celestial-horizon

Science Dictionary
celestial horizon  
A great circle on the celestial sphere having a plane that passes through the center of the Earth at a right angle to the line formed by an observer's zenith and nadir. The celestial horizon divides the celestial sphere into two equal hemispheres based on the observer's location, with one hemisphere representing the half of the sky visible to the observer at that location and the other representing the half that is hidden from the observer below the Earth's horizon. The celestial horizon is used as the reference point in determining a celestial body's altitude. Also called rational horizon. Compare sensible horizon.
American Heritage
Science Dictionary
horizon   (hə-rī'zən)  Pronunciation Key 
    1. The apparent intersection of the Earth and sky as seen by an observer. Also called apparent horizon.

    2. See celestial horizon.

    3. See sensible horizon.

    4. A specific position in a stratigraphic column, such as the location of one or more fossils, that serves to identify the stratum with a particular period.

    5. A specific layer of soil or subsoil in a vertical cross-section of land.

  1. Geology

    1. A specific position in a stratigraphic column, such as the location of one or more fossils, that serves to identify the stratum with a particular period.

    2. A specific layer of soil or subsoil in a vertical cross-section of land.

  2. Archaeology A period during which the influence of a particular culture spread rapidly over a defined area.


The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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