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

system of measurement A system of units of measurement, also known as a system of units or 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 defi ...

developed for use in

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 ...

. It was adopted by the

International Astronomical Union The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...

(IAU) in 1976 via Resolution No. 1, and has been significantly updated in 1994 and 2009 (see Astronomical constant). The system was developed because of the difficulties in measuring and expressing astronomical data in

International System of Units The International System of Units, internationally known by the abbreviation SI (from French ), is the modern form of the metric system and the world's most widely used system of measurement. It is the only system of measurement with official s ...

(

SI unit The International System of Units, internationally known by the abbreviation SI (from French ), is the modern form of the metric system and the world's most widely used system of units of measurement, system of measurement. It is the only system ...

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

that 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 as Einstein's theory of gravity, is the differential geometry, geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of grav ...

, 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 (physical quantity), dimension distance. In most systems of measurement a Base unit (measurement), base unit for length is chosen, ...

mass Mass is an Intrinsic and extrinsic properties, intrinsic property of a physical body, body. It was traditionally believed to be related to the physical quantity, quantity of matter in a body, until the discovery of the atom and particle physi ...

and

time Time is the continuous progression of existence that occurs in an apparently irreversible process, irreversible succession from the past, through the present, and into the future. It is a component quantity of various measurements used to sequ ...

. The associated astronomical constants also fix the different frames of reference that are needed to report observations. — in particular, 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) (See Barycentric and geocentric celestial reference systems.) It is a conventional system, in that neither the unit of length nor the unit of mass are true physical constants, 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 rotation period, period of a full Earth's rotation, rotation of the Earth with respect to the Sun. On average, this is 24 hours (86,400 seconds). As a day passes at a given location it experiences morning, afternoon, evening, ...

, defined as seconds. 365.25 days make up one Julian year.. Refer to subsection "Recommendations to IAU General Assembly, 1976" on pages 58–61 for specific values. 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

, used to describe the masses of other

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 ...

s and galaxies. 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 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 ...

or 1 048 times the mass of

Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a Jupiter mass, mass more than 2.5 times that of all the other planets in the Solar System combined a ...

. 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. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" was originally synonymous with "giant planet". However, in the 1990s, it became known that Uranu ...

s, such as the outer planets and extrasolar planets. It is also used in describing brown dwarfs 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 planet A terrestrial planet, tellurian 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 ...

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 the

Gaussian 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 histor ...

(''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 exactly equal to ). It is exact because, by international agreement, a metre is defined as the length of the path travelled by light in vacuum during a time i ...

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 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 increase in frequency and e ...

. 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, also called the space-time continuum, is a mathematical model that fuses the three dimensions of space and the one dimension of time into a single four-dimensional continuum. Spacetime diagrams are useful in visualiz ...

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