What Is the Horizon?
Of course you know what the horizon is. It is easiest to see the horizon at sea when out of sight of land. There, when you look in any direction from the ship to the place where the sea and the sky meet you see a line which, if you follow with your eye as you turn completely around, makes a perfect circle. It looks as though it marked the boundary of the earth. On land it is not easy to see as much of the horizon at one time, because of buildings and trees and hills in the woods and elsewhere, but if the land were perfectly smooth like the sea and there were no trees or buildings or hills in the way, you could see just as perfect a circle on land as on sea. This proves that the horizon is a movable circle. On land it is where the earth and sky appear to meet, and on water it is where sky and water appear to meet.