How far can the human eye really see? Well, it depends on how many particles of light, or photons, a distant object emits. In other words, the farthest object currently visible with the naked eye is the Andromeda galaxy, which is located 2.6-million light-years from Earth. That's right, the galaxy's 1-trillion stars emit enough light collectively for a few thousand photons to hit each square centimeter of Earth every second. Continue reading for more.