While, the astronomical and true horizons are celestial horizons. The Local horizons, sea-level horizons and geographic horizons are the sub-parts of Earth-sky horizons. There are subtypes of earth-sky horizons and celestial horizons. Especially when the observer is above the equator, as the Earth's surface can be much better modelled as an ellipsoid than as a sphere. In this case, the horizon would no longer be an exact circle, not even a plane curve like an ellipse. When it is observed from high standpoints, like as a space station, then the horizon is far away and it surrounds a much larger area of Earth's surface. For example, in standard atmosphere conditions, for an observer with eye-level above the sea level by 2 metres, the horizon is at a distance of about 6 kilometres. Its distance from the observer differs from day to day due to atmospheric refraction, which is seriously affected by weather conditions.Īlso, the horizon is far away from the observer, the higher the observer's eyes are from sea level. The horizon centre is under the observer and below sea level. The true horizon definition astronomy near the observer is generally assumed to be a circle, drawn on the surface of a spherical model of the Earth. This line is obscured by terrain, towers, trees, houses, and mountains in many places, and the resultant convergence of earth and sky is referred to as the apparent horizon in astronomy.

When looking at a sea from a seaside, then the part of the sea nearest to the horizon is called the offing. The horizon definition in astronomy is defined as, the particular line, which can only be observed when it lies on the sea surface. The study of the horizon is called horizon astronomy. The horizon is a visible line that separates all viewing lines depending on whether or not it intersects the Earth's surface.

Celestial horizon definition astronomy