This section is from the "The New Student's Reference Work Volume 5: How And Why Stories" by Elinor Atkinson.
The horizon is the boundary of as much of the world as you can see from the place in which you happen to be standing. If you are on the ocean, or on a flat plain with no houses or trees to break the line, the horizon is a perfect circle, because you are able to see the same distance in every direction. The horizon, then, is the circular line where the earth and sky seem to meet. The distance that can be swept by the human eye, depends upon the height from which the view of the world, is taken. On the sea, or a plain, a man with perfect eyes can see a distance of about two and a half miles on a clear day. That is, he sees across a circle of five miles, or around a horizon of about sixteen miles. From a sky-scraper tower, two hundred feet above the earth, the same man can see nearly nineteen miles. From a mountain five thousand two hundred and eighty feet, or a mile high, he can see nearly one hundred miles in every direction. We can see farther by going up higher, because the earth is a sphere, or globe. The surface of it slopes away on all sides. At a certain distance, on a level, the slope falls below our vision. As we go higher, farther limits come up within range of our eyes.