Horizontal parallax

Horizontal parallax, difference between geocentric coordinates and topocentric coordinates for an object that is above the horizon of the observer.

In astronomy it is the angle under which the radius of the Earth would be seen from a star when it is on the horizon. If the observer is located on the equator, then this parallax receives the special name of equatorial horizontal parallax. The value is maximum at the Earth's equator and varies with latitude, as the Earth is not completely spherical.