I know why, but a recent read on the subject led me to find a cure. Some of you might know this already. For those of us who mod and for players alike, i've always liked spinning planets. I mean it's a planet, it should spin, right? Well not in FL it seems. If you add this line to the planet code,
spin = 0, 0.035000, 0
Or similar, your planet will happily spin on it's axis, and after a certain period, grind to a halt, especially if you minimize the playing window. The bigger the number you enter, the faster the planet spins, either way it's not very realistic. The reason for this is the planet does not have inertia, which for those who are not sure what that means is the resistance of any physical object to a change in its state of motion or rest, or the tendency of an object to resist any change in its motion. In other words, the planets weight should keep it moving, but it doesn't, so how do we achieve this?
We have to do some hacking!! Adoxa as usual found this, but a simple edit using your favourite hex editor of the common.dll gives planets rotation inertia. If you goto address 0E698E and change the value from 7C -> EB, planets will keep spinning, not only that, lower the number in the spin code to something like
spin = 0, 0.015000, 0
or less, and the planets will spin nice and slowly and will never stop spinning 
I'm currently updating all my planets in my Genesis mod for my next release and they look so much better for it as they spin more slowly now and don't make me feel sick anymore lol
