The post and some commenters refer to moving targets as a problem. Archimedes was concerned with the defense of Syracuse. In that situation, the attacking ships would mostly be stationary, and if approaching, would generally not be maneuvering but heading toward the defenders, meaning that there would be only one degree of freedom in their position and that, since their speed would likely be fairly constant, it would be possible to lead them. Aiming the heat ray would therefore not be a big issue in the situation for which Archimedes is said to have invented it.