Rassilon wrote:The volume is found by an earth to alien world ratio of radiuses correct? say one world is 5000 km and earth is 6378 km that would be a percent of say 0.78. Now I cube that...multiply by pi (3.14) and then by 4 and divide that by 3.
You don't need the 4/3 and pi if you're using the ratio of the radii ... the ratio of volumes will be just the cube of the ratio of radii.
OK now I find the density...Which I think is a guess more than a formula...What do I base this on? I havebeen told earth has a density of 5.52 per cubic centemeter. Is this correct? If so what do I base my density on? Material composition?
Simplest thing is just to have a table of likely densities ... for Earth-like bodies, for Jupiter-like bodies, and so on, using the real-world value of the corresponding solar-system world. So your Earth density will be plausible for a range of terrestrial planets.
After that is found I then multiply volume by density to find my mass...
Might be easier to use "Earth densities" as your units, so you end up with a mass in Earth masses. Divide the planetary density by 5.52 before doing the sum.
Of which mass I need to compute the correct orbits of my moons. And get a correct gravity.
Surface gravity scales in proportion to the mass and inversely with the radius squared; so divide the number of Earth masses by the square of the radius in Earth radii, and you get the surface gravity in g.
The orbital period squared scales in proportion to the orbital radius cubed and inversely with the mass. You might use the Moon here ... express the orbital radius as a proportion of the Moon's, cube that figure, divide by the primary mass, take the square root, and you'll have the satellite's orbital period as a proportion of the Moon's orbital period.(Approximately ... since the Moon has quite a high mass it slightly jiggers the figures, but given the uncertainty in your density, the error won't matter.)
Grant
