Let's say you're standing on the ground looking out on a clear day, you can probably see the horizon that's about three miles away but if you were standing on a mountain or if the earth was flat (which it is not) you could see much farther. On a dark night, on a mountain top you can even see a candle flame flickering up to thirty miles or 50 kilometres away and when you look up at the night sky you can see the distant stars.
How far a person can see depends in part on how many particles of light or photons the object emits. To perceive something a minimum of five to fourteen photons need to hit the backs of your eyeballs at the retina, that's about what would happen with that candle flame thirty miles away. But of course twinkles of light aren't the only thing the eye can see. To see something spatially as opposed to just a speck of light, light from the object needs to stimulate at least two adjacent cones in our eyes.
These are the cells responsible for our ability to see color. To stimulate at least two cones that means, light must come from at least two points on an object and the two points need to form an angle of at least one arc minute which is one sixtieth of a degree. The moon is thirty arc minutes across so we can see it clearly. Venus which appears just a dot of light in the night sky is only one arc minute across.
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.
