What is the maximum distance the human eye can see when unobstructed?
Dust, water vapour and pollution in the air will rarely let you see more than 12 miles even on a clear day.
Often, the curvature of the Earth gets in the way first --- at sea level, for example, the horizon is only 2.9 miles away.
On the top of Mount Everest, you could theoretically see for 211 miles, but in practice cloud gets in the way.
For a truly unobstructed view, look up.
On a perfectly clear night with no Moon in the sky you can see the Andromeda galaxy, which is 2.25 million light years away.
