| diurnal parallax (geocentric parallax) |
|
The apparent difference between the position of a celestial object measured from the Earth's surface and the position that would be recorded by a hypothetical observer at the centre of the Earth.
The effect of diurnal parallax causes nearby celestial objects to move against the background of more distant stars as the Earth rotates daily on its axis. In practice, the effect is so small as to be detectable only for objects within the solar system. It varies from a maximum when the object is on the horizon to a minimum when it is on the meridian. The maximum value is called the horizontal parallax. Since the Earth is not spherical,
the value of horizontal parallax varies with latitude. The standard value quoted is normally that at the equator, the equatorial horizontal parallax. See also: parallax. |