They strap some poor bastard's feet to bicycle pedals that are wired to the 5 HP propeller engine and tell him to get on with it.
Also, it's in orbit around a planet. It can just shoot anything from where it is I presume.
They do call it a weapon all the time, and not a ship (unless I'm mistaken).
EDIT: Wikipedia says it's a space station, so it wouldn't need to move.
Oh, I have a question though:
If the RotJ is maybe a year after Empire (if that), then how has the second Death Star been half built by then already?
The first was only a skeleton at the end of Ep. 3, and 20 or so years later in TFU it's still not fully finished. And the second was supposedly much bigger too!