An Etymological Dictionary of Astronomy and Astrophysics
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The distance separating an observer and the → apparent horizon of the place. Neglecting the → atmospheric refraction, it is given by: d = (2Rh)^1/2, where R is the radius of the Earth and h is the observer's height. This can be approximated to: d (km) = 3.57(h)^1/2 for a typical value of R = 6378 km. The atmospheric refraction, however, makes the thing more complex, depending on the temperature and density variations along the line of sight. Generally, refraction pushes the apparent horizon about 10% farther.

→ distance; → horizon.