Multiple moons and tidal forces

[nameless]

If you really want to model it, it shouldn't be too hard to write a computer program to crunch the positions of the moons and find the resulting tidal force at any particular time. If you give the program periods of revolution for the moons, you can reduce the input to simply a day and time, and the output to an ellipse.

I'm thinking vectors for the individual moons, and crunching the 5 vectors together to determine an overall vector force. For whatever reason, tides bulge on opposite sides of the planet; the vector in one direction and the complementary one in the opposite direction give the foci and eccentricity of an ellipse.

Throw some conversion factors in against that ellipse and BAM!, you've got tidal highs and lows in any particular part of the planet at any particular time. I'd say it's simple, but it's actually a lot of math. Luckily computers don't care about doing math so much.

-nameless

 

[Umbran]

Why would the moons have to be very small in relation to the planet, I don't understand this. Probably has something to do with gravitational forces, would be nice to have it explained though.

Because, if they are not small, their orbits depend not only on the planet, but on each other. The orbit of one moon depends ont he planet, and the other moon. But, the orbit of the other moon depends ont eh first moon. It very quickly gets hairy.

In physics, it's referred to as a "many body problem", and they are notoriously difficult to solve. Figuring orbits is easy if there are only two objects. You can figure in the effects of a third or more, if the effects are small - either the object is small, or is far away. But if they are all pretty large, the math gets nigh intractable.

 

[Avatar28]

Again, though, computers don't care much about the math. BUT, you also would have to figure in mass, distance, etc into all that. But, once again, it IS doable. Just don't do it by hand, that's all. I wouldn't be surprised if there's a computer program or a java applet somewhere to do all that.

Hell, if nothing else, go to your local university and talk to someone in the astronomy dept.

 

[nameless]

Well, computers don't mind doing the math, but they ARE limited by processing speeds. A 6-body system is a very complex interaction (and that's not counting the sun, or other planets, etc.). It might realistically take a supercomputer to crunch the numbers in a useful amount of time.

That said, I'd hand-wave the many body problem and let the moons have circular orbits. Ditto for friction and viscosity and other minor force corrections. Then it becomes a pretty simple process, one that out math-o-phile PCs will do all day.

 

[Sulimo]

If you really want to model it, it shouldn't be too hard to write a computer program to crunch the positions of the moons and find the resulting tidal force at any particular time. If you give the program periods of revolution for the moons, you can reduce the input to simply a day and time, and the output to an ellipse.

For this there is Weathermaster . I havent used it personally, but it was brought up in a discussion on the Shadow World (a Rolemaster setting) list. Here's one posting to give a feel for the program:

"Wrom: AFXISHJEXXIMQZUIV
[mailto:shadow_world@s...]On Behalf Of bcd@p...
Sent: Saturday, December 29, 2001 3:57 PM
To: recipient list not shown:
Subject: WeatherMaster and Shadow World


Having briefly looked at the WeatherMaster software, I have the following observations;

It looks like decent software for working out weather and weather-related details. Of course, it has its problems.

First, when making a new world, you're not allowed to use an odd number of hours per day. Rather a weird limitation, and it rules out Kulthea unless, of course, you just double it to 50 and remember to halve the times it gives you.

Second, while you can customize the climate zones to your liking, I find this to be very tedious work using their software (GUIs simply aren't very well suited to bulk data editing). The data also appears to be stored in a way that precludes easy editing with the text editor of your choice (I would have liked import/export from/to XML or similiar). Another consequence of this last is that the x hours you sink into setting up the data in WeatherMaster are lost if you ever choose to change software (or use supplementary software). The data simply isn't in a format that allows you to take it with you to your new software. I hate that. Of course, if you have good tools for working with mdb files (I believe that is a common Microsoft database format) and their table design is sensible, some work could possibly allow you to make an export utility of your own.

Third, it appears that any one database is not meant to be for "one world" as much as it is meant to be for "one campaign in one world". That is, if you have two different campaigns in the same world happening at the same time, you will need two separate databases for the two campaigns, simply because each day of weather can be generated in one single location only. For instance, I will have an entry for "Day 23 of year 6040" and assoicated with this entry will be 1) longitude and latitude and 2) weather for that day in that location. It is not possible to have another entry for "Day 23 of year 6040" that is in another long/lat and/or with different weather. I do not know how easy it is to make a duplicate database for this purpose. The software does not appear to be able to do it for you, but it may be as simple as copying the mdb file. A similiar problem will be seen for world-hopping campaigns since each database is rooted to one single world and so you have to switch databases when the PCs
change worlds. This is probably easier to deal with though.

