The horizon or skyline is the apparent line that separates earth from sky, the line that divides all visible directions into two categories: those that intersect the Earth’s surface, and those that do not.
What is the horizon?
At many locations, the true horizon is obscured by trees, buildings, mountains, etc., and the resulting intersection of earth and sky is called the visible horizon. When looking at a sea from a shore, the part of the sea closest to the horizon is called the offing.
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.
Historically, the distance to the visible skyline has long been vital to survival and successful navigation, especially at sea, because it determined an observer’s maximum range of vision and thus of communication, with all the obvious consequences for safety and the transmission of information that this range implied. This importance lessened with the development of the radio and the telegraph, but even today, when flying an aircraft under Visual Flight Rules, a technique called attitude flying is used to control the aircraft, where the pilot uses the visual relationship between the aircraft’s nose and the horizon to control the aircraft. A pilot can also retain his or her spatial orientation by referring to it.
On terrestrial planets and other solid celestial bodies with negligible atmospheric effects, the distance to the horizon for a “standard observer” varies as the square root of the planet’s radius. Thus, theskyline on Mercury is 62% as far away from the observer as it is on Earth, on Mars the figure is 73%, on the Moon the figure is 52%, on Mimas the figure is 18%, and so on.
Text: Wikipedia contributors. “Horizon.” Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 23 Jul. 2017. Web. 9 Aug. 2017.