Sit down and learn calculus!
Though you could just learn programming, and create a formula that calculates the appropriate hit of two curves, being the planet and the ship.
@ Steve: It still should be an update once per day for colonized planets, because otherwise ships might find themselves in a position where they either try to go where the planet was 2 days ago, and miss it, or go where it's supposed to be, and wait there for 2 days.^^
Aside, in the case of traveling to a close moon, teleporting the ships with it might be a valid option, but what if those ships are shot at?
Do you teleport the missiles as well?
And any support fleets behind the planet, so they don't suddenly get in range to blast away those missiles?