How far away is the horizon?

Question

How far away is the horizon? Or, how far out can you see on a clear day with no obstructions, assuming you have 20/20 vision.

Answer 1

If we ignore atmospheric refraction, an approximation is d(in miles)≈1.22(height above ground level in feet)^.5.

So for example, if our observer's eyes are 6' off the ground: d=1.22*6^.5=2.99 miles.