User blog comment:RandomGuy96/So how do planetary defense cannons work?/@comment-5436239-20130127223836/@comment-4967682-20130127230440

Your laptop can calculate and compensate for the exact shot it would take to hit something when the projectile travels in a straight line. There should be no accuracy problems at all.

Usually, they'd want the planet. Not all guns are city-destroyers like dreadnoughts; simply destroying the enemy defenses from far away with small guns would make them useless.