Celestia Forums

can i put in celestia two planets orbiting a star like this:

Planet |Distance from star |Rotation Period
-Planet1 | d | T
-Planet2 |2d | T/2

phisycaly its impossible because of the kepller law, but can i do it in celestia?

You can do it in Celestia. Why not just try it and see?

Grant

i thought that i may be able to do it, but if i can, then celestia doesn't use the kepler law?

When I was first mucking about with celestia, I too was confused by the lack of Kepler's law (the one regarding orbital distance versus orbital period ) in the program itself.

However, having seen some of the ideas people have been trying to develop, I see the wisdom of being able to define 'impossible' orbits.

One example of this is placing an object at the Lagrange 1 point (a point between two bodies that orbit their common centre of gravity). If Kepler's Law was built into Celestia, this would be impossible without a lot of extra code to use gravity (see other threads regarding the implementation or lack of gravity in Celestia)

Besides, Kepler's Law isn't difficult to work out, given the mass of the central body and the distance.

Regards,

Cormoran

Personally, I think it would be nice to have the best of both worlds. If Celestia understood the masses of objects, the user could specify either a SemiMajorAxis or a Period, and Celestia could then calculate the missing value. But if the user chose to specify both these values, Celestia would accept them without modification.

Grant

Cormoran wrote:
One example of this is placing an object at the Lagrange 1 point (a point between two bodies that orbit their common centre of gravity). If Kepler's Law was built into Celestia, this would be impossible without a lot of extra code to use gravity (see other threads regarding the implementation or lack of gravity in Celestia)

Cormoran

The positions of the Lagrange points can be calculated iteratively if the mass ratio of the bodies involved, and the distance between them, is known. It could be nice to have the option to view, and perhaps specify spacecraft positions relative to them in Celestia.

For me, half the fun of Celestia is doing calculations for orbits myself (weird, huh?).

I think other folks have commented that a Lagrange point could be specified as an invisible object, and then bodies can be placed orbiting that.

I haven't used the L1 point anywhere yet, so I'd have to look up the formulae. I'm sure its out there on the web somewhere

Cheers,

Cormoran