65 relations: Aberration of light, Apsis, Astrometry, Astronomical object, Astronomy, Axial precession, Bede, Celestial coordinate system, Celestial mechanics, Civil time, Constellation, Coordinated Universal Time, Day, Earth, Ecliptic, Ecliptic coordinate system, Ephemeris, Epoch (reference date), Equator, Equatorial coordinate system, Equinox (celestial coordinates), Friedrich Bessel, Greenwich Mean Time, Gregorian calendar, Hebrew calendar, Heliacal rising, Henry Draper Catalogue, Hipparcos, Infinity, International Astronomical Union, International Atomic Time, International Celestial Reference Frame, International Celestial Reference System, Islamic calendar, ΔT, Julian calendar, Julian day, Julian year (astronomy), Longitude of the ascending node, Lunar calendar, Lunisolar calendar, March equinox, Mean anomaly, Mean longitude, Meridian (astronomy), Midnight, Minor planet, Noon, Orbital elements, Orbital mechanics, ..., Osculating orbit, Perihelion and aphelion, Perturbation (astronomy), Plane of reference, Polynomial, Precession, Proper motion, Semi-major axis, Sirius, SOFA (astronomy), Sun, Terrestrial Time, Universal Time, Year, 5145 Pholus. Expand index (15 more) »

## Aberration of light

The aberration of light (also referred to as astronomical aberration or stellar aberration) is an astronomical phenomenon which produces an apparent motion of celestial objects about their locations dependent on the velocity of the observer.

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## Apsis

The apsis (Greek ἁψίς), plural apsides (Greek: ἁψίδες) is an extreme point in an object's orbit.

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## Astrometry

Astrometry is the branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies.

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## Astronomical object

An astronomical object or celestial object is a naturally occurring physical entity, association, or structure that current science has demonstrated to exist in the observable universe.

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## Astronomy

Astronomy is a natural science which is the study of celestial objects (such as stars, galaxies, planets, moons, asteroids, comets and nebulae), the physics, chemistry, and evolution of such objects, and phenomena that originate outside the atmosphere of Earth, including supernovae explosions, gamma ray bursts, and cosmic microwave background radiation.

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## Axial precession

In astronomy, axial precession is a gravity-induced, slow, and continuous change in the orientation of an astronomical body's rotational axis.

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## Bede

Bede (Bǣda or Bēda; 672/673 – 26 May 735), also referred to as Saint Bede or the Venerable Bede (Bēda Venerābilis), was an English monk at the monastery of Saint Peter at Monkwearmouth and its companion monastery, Saint Paul's, in modern Jarrow (see Monkwearmouth-Jarrow), County Durham, both of which were then in the Kingdom of Northumbria.

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## Celestial coordinate system

In astronomy, a celestial coordinate system is a system for specifying positions of celestial objects: satellites, planets, stars, galaxies, and so on.

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## Celestial mechanics

Celestial mechanics is the branch of astronomy that deals with the motions of celestial objects.

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## Civil time

In modern usage, civil time refers to statutory time scales designated by civilian authorities, or to local time indicated by clocks.

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## Constellation

In modern astronomy, a constellation is a specific area of the celestial sphere as defined by the International Astronomical Union (IAU).

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## Coordinated Universal Time

Coordinated Universal Time (temps universel coordonné), abbreviated as UTC, is the primary time standard by which the world regulates clocks and time.

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## Day

A day is a unit of time.

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## Earth

Earth (also the world, in Greek: Gaia, or in Latin: Terra), is the third planet from the Sun, the densest planet in the Solar System, the largest of the Solar System's four terrestrial planets, and the only astronomical object known to accommodate life.

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## Ecliptic

The ecliptic is the apparent path of the Sun on the celestial sphere, and is the basis for the ecliptic coordinate system.

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## Ecliptic coordinate system

The ecliptic coordinate system is a celestial coordinate system commonly used for representing the positions and orbits of Solar System objects.

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## Ephemeris

In astronomy and celestial navigation, an ephemeris (plural: ephemerides; from Latin ephemeris, "diary", from Greek ἐφημερίς, ephēmeris, "diary, calendar") gives the positions of naturally occurring astronomical objects as well as artificial satellites in the sky at a given time or times.

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## Epoch (reference date)

In the fields of chronology and periodization, an epoch is an instant in time chosen as the origin of a particular era.

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## Equator

An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and midway between the poles.

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## Equatorial coordinate system

The equatorial coordinate system is a widely used celestial coordinate system used to specify the positions of celestial objects.

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## Equinox (celestial coordinates)

In astronomy, equinox is a moment in time at which the vernal point, celestial equator, and other such elements are taken to be used in the definition of a celestial coordinate system.

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## Friedrich Bessel

Friedrich Wilhelm Bessel (22 July 1784 – 17 March 1846) was a German astronomer, mathematician, physicist and geodesist.

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## Greenwich Mean Time

Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London.

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## Gregorian calendar

The Gregorian calendar, also called the Western calendar and the Christian calendar, is internationally the most widely used civil calendar.

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## Hebrew calendar

The Hebrew or Jewish calendar (ha'luach ha'ivri) is a lunisolar calendar used today predominantly for Jewish religious observances.

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## Heliacal rising

The heliacal rising of a star occurs annually when it first becomes visible above the eastern horizon for a brief moment just before sunrise, after a period of time when it had not been visible.

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## Henry Draper Catalogue

The Henry Draper Catalogue (HD) is an astronomical star catalogue published between 1918 and 1924, giving spectroscopic classifications for 225,300 stars; it was later expanded by the Henry Draper Extension (HDE), published between 1925 and 1936, which gave classifications for 46,850 more stars, and by the Henry Draper Extension Charts (HDEC), published from 1937 to 1949 in the form of charts, which gave classifications for 86,933 more stars.

