as far away as Saturn and doesn't effectively contribute daylight.".

It might be possible to have the suns orbiting each other and the planet be in one of the 4th or 5th Trojan spots. That would have the planet in a stable orbit trailing 60 degrees behind one of the suns in its' orbit.

Multiple moons are a lot easier but note that small moons like Mars' are too small and dim to be very visible. Any close moons would probably also be in the planet's shadow cone a lot of the time (like lunar eclipse only more so) and therefore blacked out.

I had suspected it wouldn't be possible, but it's an idea which captured my imagination. Looking through my old notes, I apparently had a few different sketches about how the set up was supposed to work. The first sketch I already mentioned.

The second sketch (which is the only other one I seem to have spent time working out) was pretty much the same idea except somehow the two equal sized suns circled around the planet. Apparenltly my imagination at the time had them orbiting each other (somewhat similar to the moon idea for the current world I'm working on,) and the planet just happened to be situated in the middle; held in place by the opposing forces of the sun... a sort of eternal celestial tug of war for a lack of better words.

In either case, I think I was trying to work something out where they were far enough away that the planet would recieve roughly the same amount of solar energy as the sun -so that the world wouldn't burn up. As you said, this would create an eternal twilight of sorts. The people of the world believed the two suns to be the eyes of the pantheon's over-god; eternally watching.

----------------------------------------------------------------------

Would the moon idea I mentioned for the world I'm currently working on be feasible?

If yes, from a GURPS rules perspective, do you feel it would be far to say that Lunacy would give a -4 to self control instead of a -2 whenever both moons were full?
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