Shipman, Harry L. Department of Physics, University of Delaware, Newark, Delaware.
Freedman, Wendy L. Observatories of the Carnegie Institution for Science, Pasadena, California.
Last reviewed:December 2019
- Black holes
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The apparent boundary between earth and sky. Without complicating factors, it would be a line located 90° from the zenith or overhead point; because of trees, hills, or buildings, the visible horizon differs from the ideal one. Even with a perfectly flat surface, as in a view over the ocean, the horizon is slightly depressed from 90° by an angle corresponding to the observer's altitude and further slightly depressed if the curvature of the Earth is taken into account. See also: Astronomical coordinate systems; Celestial sphere
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