create the earth and the sun as two spheres and move the earth to where you want it to be in relation to the sun.
So after you move the earth select it and hit "ctrl+g" to group it. Now instead of moving the pivot point of the earth you can see that you have a pivot at the origin to rotate about the sun and you can use the earth's own "rotate y" atribute to spin it on it's own axis.
You could also add all the other planets/moons in the same way
give it a try, i'll post a scene if you need it.
A
Technical Director - Framestore
Currently working on: Your Highness
IMDB