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

. 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, outreac ...

(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 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. ...

( SI units). In particular, there is a huge quantity of very precise data relating to the positions of objects within 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 ...

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 General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics ...

, which is a necessary addition to the

in order to accurately treat astronomical data. The astronomical system of units is a tridimensional system, in that it defines units of

length Length is a measure of distance. In the International System of Quantities, length is a quantity with dimension distance. In most systems of measurement a base unit for length is chosen, from which all other units are derived. In the Inte ...

mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different ele ...

and

time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past, through the present, into the future. It is a component quantity of various me ...

. The associated

s also fix the different frames of reference 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 A day is the time period of a full rotation of the Earth with respect to the Sun. On average, this is 24 hours, 1440 minutes, or 86,400 seconds. In everyday life, the word "day" often refers to a solar day, which is the length between two ...

, defined as

second 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 ea ...

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 solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass ...

. The symbol is often used to refer to this unit. The solar mass (), , is a standard way to express

, used to describe the masses of other

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 ...

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 System ...

. It is equal to the mass of the Sun, about times the mass of the

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 ...

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-thousand ...

. 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''_JUP), is the unit of

equal to the total mass of the planet

, . Jupiter mass is used to describe masses of the

gas giant A gas giant is a giant planet composed mainly of hydrogen and helium. Gas giants are also called failed stars because they contain the same basic elements as a star. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" ...

s, such as the outer planets 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 mos ...

and Neptune-mass planets.

Earth mass

Earth mass () is the unit of

equal to that of the

. 1 = . Earth mass is often used to describe masses of rocky terrestrial planets. 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 the Gaussian gravitational constant (''k'') takes the value when the units of measurement are the astronomical units of length, mass and time. The dimensions of ''k''² are those of the constant of gravitation (''G''), i.e., L³M⁻¹T⁻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 The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit fo ...

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

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''_A.

Other units for astronomical distances

The distances to distant galaxies are typically not quoted in distance units at all, but rather in terms of

redshift 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 in fr ...

. The reasons for this are that converting redshift to distance requires knowledge of the

Hubble constant Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. In other words, the farther they are, the faster they are moving ...

, 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 dimensions of space and one dimension of time into a single four-dimensional manifold. Spacetime diagrams can be used to visualize relativistic effects, such as why differ ...

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.

References

External links

The IAU and astronomical units

2014 Selected Astronomical Constants
in . {{Authority control * Systems of units