The Horizon

The *Horizon* is the line that separates Earth from Sky.
More precisely, it is the line that divides all of the directions you can
possibly look into two categories: those which intersect the Earth,
and those which do not. At many locations, the Horizon is
obscured by trees, buildings, mountains, etc. However, if you are on
a ship at sea, the Horizon is strikingly apparent.

The horizon is the *Fundamental Plane* of the Horizontal Coordinate System. In other
words, it is the locus of points which have an *Altitude*
of zero degrees.