- The apparent line where the sky meets the land or sea. At sea, its
distance varies in proportion to the square root of the height of the
observer's eyes above sea level: if this is, say, 2 meters the horizon
will be about 5.57 km distant.
- Astronomical horizon (or sensible horizon):
the great circle formed by the
intersection of the celestial sphere
with a plane perpendicular to the line from an observer to the zenith;
in other words, the great circle whose poles are the nadir
- Cosmological horizon: the maximum distance from us
that light has traveled since the beginning of the universe; objects
farther away are invisible to us because there has not been enough time
for light to travel from them to Earth.