Standing on a beach, the horizon is only about five kilometres away
That distant line where sea meets sky feels infinitely far, yet for someone of average height standing at the water's edge it lies just under 5 kilometres off. Climb Mount Everest and it stretches to about 336 kilometres. On the Moon, a smaller ball, the same person would see only about half as far.
The word comes from a Greek phrase meaning separating circle, rooted in a term for a boundary stone. Astronomers and navigators split the idea into layers. The geometric horizon is what you would see on a perfect sphere with no air. The apparent horizon is that line bent by atmospheric refraction, which usually lifts distant objects a little and occasionally makes them loom into view or sink out of sight. The visible horizon is what remains after hills, trees and buildings get in the way.
Working out the distance needs only Pythagoras. Your line of sight grazes the Earth at a right angle to its radius, forming a triangle whose longest side is the planet's radius plus your height. For anyone close to the ground there is a handy shortcut: take the square root of your eye height in metres and multiply by about 3.57 to get kilometres. It stays accurate to within 1 percent for views from mountaintops, aircraft and high balloons.
The numbers climb faster than intuition suggests. From a 30-metre tower you see about 19.6 kilometres; from the roof of the Burj Khalifa, about 103. Passengers cruising at 35,000 feet can see roughly 369 kilometres, and a U-2 pilot at 21,000 metres about 517. Weather matters too, since refraction changes from day to day. From a space station the horizon is no longer even a neat circle, because Earth bulges at the equator.
Other worlds shrink the view in proportion to the square root of their size. Mercury's horizon sits at 62 percent of Earth's distance and Mars at 73 percent, while tiny Mimas, a moon of Saturn, offers just 18 percent. Artists borrow the idea too: in perspective drawing, the horizon is the eye-level line toward which parallel edges converge.
Source: Horizon
