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.
Position of a star
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:
- Annual aberration — a deflection caused by the velocity of the Earth's motion around the Sun, relative to an inertial frame. This is independent of the distance of the star from the Earth.
- Annual parallax — the apparent change in position due to the star being viewed from different places as the Earth orbits the Sun in the course of a year. Unlike aberration, this effect depends on the distance of the star, being larger for nearby stars.
- Precession — a long-term (ca. 26,000 years) variation in the direction of the Earth's axis of rotation.
- Nutation — shorter-term variations in the direction of the Earth's axis of rotation.
- Gravitational light deflection — the deflection of the path of light from the object by one or more massive bodies (an effect of General relativity)
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.
Solar System objects
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:
- Diurnal aberration — a deflection caused by the velocity of the observer's motion around the Earth's centre, due to its rotation.
- Diurnal parallax — the apparent change in position due to the object being viewed from different places as the observer's position rotates around the Earth's axis.
- Polar motion — small changes in the position of the Earth's axis of rotation relative to its surface.
- Atmospheric refraction — a deflection of the light from the object caused by its passage through the Earth's atmosphere.
- Geodetic astronomy
- Local apparent time
- Meridian circle
- Passage instrument
- Star position
- Zenith camera
- P. Kenneth Seidelmann (ed.), pp 99-140, Explanatory Supplement to the Astronomical Almanac: A Revision to the Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac (Sausalito [CA]: University Science Books, 1992).