The displacement in apparent direction of a celestial object caused by the refraction of its light in passing through the Earth's atmosphere. Refraction increases the observed altitude of an object. For an object on the horizon, the amount is about 34′ (i.e. just over half a degree). According to a simple atmospheric model, the refraction at zenith distances up to about 45° is proportional to the tangent of the zenith distance, but the exact figure depends on the atmospheric pressure, temperature, and humidity. See also Horizontal Refraction.
Subjects: Astronomy and Astrophysics.