What we actually see is the visible horizon, but for our calculation we will just use the geometrical horizon.Īngular size of Earth limb for an observer in altitude. The geometrical horizon is reduced by the skyline (buildings, trees, mountains.) and increased by atmospheric refraction. Geoidal, sensible and geometrical horizons
The angular size of the limb is indeed less than 180°, and smaller as altitude increases. Astronomical or sensible horizon is nearly identical, this is the one from eyes level, and the reference to read the height of a body in the sky with a sextant.īut as soon as the observer gains altitude the separation between Earth and sky moves below geoidal horizon to form a circle, known as the limb, which determines the true, geometrical or geographic horizon. This geoidal horizon (tangential to the geoid) hides half of the sky.
Horizon at ground level appears indeed as a plane taking up 180° of our field of view when looking at nadir.
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