em keyframe the rotate Y attribute or write a simple expression that just adds a constant value to the y rotation. Assuming you call the sphere "earth" it would be
expression -s "earth.rotateY += 5;" -o earth -ae 1 -uc all ;
put that in the scrip editor and execute it.
And for the sun, man do whatever you want... a big yellow sphere is probably a good starting point
A
Technical Director - Framestore
Currently working on: Your Highness
IMDB