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## Hipparcos

Hipparcos was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993.

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## Infinity

Infinity (symbol) is an abstract concept describing something without any limit and is relevant in a number of fields, predominantly mathematics and physics.

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## International Astronomical Union

The International Astronomical Union (IAU; Union astronomique internationale, UAI) is a collection of professional astronomers, at the PhD level and beyond, active in professional research and education in astronomy.

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## International Atomic Time

International Atomic Time (TAI, from the French name Temps Atomique International) is a high-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid.

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## International Celestial Reference Frame

The International Celestial Reference Frame (ICRF) is a quasi-inertial frame of reference centered at the barycenter of the Solar System, defined by the measured positions of 212 extragalactic sources (mainly quasars).

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## International Celestial Reference System

The International Celestial Reference System (ICRS) is the current standard celestial reference system adopted by the International Astronomical Union (IAU).

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## Islamic calendar

The Islamic calendar, Muslim calendar or Hijri calendar (AH) is a lunar calendar consisting of 12 months in a year of 354 days.

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## ΔT

In astrometry, ΔT (Delta T, delta-T, deltaT, or DT) is the time difference obtained by subtracting Universal Time (UT) from Terrestrial Time (TT): ΔT.

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## Julian calendar

The Julian calendar, introduced by Julius Caesar in 46 BC (708 AUC), was a reform of the Roman calendar.

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## Julian day

Julian day is the continuous count of days since the beginning of the Julian Period used primarily by astronomers.

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## Julian year (astronomy)

In astronomy, a Julian year (symbol: a) is a unit of measurement of time defined as exactly 365.25 days of SI seconds each.

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## Longitude of the ascending node

The longitude of the ascending node (☊ or Ω) is one of the orbital elements used to specify the orbit of an object in space.

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## Lunar calendar

A lunar calendar is a calendar that is based on cycles of the lunar phases.

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## Lunisolar calendar

A lunisolar calendar is a calendar in many cultures whose date indicates both the moon phase and the time of the solar year.

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## March equinox

The March equinox or Northward equinox is the equinox on the earth when the Sun appears to cross the celestial equator, heading northward.

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## Mean anomaly

In celestial mechanics, mean anomaly is a parameter relating position and time for a body moving in a Kepler orbit.

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## Mean longitude

Mean longitude is the ecliptic longitude at which an orbiting body could be found if its orbit were circular, and free of perturbations, and if its inclination were zero.

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## Meridian (astronomy)

A meridian is the great circle passing through the celestial poles, the zenith, and the nadir of a particular location.

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## Midnight

Midnight is the transition time period from one day to the next: the moment when the date changes.

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## Minor planet

A minor planet is an astronomical object in direct orbit around the Sun that is neither a planet nor originally classified as a comet.

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## Noon

Noon (also midday or noon time) is usually defined as 12 o'clock in the daytime.

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## Orbital elements

Orbital elements are the parameters required to uniquely identify a specific orbit.

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## Orbital mechanics

Orbital mechanics or astrodynamics is the application of ballistics and celestial mechanics to the practical problems concerning the motion of rockets and other spacecraft.

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## Osculating orbit

In astronomy, and in particular in astrodynamics, the osculating orbit of an object in space at a given moment in time is the gravitational Kepler orbit (i.e. ellipse or other conic) that it would have about its central body if perturbations were not present.

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## Perihelion and aphelion

The perihelion is the point in the orbit of a planet, asteroid, comet or other star-orbiting body where it is nearest to its star.

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## Perturbation (astronomy)

In astronomy, perturbation is the complex motion of a massive body subject to forces other than the gravitational attraction of a single other massive body.

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## Plane of reference

In celestial mechanics, the plane of reference is the plane used to define orbital elements (positions).

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## Polynomial

In mathematics, a polynomial is an expression consisting of variables (or indeterminates) and coefficients, that involves only the operations of addition, subtraction, multiplication, and non-negative integer exponents.

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## Precession

Precession is a change in the orientation of the rotational axis of a rotating body.

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## Proper motion

Proper motion is the astronomical measure of the observed changes in apparent positions of stars in the sky as seen from the center of mass of the Solar System, as compared to the imaginary fixed background of the more distant stars.

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## Semi-major axis

In geometry, the major axis of an ellipse is its longest diameter: a line segment that runs through the center and both foci, with ends at the widest points of the perimeter.

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## Sirius

Sirius is the brightest star (in fact, a star system) in the Earth's night sky.

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## SOFA (astronomy)

The SOFA (Standards of Fundamental Astronomy) software libraries are a collection of subroutines that implement official IAU algorithms for astronomical computations.

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## Sun

The Sun (in Greek: Helios, in Latin: Sol) is the star at the center of the Solar System and is by far the most important source of energy for life on Earth.

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## Terrestrial Time

Terrestrial Time (TT) is a modern astronomical time standard defined by the International Astronomical Union, primarily for time-measurements of astronomical observations made from the surface of Earth.

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## Universal Time

Universal Time (UT) is a time standard based on Earth's rotation.

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## Year

A year is the orbital period of the Earth moving in its orbit around the Sun.

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## 5145 Pholus

5145 Pholus (from) is a centaur in an eccentric orbit, with a perihelion less than Saturn's and aphelion greater than Neptune's.

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## Redirects here:

1875.0, 1900.0, 1950.0, 2000.0, Astronomical epoch, B1875, B1875.0, B1900, B1900.0, B1950, B1950.0, B2000.0, Besselian epoch, Besselian year, Epoch astronomy, Epochal moment, J1900, J1900.0, J1950, J1950.0, J2000, J2000.0, Julian epoch, Standard epoch 2000.

## References

[1] https://en.wikipedia.org/wiki/Epoch_(astronomy)