The astronomical system of units, formerly called the IAU (1976) System of Astronomical Constants, is a

_{JUP}), is the unit of mass equal to the total mass of the planet

^{2} are those of the constant of gravitation (''G''), i.e., L^{3}M^{−1}T^{−2}. The term “unit distance” is also used for the length ''A'' while, in general usage, it is usually referred to simply as the “astronomical unit”, symbol au.
An equivalent formulation of the old definition of the astronomical unit is the radius of an unperturbed circular Newtonian orbit about the Sun of a particle having infinitesimal mass, moving with a mean motion of radians per day..
The speed of light in IAU is the defined value ''c''_{0} = of the SI units. In terms of this speed, the old definition of the astronomical unit of length had the accepted value: 1 au = ''c''_{0}''τ''_{A} = () m, where ''τ''_{A} is the transit time of light across the astronomical unit. The astronomical unit of length was determined by the condition that the measured data in the _{A}.

The IAU and astronomical units

2014 Selected Astronomical Constants

in . {{Authority control * Systems of units

system of measurement
A system of measurement is a collection of units of measurement and rules relating them to each other. Systems of measurement have historically been important, regulated and defined for the purposes of science and commerce. Systems of measurement i ...

developed for use in 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, galax ...

. It was adopted by the International Astronomical Union
The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach ...

(IAU) in 1976 via Resolution No. 1, and has been significantly updated in 1994 and 2009 (see astronomical constant An astronomical constant is any of several physical constants used in astronomy. Formal sets of constants, along with recommended values, have been defined by the International Astronomical Union (IAU) several times: in 1964Resolution No.4 of thXIIt ...

).
The system was developed because of the difficulties in measuring and expressing astronomical data in International System of Units (SI unit
The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric system and the world's most widely used system of measurement. E ...

s). In particular, there is a huge quantity of very precise data relating to the positions of objects within the Solar System which cannot conveniently be expressed or processed in SI units. Through a number of modifications, the astronomical system of units now explicitly recognizes the consequences of general relativity, which is a necessary addition to the International System of Units in order to accurately treat astronomical data.
The astronomical system of units is a tridimensional system, in that it defines units of length, mass and time. The associated astronomical constant An astronomical constant is any of several physical constants used in astronomy. Formal sets of constants, along with recommended values, have been defined by the International Astronomical Union (IAU) several times: in 1964Resolution No.4 of thXIIt ...

s also fix the different frames of reference
In physics and astronomy, a frame of reference (or reference frame) is an abstract coordinate system whose origin, orientation, and scale are specified by a set of reference points― geometric points whose position is identified both mathe ...

that are needed to report observations.
In particular, there is the ''barycentric celestial reference system'' (BCRS) centered at the barycenter of the Solar System, and the ''geocentric celestial reference system'' (GCRS) centered at the center of mass of the Earth (including its fluid envelopes)
The system is a conventional system, in that neither the unit of length nor the unit of mass are true physical constant
A physical constant, sometimes fundamental physical constant or universal constant, is a physical quantity that is generally believed to be both universal in nature and have constant value in time. It is contrasted with a mathematical constant, ...

s, and there are at least three different measures of time.
Astronomical unit of time

The astronomical unit of time is the day, defined assecond
The second (symbol: s) is the unit of time in the International System of Units (SI), historically defined as of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds each ...

s. 365.25 days make up one Julian year. The symbol ''D'' is used in astronomy to refer to this unit.
Astronomical unit of mass

The astronomical unit of mass is the solar mass. The symbol is often used to refer to this unit. The solar mass (), , is a standard way to express mass inastronomy
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, galax ...

, used to describe the masses of other star
A star is an astronomical object comprising a luminous spheroid of plasma held together by its gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night, but their immense distances from Eart ...

s and galaxies
A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar Sys ...

. It is equal to the mass of the Sun, about times the mass of the Earth or 1 048 times the mass of Jupiter
Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousandth th ...

.
In practice, the masses of celestial bodies appear in the dynamics of the Solar System only through the products ''GM'', where ''G'' is the constant of gravitation. In the past, ''GM'' of the Sun could be determined experimentally with only limited accuracy. Its present accepted value is .
For complete document see
Jupiter mass

Jupiter mass ( or ''M''Jupiter
Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousandth th ...

, . Jupiter mass is used to describe masses of the gas giants, such as the outer planet
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 "Solar S ...

s and extrasolar planets. It is also used in describing brown dwarfs
Brown dwarfs (also called failed stars) are substellar objects that are not massive enough to sustain nuclear fusion of ordinary hydrogen ( 1H) into helium in their cores, unlike a main-sequence star. Instead, they have a mass between the most ...

and Neptune-mass planets.
Earth mass

Earth mass () is the unit of mass equal to that of the Earth. 1 = . Earth mass is often used to describe masses of rockyterrestrial planet
A terrestrial planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the Sun: Mercury, ...

s. It is also used to describe Neptune-mass planets. One Earth mass is times a Jupiter mass.
Astronomical unit of length

The astronomical unit of length is now defined as exactly 149 597 870 700 meters. It is approximately equal to the mean Earth–Sun distance. It was formerly defined as that length for which theGaussian gravitational constant
The Gaussian gravitational constant (symbol ) is a parameter used in the orbital mechanics of the Solar System.
It relates the orbital period to the orbit's semi-major axis and the mass of the orbiting body in Solar masses.
The value of hist ...

(''k'') takes the value when the units of measurement are the astronomical units of length, mass and time. The dimensions of ''k''ephemeris
In astronomy and celestial navigation, an ephemeris (pl. ephemerides; ) is a book with tables that gives the trajectory of naturally occurring astronomical objects as well as artificial satellites in the sky, i.e., the position (and possibly ...

match observations, and that in turn decides the transit time ''τ''Other units for astronomical distances

The distances to distant galaxies are typically not quoted in distance units at all, but rather in terms ofredshift
In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and simultaneous increase i ...

. The reasons for this are that converting redshift to distance requires knowledge of the Hubble constant, which was not accurately measured until the early 21st century, and that at cosmological distances, the curvature of spacetime
In physics, spacetime is a mathematical model that combines the three-dimensional space, three dimensions of space and one dimension of time into a single four-dimensional manifold. Minkowski diagram, Spacetime diagrams can be used to visualize S ...

allows one to come up with multiple definitions for distance. For example, the distance as defined by the amount of time it takes for a light beam to travel to an observer is different from the distance as defined by the apparent size of an object.
See also

*Astronomical constant An astronomical constant is any of several physical constants used in astronomy. Formal sets of constants, along with recommended values, have been defined by the International Astronomical Union (IAU) several times: in 1964Resolution No.4 of thXIIt ...

* Standard gravitational parameter
In celestial mechanics, the standard gravitational parameter ''μ'' of a celestial body is the product of the gravitational constant ''G'' and the mass ''M'' of the bodies. For two bodies the parameter may be expressed as G(m1+m2), or as GM when ...

* Planetary mass
* Natural units
References

External links

The IAU and astronomical units

2014 Selected Astronomical Constants

in . {{Authority control * Systems of units