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 a ...

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 the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry in order to explain their origin and their overall evolution. Objects of interest includ ...

, 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 a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

(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 and standardize the position of the points or other geometric elements on a manifold such as Euclidean space. The coordinates are ...

) 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 b ...

, together with its actual motion over time (known as

proper motion Proper motion is the astrometric measure of changes in the apparent places of stars or other celestial objects as they move relative to the center of mass of the Solar System. It is measured relative to the distant stars or a stable referenc ...

). 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 Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

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

velocity Velocity is a measurement of speed in a certain direction of motion. It is a fundamental concept in kinematics, the branch of classical mechanics that describes the motion of physical objects. Velocity is a vector (geometry), vector Physical q ...

of the Earth's motion around the

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

, relative to an

inertial frame of reference In classical physics and special relativity, an inertial frame of reference (also called an inertial space or a Galilean reference frame) is a frame of reference in which objects exhibit inertia: they remain at rest or in uniform motion relative ...

. 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 sightline, lines of sight and is measured by the angle or half-angle of inclination between those two lines. Due to perspective (graphica ...

– 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 is a unit of time based on how long it takes the Earth to orbit the Sun. In scientific use, the tropical year (approximately 365 Synodic day, solar days, 5 hours, 48 minutes, 45 seconds) and the sidereal year (about 20 minutes longer) ...

. 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 o ...

– a long-term (ca. 26,000 years) variation in the direction of the Earth's axis of rotation. *

Nutation Nutation () is a rocking, swaying, or nodding motion in the axis of rotation of a largely axially symmetric object, such as a gyroscope, planet, or bullet in flight, or as an intended behaviour of a mechanism. In an appropriate reference fra ...

– 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 Annual publication, 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 ...

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

Heidelberg Heidelberg (; ; ) is the List of cities in Baden-Württemberg by population, fifth-largest city in the States of Germany, German state of Baden-Württemberg, and with a population of about 163,000, of which roughly a quarter consists of studen ...

, Germany. It lists the apparent place of about 1000

fundamental star The Catalogue of Fundamental Stars is a series of six astrometric catalogues of high precision positional data for a small selection of stars to define a celestial reference frame, which is a standard coordinate system for measuring positions of ...

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 network, global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a internetworking, network of networks ...

Solar System objects

The apparent position of a

planet A planet is a large, Hydrostatic equilibrium, rounded Astronomical object, astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets b ...

or other object in the

Solar System The Solar SystemCapitalization 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 "Sola ...

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 cor ...

, 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 rotational/rotary motion is the circular movement of an object around a central line, known as an ''axis of rotation''. A plane figure can rotate in either a clockwise or counterclockwise sense around a perpendicular axis intersect ...

. * 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 Polar motion of the Earth is the motion of the Earth's rotation, Earth's rotational axis relative to its Earth's crust, crust. This is measured with respect to a reference frame in which the solid Earth is fixed (a so-called ''Earth-centered, Ea ...

– small changes in the position of the Earth's

axis of rotation Rotation or rotational/rotary motion is the circular movement of an object around a central line, known as an ''axis of rotation''. A plane figure can rotate in either a clockwise or counterclockwise sense around a perpendicular axis intersect ...

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 light ...

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

atmosphere An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...

References

External links