Improper motion

The apparent place of an object is its position in space as seen by the observer. Because of physical and/or geometrical effects it differs from the "true" or "geometric" position.

In astronomy, a distinction is made between the true position, apparent position and topocentric position of an object.

The true position of a star is derived from its fixed position at an arbitrary epoch, together with its actual motion over time (known as Proper motion). The apparent position means its visual or photographic position, as seen by a theoretical observer at the centre of the moving Earth, relative to the observer's adopted coordinate system. Several effects cause the apparent position to differ from the true position:

The Apparent Places of Fundamental Stars (commonly abbreviated APFS) is an astronomical yearbook, which is published one year in advance by the Astronomisches Rechen-Institut in Heidelberg, Germany. It lists the apparent place of about 1000 fundamental stars for every 10 days and is published as a book and in a more extensive version on the Internet.

The apparent position of a planet or other object in the Solar System is also affected by Light-time correction, which is caused by the finite time it takes light from a moving body to reach the observer. Simply put, the observer sees the object in the position where it was when the light left it.

Theoretically, light-time correction could also be calculated for more distant objects, such as stars, but in practice it is ignored because the distance and relative motion of the object are usually not known with sufficient precision. Therefore, the adopted "true position" already includes the effect of this imprecisely-known light-time correction.

The topocentric position of a body is that seen by an actual observer on the Earth, and differs from the apparent position as a result of the following effects:

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