How far can our eye see?


The Earth's surface curves out of sight at a distance of 3.1 miles, or 5 kilometers. But our visual acuity extends far beyond the horizon. If Earth were flat, or if you were standing atop a mountain surveying a larger-than-usual patch of the planet, you could perceive bright lights hundreds of miles distant.
On a dark night, you could even see a candle flame flickering up to 48 kms away.

How far the human eye can see depends on how many particles of light, or photons, a distant object emits. 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.