• 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.
    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.

  • Term Part of Speech Definition Encyclopedic Entry
    altitude Noun

    the distance above sea level.

    Encyclopedic Entry: altitude
    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.

    beach Noun

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

    Encyclopedic Entry: beach
    boundary Noun

    line separating geographical areas.

    Encyclopedic Entry: boundary
    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


    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.

    horizon Noun

    line where the Earth and the sky seem to meet.

    Encyclopedic Entry: horizon
    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.

    sea level Noun

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

    Encyclopedic Entry: sea level
    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.