Whats up with UFOs in paintings?

If you look at some Renassaince paintings you can see UFOs in the background. Not just blobs either but full on spaceships with flames and engines or flying saucer shaped ships (Google Renassaince painting UFOs for good pictures). Theres even one of a "star battle" (actual quote) that was witnessed by hundreds. So i want to know why, in a time before photoshop, airplanes or even the knowledge of what space was, are painters painting detailed examples of UFOs?