Horizon
The horizon (from Greek orizein, to limit) is the line that separates earth from sky. But at many places the true horizon cannot be seen as there are trees, buildings, mountains and so forth. The line is then called visible horizon.
Horizon Media
The curvature of the horizon is easily seen in this 2008 photograph, taken from a Space Shuttle at an altitude of 226 km (140 mi).
View of the ocean with two ships: one in the foreground and one to the left of it on the horizon
Geometrical basis for calculating the distance to the horizon, tangent-secant theorem
Geometrical distance to the horizon, Pythagorean theorem
Three types of horizon. An approximate formula for the distance to the true horizon in metric units (height in meters and distance in kilometers), accurate only for small values of h.
How far away is the horizon
Maximum zenith angle for elevated observer in homogeneous spherical atmosphere
Geometrical horizon distance
A view across a 20-km-wide bay in the coast of Spain. Note the curvature of the Earth hiding the base of the buildings on the far shore.
.JPG)
