Designing detailed Astronomy?

[Steel General]

This is a interesting discussion but way over my head (no pun intended).

[jezelf]

it is for me too - in both respects . Nevertheless I was thinking of having a dual moon system in a story I'm writing and I did find this from a quick Google.

Part of my world building is that the planet my story will be on is in our universe and in the here and now - that they see the same stars we see.

Is there a program that can extrapolate constellations we see and look at them from another location? I would expect the alignment would be very different from another point of view, but my characters would will be looking at the same stars.

cheers

[Talroth]

Is there a program that can extrapolate constellations we see and look at them from another location? I would expect the alignment would be very different from another point of view, but my characters would will be looking at the same stars.

cheers

For the most part, unless the characters were on a planet orbiting a star very near earth, picking out the stars that belong to given constellations would be rather useless. They are based on what appears to be some of the brightest stars To Us, but not what are actually the brightest stars. Things like the north star would be useless, becoming just another random star out there. Bright stars would dim, dim stars could brighten. If you really wanted to and you had the data, then it wouldn't be all that hard to calculate the primary visible stars from the new location, and then high light the primary stars as seen from earth.

Now, that is actually a far easier task than orbits, if you didn't want things overly fancy or precise. Did you have a project that you wanted such details for? If I can find a set of galaxy data that is compact enough to work with, but nicely detailed, then I might be able to do a program to render your new night sky view from any given star within the data set.

[torstan]

Now I haven't done this stuff for a while so this is only loosely remembered. As far as I can remember if you have two large moons then you have to fully calculate the effects of a three-body gravitational system - which is a serious headache. It is also pretty chaotic as the gravitational potential felt by either of the moons will change as their relative distance changes.

There are clear exceptions:
1. The moons are precisely opposite each other on either side of the planet.

Therefore the gravitational pull of the other moon is directly through the centre of the planet and the orbit acts as if the planet is just a little heavier than it is.

This case is really unstable, because if the moon is perturbed from that perfect alignment then the system once again becomes chaotic and degenerates (something crashes into something else). Another issue with this instability is that the likelihood of it being created in the first place is really small.

2. The moons are very small compared to the planet.

This means that the gravitational force is dominated by the planet and the moons have a negligible effect on the force felt by the other - hence teh gas giant moons.

3. One moon is close, the other is far away, and if one moon is really light, that helps too.

This means that the moon close to the planet feels the gravitational potential of the planet with only a small effect from the other moon (better if the far away moon is small as well). Then you can approximate the situation as single body rotation. The far away one feels the planet and moon gravitational forces combined, however it is so far away, the variation of the potential due to the orbit of the closer moon is very small.

4. There is some deeply complex chaotic orbit that is stable over long periods of time and never allows one body to hit another.

I'm sure that these sort of orbits will exist as solutions. However the chances of being left in such a configuration from random creation of moons from impacts is pretty tiny. If you want to go down this route I would suggest justifying it by divine intervention This would make for a cool planet though as the tides would be random and unpredictable. You'd need some fairly serious mathematical understanding to evolve before you could predict the future behaviour of the moons. Until then your tides and weather are going to seem like cruel choices of fickle gods and the moons can be seen as heralds of that fate (the moon grows large in the sky and the tides rise to swallow the village - you'd think the moon was an evil god too).

[Greason Wolfe]

Is there a program that can extrapolate constellations we see and look at them from another location? I would expect the alignment would be very different from another point of view, but my characters would will be looking at the same stars.

cheers

There is a program called Celestia (a free download, or at least it was when I got it and it had several add-ons) that might help you with this. I had it for a while, but haven't downloaded it again since the great crash of October. It is basically an interactive planetarium that allows you to "travel" to other stars and take a look at things from their point of view. It wouldn't be entirely accurate from a planetary perspective, but you could "turn on" the constellations and see what they might look like from another location in the neighborhood. And, if I remember correctly, it defined and rendered the stars based on luminosity and tried to represent their real visibility from every point of view and distance.

http://www.shatters.net/celestia/

[Greason Wolfe]

Now, getting back to the original question . . .

