1. ## Celestial Equations.

On my game (see link below sig) I want to set up things in the world as realistic as possible. It has three moons, one large, though not quite as large as our, and two smaller. What I need to know, is how can I determine the orbital periods and respective size appearance in the sky of the three so that they are pretty much in position to not interfer too much with each other's orbits.

I also need a way to create an equation to describe the planet's position against the stars. Stars I can simply do as a circle about the pole, but how do I get the planets to move correctly with respect to the stars?

2. This site isn't perfect, but it's good enough for beginners. It won't tell you whether an orbit will be stable or not, but it's a start:

http://curriculum.calstatela.edu/courses/builders/

Estimating the planet's position against the stars--I'm assuming your talking about something like the Sun's path along the ecliptic--is pretty straightforward. Just divide the length of the year by 360 degrees, and plot the points on a 360-degree sky map; this can give you how many degrees your world moves per day, week, or month, what have you. Depending on how inclined the planet's equator is to the ecliptic, you can make a shallow or steep sinusoidal curve to mark it.

3. Thanks, it's been very helpful.

Next Question. Now I have a co-ordinate system for my sun and moons, I need to work out how big the Planet's and the moon's shadows would be so I can determine when and how long eclipes of both sun and moon occur. Does anyone know a good site with some formulas that I can use to determine the shadow's sizes and postions?

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