How Far Away Is the Horizon?
The distance to the horizon, the imaginary line where the sky seems to meet the earth, depends on how high our eyes are above the level of the land we are looking across. This is because the earth is a globe, and the land curves away from us.
On a clear day, a person standing on flat ground can see to a horizon about three miles distant. From a lighthouse 100 feet above sea level, you could see just a little more than 13 miles before the earth’s curvature cuts off your view. And from an airplane a mile high, you could see to a horizon almost 100 miles away.
The horizon is a semi-mythical distance, used in poetry as a metaphor for a philosophical division of some kind. But in fact it’s a real thing, and the distance to it can be determined.
All it takes is a little knowledge of geometry which tells us that the distance of the horizon – i.e. the farthest point the eye can see before Earth curves out beneath our view – depends simply on the height of the observer.