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late 14c., orisoun, from Old French orizon (14c., Modern French horizon), earlier orizonte (13c.), from Latin horizontem (nominative horizon), from Greek horizon kyklos "bounding circle," from horizein "bound, limit, divide, separate," from horos "boundary." The h- was restored 17c. in imitation of Latin.
|sensible horizon |
The plane of an observer's position lying at a right angle to the line formed by the observer's zenith and nadir. The plane of the sensible horizon is parallel to the plane of the observer's celestial horizon but is tangential to the Earth's surface rather than passing through the Earth's center. Both the celestial and sensible horizons change with the observer's position. Compare celestial horizon.