How Far Can You See?
How far the human eye can see depends on how many particles of light, or photons, a distant object emits. The farthest object visible with the naked eye is the Andromeda galaxy, located an astonishing 2.6 million light-years from Earth. The galaxy’s 1 trillion stars collectively emit enough light for a few thousand photons to hit each square centimeter of Earth every second; on a dark night, that’s plenty to excite our retinas.
If you stood on the shore looking across the sea to the horizon (the line appearing to separate earth from sky), you might be able to see about two and a half miles. But the higher you stood the farther you would be able to see. As the earth is curved, the horizon would appear farther away with every increase in height above sea level. At a height of 20 feet you might see for six miles.
From the top of a 300-foot cliff your view could extend for 23 miles, while on the summit of a 3500-foot Mountain, it could lengthen to 80 miles. If you look straight up into the sky, the distance you can see is immense. The moon is about 239,000 miles away and the stars are millions of miles distant.
From an aircraft flying at 16,000 feet you might have an uninterrupted panorama for 165 miles. Pan American clipper pilots have found that most of their passengers have surprisingly hazy notions of how far they can see in the air. One passenger, 20,000 feet above Brazil, insisted that she could see the coast of Africa, 1,822 miles away.
One typically sees further along the Earth’s curved surface than a simple geometric calculation allows for because of refraction error. If the ground, or water, surface is colder than the air above it, a cold, dense layer of air forms close to the surface, causing light to be refracted downward as it travels, and therefore, to some extent, to go around the curvature of the Earth.
The reverse happens if the ground is hotter than the air above it, as often happens in deserts, producing mirages. As an approximate compensation for refraction, surveyors measuring longer distances than 300 feet subtract 14% from the calculated curvature error and ensure lines of sight are at least 5 feet from the ground, to reduce random errors created by refraction.