Fourth, you can move around the location of the next day of weather on a world map. However, there does not seem to be any way to associate climate codes with world locations, so the only thing tha software appears to take from your change of location is longitude and latitude for determining sun up/down etc. I would have like to be able to designate a region of the world to be "desert of Zor climate", another part to be "Straits of Meluria climate", etc. This way, it would have been one little drag motion to set all the particulars, in stead of one drag and several climate selections. It would probably also necessitate a zoomable map to allow sufficiently precise locationing. That would be really neat.


All for now. I am as yet undecided myself.

Cheers
Bent D"

 

[Umbran]

Again, though, computers don't care much about the math.

Computers don't get tired or bored while doing the math, but that doesn't mean the process is simple or quick. The computer can only do the math as well as the human can program it to do so.

The many-body problem is one that frequently goes into the realm of "chaotic dynamics". Depending on the details, attacking a chaotic system in the wrong way can yield anyting from nonesense answers to outright crashing of your program. Saying, "It's math, a computer can do it" is a vast oversimplification.

 

[Avatar28]

Well, obviously it would have to be programmed correctly, which would require an understanding both of the math involved and the programming.

As for a six body system requiring a super computer to calculate, I doubt it. In the days of the PC-XT, maybe, but not with these multi-gigahertz machines we have now. Keep in mind that a modern PC has almost as much processing power as a supercomputer from a few years ago. Heck, weather forecasting is VERY complex, and yet it is still done on normal desktop PCs, not supercomputers. It still might take awhile, but that's why you just let it run overnight or whatever.

For what it's worth, I've seen a java program that has a model of the solar system. It's whole purpose is to model what would happen if a rogue star passed through the solar system. It has a number of specs such as speed of the star, mass, etc. AFAIK, it also takes into account the gravity of each of the planets if their orbit gets distrubed and they get closer to each other, etc. Don't have the link though, sorry. I think I originally got it at that bad astronomy site. If you wanted to ask over there, one of those guys might be able to point you in the right direction.

 

[ninthcouncil]

You want realism?

The tides are easy. There are none.

Why?

Because in order for a planet to have multiple moons in stable orbits, they must be very small in relation to the planet. This means that the major tidal influence is the sun, so the high tides come at noon and midnight.

I'm with you on this one - Earth is very unusual in the size of its satellite. The dramatic gravitational interactions between multiple satellites of such large size and close orbit would rip such a system up in no time flat. Looking at the rest of the solar system, we see planets with multiple tiny satellites (relative to the parent - some of the gas giant moons are big, but they're in wide orbits and are miniscule in relation to the primary, and Mars' satellites are pinpricks), or none. Pluto and Charon are... well, the subject of some debate, with a lot of people not regarding Pluto as a proper planet at all. Best left out of the reckoning.

Except that a world without a large moon, within the "life zone" of a star that is old enough to have life, is going to be tide-locked to that star. That means that one side will ALWAYS be noon, and the other will ALWAYS be midnight.

So no tides. No oceans, either. Just a narrow band of habitable land in the "dusk zone" between the nearside desert and the cold farside wastes.

Are you sure? I'm prepared to be wrong (my knowledge on these things is 20 years out of date), but is that true? Mercury, after all, though it has a very long "day"(58 Earth days vs. a year of 88 Earth days), is not tidally locked to the Sun, and it's a lot closer than Earth. Venus is damn close to being locked (224 day year, 243 day "day"), and was believed to be so around the turn of the last century, since some of its atmospheric features ARE tidally locked - even so it's not quite there. So it doesn't even appear to be true of other candidates for tidal locking in our own system.

A "ribbon" world makes a potentially interesting setting, though....

 

[evil_rmf]

This is not a complete answer, but may get you off on the right foot. I found this on an Elfquest website:

http://ourworld.compuserve.com/homepages/Sweetwind/sendings/tides1.htm

 

[Umbran]

Heck, weather forecasting is VERY complex, and yet it is still done on normal desktop PCs, not supercomputers. It still might take awhile, but that's why you just let it run overnight or whatever.

Actually, I'm prety sure the cutting edge of weather forecasting is still done with supercomputers.

The problem with computing is very simple - the more your machine can do, the more we ask it to do. Yes, everything from desktops to supercomputers have advanced, but so have the mathematical models. So have the applications and programs we run on them. Take a look at how much space common word processors now take up, as compared to XT days...