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.
horizon
Planets appear above the true horizon.

Flat Earth
Many people continued to believe the Earth was flat until the late Middle Ages. The horizon was interpreted as the sharp edge of the Earthalthough no one ever reached it.

The horizon also helped prove that the Earth was a sphere. As early as 330 BCE, the Greek philosopher Aristotle noticed that certain constellations in the southern sky rose higher from the horizon as he traveled south. This could only be possible if the Earth was round.

Noun

the distance above sea level.

apparent
Adjective

clear or obvious.

astronomer
Noun

person who studies space and the universe beyond Earth's atmosphere.

astronomical horizon
Noun

imaginary horizontal plane always at a 90-degree angle from the observer's zenith (the point directly above the observer).

aviation
Noun

the art and science of creating and operating aircraft.

Noun

narrow strip of land that lies along a body of water.

Noun

line separating geographical areas.

celestial horizon
Noun

line or planes used as reference for observation and measurement relative to a given location on the surface of the earth or another celestial body, and referenced to positions at right angles to the location's zenith.

celestial navigation
Noun

determining an object's position using the stars and planets as guides.

communication
Noun

sharing of information and ideas.

concept
Noun

idea.

constellation
Noun

group of stars that form a recognizable shape.

depict
Verb

to illustrate or show.

elevate
Verb

to raise higher than the surrounding area.

geographic horizon
Noun
apparent boundary between the Earth and sky, with local prominences such as mountains and trees excluded.
Global Positioning System (GPS)
Noun

system of satellites and receiving devices used to determine the location of something on Earth.

Great Circle
Noun

largest circle that can be drawn around a sphere, such as the Equator.

Noun

line where the Earth and the sky seem to meet.

horizontal
Adjective

left-right direction or parallel to the Earth and the horizon.

illusion
Noun

plan or occurrence that creates a false belief.

local horizon
Noun

line where Earth and sky seem to meet but are blocked by elevated features of the landscape, such as trees or mountains. Also called the visible horizon.

measurement
Noun

process of determining length, width, mass (weight), volume, distance or some other quality or size.

navigate
Verb

to plan and direct the course of a journey.

nose
Noun

front end of an aircraft.

offing
Noun

part of a body of water that touches the horizon.

orbit
Noun

path of one object around a more massive object.

perpendicular
Noun

at a right angle to something.

perspective
Noun

representation of volume or depth on a flat surface.

pilot
Noun

person who steers a ship or aircraft.

plane
Noun

flat surface of two dimensions (length and width).

radio
Noun

wireless transmission based on electromagnetic waves.

radius
Noun

ray extending from the center of a circle or sphere to its surface or circumference.

sailor
Noun

person who works aboard a ship.

Noun

base level for measuring elevations. Sea level is determined by measurements taken over a 19-year cycle.

sea-level horizon
Noun
apparent boundary between the Earth and sky at sea level, usually a measurement taken from a beach on on the ocean itself.
signal
Verb

to communicate using signs.

Southern Cross
Noun

constellation visible in the Southern Hemisphere.

spherical
Adjective

rounded and three-dimensional.

telegraph
Noun

system of communication involving devices connected through electrical wires.

true horizon
Noun

imaginary plane that passes through the center of the Earth, perpendicular to the radius of the Earth.

visible
Adjective

able to be seen.

visible horizon
Noun

line where Earth and sky seem to meet but are blocked by elevated features of the landscape, such as trees or mountains. Also called the local horizon.

zenith
Noun

point on the celestial sphere directly above a given position.