The horizon is the line that separates the Earth from the sky. There two main types of horizons—Earth-sky horizons and celestial horizons. Both Earth-sky and celestial horizons have different sub-types of horizons. The local horizon, geographic horizon, and sea-level horizon are all Earth-sky horizons. The astronomical horizon and true horizon are celestial horizons. Earth-Sky Horizons The local horizon, also called the geometrical horizon, is the visible boundary between the Earth and sky. The local horizon may include trees, buildings, and mountains. The geographic horizon is the apparent boundary between the Earth and sky. Mountains, trees, and other elevated features are not considered part of the geographic horizon. The sea-level horizon is the geographic horizon at sea level. One of the best places to see the sea-level horizon is a beach. The ocean and the sky provide a clean, flat line where the Earth seems to meet the sky. If you're standing on the beach looking out at the sea, the part of the sea that "touches" the horizon is called the offing. Celestial Horizons Celestial horizons are used by astronomers. They are measurements of the position of the Earth relative to the rest of the sky. The astronomical horizon is the imaginary horizontal plane always at a 90-degree angle from the observer's zenith (the point directly above the observer). Astronomical horizons are great circles that surround the observer. The true horizon is the imaginary plane that passes through the center of the Earth, perpendicular to its radius. From orbit, the true horizon is spherical, following the shape of the Earth. Importance of the Horizon The concept of the horizon is important to different types of work, including aviation, navigation, and art. Pilots use the horizon to keep aircraft level while in the air. Using a method called "attitude flying," they can control their aircraft by determining the relationship between the aircraft's nose, or front end, and the horizon. Pilots can change their altitude or flight pattern by changing the horizon to be composed of mostly sky (increasing their altitude) or mostly ground (lowering their altitude). Before the introduction of modern tools such as global positioning system (GPS) devices, sailors depended on a clear view of the horizon to navigate the ocean. The sun's position to the horizon told sailors what time of day it was and what direction they were sailing. At night, sailors could use celestial navigation, or the appearance of certain stars or planets relative to the horizon. As the Earth turns, stars and constellations rise and set on the horizon, just like the sun. Different constellations appear at different times of the year, or are only visible from certain places. The rising of the constellation of the Southern Cross, for instance, signaled that sailors were in the Southern Hemisphere. Artists use the concept of the horizon to create perspective in paintings and drawings. When depicted with shapes getting smaller and less detailed closer to it, a horizon can create the illusion of depth on a flat canvas. The horizon was also important for early methods of communication. Before the radio and telegraph were invented, people couldn't communicate with anyone farther away than the local horizon.