An imaginary circle that delimits the sky and the Earth.
The fundamental great circle of the → horizon system,
defined by the intersection of the → celestial sphere
and a level plane passing through the observer.
→ celestial horizon.
In → Robertson-Walker models, the boundary separating
objects already observed from those not yet observed, or the boundary separating objects
observable from unobservable (J. Plebanski, A. Krasinski, 2006, An Introduction to
General Relativity and Cosmology, Cambridge Univ. Press).
→ cosmic horizon.
→ event horizon.
See also:
→ apparent horizon,
→ artificial horizon,
→ astronomical horizon,
→ dip of the horizon,
→ distance to the horizon,
→ geometric horizon,
→ horizon coordinate system,
→ horizon problem,
→ horizon system,
→ particle horizon,
→ sea horizon,
→ sensible horizon,
→ sound horizon,
→ true horizon,
→ visible horizon.
Etymology (EN): From O.Fr. orizon, from orizonte, from L. horizontem
(nom. horizon), from Gk. horizon kyklos “bounding circle,”
from horizein “bound, limit, divide, separate,” from horos “boundary.”
Etymology (PE): Ofoq, from Ar.
