The apparent place of an

object Object may refer to: General meanings * Object (philosophy), a thing, being, or concept ** Object (abstract), an object which does not exist at any particular time or place ** Physical object, an identifiable collection of matter * Goal, an ...

is its position in space as seen by an observer. Because of physical and geometrical effects it may differ from the "true" or "geometric" position.

Astronomy

astronomy Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, g ...

, a distinction is made between the ''mean position'', ''apparent position'' and ''topocentric position'' of an object.

Position of a star

The mean position of a

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

(relative to the observer's adopted

coordinate system In geometry, a coordinate system is a system that uses one or more numbers, or coordinates, to uniquely determine the position of the points or other geometric elements on a manifold such as Euclidean space. The order of the coordinates is sig ...

) can be calculated from its value at an arbitrary

epoch In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured. The moment of epoch is usually decided ...

, together with its actual motion over time (known as

proper motion Proper motion is the astrometric measure of the observed changes in the apparent places of stars or other celestial objects in the sky, as seen from the center of mass of the Solar System, compared to the abstract background of the more distan ...

). The ''apparent position'' is its position as seen by a theoretical observer at the centre of the moving

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...

. Several effects cause the apparent position to differ from the mean position: * Annual aberration – a deflection caused by the

velocity Velocity is the directional speed of an object in motion as an indication of its rate of change in position as observed from a particular frame of reference and as measured by a particular standard of time (e.g. northbound). Velocity i ...

of the Earth's motion around the Sun, relative to an

inertial frame of reference In classical physics and special relativity, an inertial frame of reference (also called inertial reference frame, inertial frame, inertial space, or Galilean reference frame) is a frame of reference that is not undergoing any acceleration. ...

. This is independent of the distance of the star from the Earth. * Annual

parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between those two lines. Due to foreshortening, nearby object ...

– 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 A year or annus is the orbital period of a planetary body, for example, the Earth, moving in its orbit around the Sun. Due to the Earth's axial tilt, the course of a year sees the passing of the seasons, marked by change in weather, the h ...

. Unlike aberration, this effect depends on the distance of the star, being larger for nearby stars. *

Precession Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In oth ...

– 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. The ''Apparent Places of Fundamental Stars'' is an astronomical

yearbook A yearbook, also known as an annual, is a type of a book published annually. One use is to record, highlight, and commemorate the past year of a school. The term also refers to a book of statistics or facts published annually. A yearbook often ...

, which is published one year in advance by the Astronomical Calculation Institute (Heidelberg University) in

Heidelberg Heidelberg (; Palatine German: ') is a city in the German state of Baden-Württemberg, situated on the river Neckar in south-west Germany. As of the 2016 census, its population was 159,914, of which roughly a quarter consisted of students ...

, Germany. It lists the apparent place of about 1000

fundamental star Fundamental may refer to: * Foundation of reality * Fundamental frequency, as in music or phonetics, often referred to as simply a "fundamental" * Fundamentalism, the belief in, and usually the strict adherence to, the simple or "fundamental" idea ...

s for every 10 days and is published as a book and in a more extensive version on the

Internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a '' network of networks'' that consists of private, p ...

Solar System objects

The apparent position of a

planet A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...

or other object in the

Solar System The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...

is also affected by

light-time correction Light-time correction is a displacement in the ''apparent'' position of a celestial object from its ''true'' position (or geometric position) caused by the object's motion during the time it takes its light to reach an observer. Light-time corr ...

, 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. The movement of an object since the light left it is not needed because the mean position is the mean position of where it appears to be, not of where it once was. Unlike planets, these objects basically appear to move in straight lines, so for normal use no complicated calculation is needed to find their mean position.

Topocentric position

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

of the observer's motion around the Earth's centre, due to its

rotation Rotation, or spin, is the circular movement of an object around a '' central axis''. A two-dimensional rotating object has only one possible central axis and can rotate in either a clockwise or counterclockwise direction. A three-dimensional ...

. * Diurnal

– 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 Rotation around a fixed axis is a special case of rotational motion. The fixed- axis hypothesis excludes the possibility of an axis changing its orientation and cannot describe such phenomena as wobbling or precession. According to Euler's r ...

relative to its surface. *

Atmospheric refraction Atmospheric refraction is the deviation of light or other electromagnetic wave from a straight line as it passes through the atmosphere due to the variation in air density as a function of height. This refraction is due to the velocity of ligh ...

– a deflection of the light from the object caused by its passage through the Earth's

atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A ...

References

External links