I suppose that one approach you could take would be to think of it as a binary star system with a planet orbiting one of the stars (there are a few "rules" governing this possibility). Alternatively, you could approach it from the view point of that same binary system with a planet orbiting the central mass of both stars. Again, there are some rules about the mechanics of this and, at one point, I had a link to some information about this sort of thing, but alas, the great October crash did much more damage that I thought.

In any event, you might look into things from that angle and see if you can find a suitable solution. There is still a lot of mathematics involved, but following one of these two solutions might do the trick for you.

[Redrobes]

A lot of these theories that the moon orbits degenerate because of collision and stuff is based on a small but significant probability that there is a collision so running for a long time makes the orbit not possible. But if your in a fantasy setting then I would say that if the orbit is not probably going to cause that collision then just fudge it and say they don't collide even tho there is a small chance they might. It would be like saying that in my fantasy setting with earth and moon setup there are never any solar eclipses. It wouldn't drastically affect the game to make that assertion.

[Talroth]

A lot of these theories that the moon orbits degenerate because of collision and stuff is based on a small but significant probability that there is a collision so running for a long time makes the orbit not possible. But if your in a fantasy setting then I would say that if the orbit is not probably going to cause that collision then just fudge it and say they don't collide even tho there is a small chance they might. It would be like saying that in my fantasy setting with earth and moon setup there are never any solar eclipses. It wouldn't drastically affect the game to make that assertion.

Well, this is kind of the interesting thing about planets and their moons. Even Earth's moon is degrading and will eventually leave orbit (I forget if it is coming toward us, or going to move away and out of orbit) and ones that orbit around Mars are even more unstable.

For a large moon like Earth's, that most likely starts as a major impact, then you ideally have it stable for several million years. But for small ones, they can be 'stable' for far shorter periods of time, even if they're only stable for hundreds of years before going back out of orbit they can still have an impact on society and life.

[Ascension]

The Science Channel used to show Carl Sagan's "Cosmos" quite frequently until being retired this summer. One of his shows was about constellations and how they look totally different from different perspectives...not only in space but also in time and culture. Run the program backwards and Ursa Major doesn't look like Ursa Major at all. Viewed by the ancient Chinese, it was actually three figures (a dragon, a cart, and a tax collector) instead of a bear. Since everything is moving, over time the shape will change and for us the bear will become something else (although that time is thousands of years off).

As to the moon retreating away from us, there was a History Channel show called "If We Had No Moon" (narrated by Patrick Stewart) that says the moon is moving away from earth at about 1 mile per year.

[jezelf]

Talroth: yeah - the stars would be brighter and dimmer, thanks for pointing that out. Thanks for offereing to help out. At the moment, it's nothing more than ideas and note making, structuring creation etc. I might do some illustrations in time with a night sky, but I will also be tempted to work out the night sky per season.

Greason Wolfe: Cool program- Thanks! I had a play about with it. Need to explore more, but that could be just what Im looking for now.

Ascension: That sounds good. I'll have to look into that. The stars representing something else is something I did think off too. I did have a Night Sky program, you could look back and forward in time and see the stars in a similar way

My story is a fantasy epic. Just liked the idea on something going on in our unvierse on another planet in our galaxy, right now. Its more for a world building note to self than appearing in the story. The stars might crop up in the story as wizards or characters naviagte and I'll give them mythical names etc.

I thought it might be cool to work out what those constellations would look like and see if I can create new ones for my characters based on their persective. Might also inspire mythical creatures for my story's history.

I guess as Im creating a fictional planet, putting it in a known part of space might conflict with any suspension of reality...so it might be a flawed idea. - and probably too much detail, but I'm a sucker for it, even if it's all just for my own reference.

thanks for the help!