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The astronomical system of units, formally called the IAU (1976) System of Astronomical Constants, is a system of measurement developed for use in astronomy.

The astronomical system of units is a tridimensional system, in that it defines units of length, mass and time. 

Astronomical unit of time

The astronomical unit of time is the day, defined as 86400 seconds. 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 M is often used to refer to this unit.

The solar mass (M), 1.98892×1030 kg, is a standard way to express mass in astronomy, used to describe the masses of other stars and galaxies.

It is equal to the mass of the Sun, about 333,000 times the mass of the Earth or 1,048 times the mass of Jupiter.

Astronomical unit of length

In general usage, it is usually referred to simply as the “astronomical unit”, symbol au, AU or ua.

The speed of light in IAU is the defined value co = 299 792 458 m/s of the SI units.

Astronomical units for distances:

Astronomical Range

Typical Units

Distances to satellites


Distances to near-Earth objects

lunar distance


astronomical units, gigametres

Distances to nearby stars

parsecs, light-years

Distances at the galactic scale


Distances to nearby galaxies



One parsec is the distance at which one astronomical unit subtends an angle of one arc second.

A parsec is equal to about 3.26 light-years (31 trillion kilometres or 19 trillion miles) in length.

The nearest star, Proxima Centauri, is about 1.3 parsecs from the Sun.

Light Year

A light-year is a unit of length used informally to express astronomical distances.

It is approximately 10 trillion kilometres (or about 6 trillion miles),

As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in vacuum in one Julian year.

The light-year is most often used when expressing distances to stars and other distances on a galactic scale, especially in non-specialist and popular science publications.

The unit usually used in professional astrometry is the parsec (symbol: pc, approximately 3.26 light-years; the distance at which one astronomical unit subtends an angle of one second of arc).

Earth mass

Earth mass (M) is the unit of mass equal to that of the Earth. 1 M = 5.9742 × 1024 kg.

Earth mass is often used to describe masses of rocky terrestrial planets.

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