Horizon, the visible boundary between earth and sky. Because the earth is a globe, its surface curves out of sight. Beyond and below the horizon the earth is hidden. Above is limitless space. On land, the horizon is irregular in outline and at varying distances from the observer, depending on the direction he or she faces. This is because the true horizon is usually obscured by hills, buildings, trees, and the like. At sea, the horizon is at an equal distance in every direction from the observer.

The distance to the horizon varies with the altitude of the observer. At ground level a person six feet (1.83 m) tall can see about three miles (5 km). From the top of a 100-foot (30-m) tower the horizon is 12 1/2 miles (20 km) away. Above 100,000 feet (30,000 m), a very broad sector of the globe is visible, and the horizon distinctly shows the earth's curvature. A traveler 18,640 miles (30,000 km) in space would see the horizon as the outline of the whole earth.