How far the human eye can see depends on how many particles of light, or photons, a distant object emits. Dust, water vapour and pollution in the air will rarely let you see more than 20 kilometres away even on a clear day.
Often, the curvature of the Earth gets in the way first. For example, at sea level, the horizon is only 4.8 kilometres away. On the top of Mount Everest, you could theoretically see for 339 kilometres, but in practice clouds get in the way.
For a truly unobstructed view, the farthest object visible with the naked eye is the Andromeda galaxy, located an astonishing 2.6 million light-years from Earth. The galaxy’s 1 trillion stars collectively emit enough light for a few thousand photons to hit each square centimeter of Earth every second; on a dark night, that’s plenty to excite our retinas
