Astronomical system of units

The astronomical system of units, formally called the IAU (1976) System of Astronomical Constants), is a system of units developed for use in astronomy. It was adopted by the International Astronomical Union (IAU) in 1976, and has been slightly updated since then.

The system was developed due to the different difficulties in the measurement and expression of astronomical data in the International System of Units (SI units), when dealing with very large magnitudes. In particular, there is an enormous amount of very precise data relating to the position of objects within the solar system that cannot be expressed, or conveniently treated, in SI units. Through a series of modifications, the astronomical system of units now explicitly recognizes the consequences of general relativity, which is a necessary complement to the International System of Units, in order to accurately deal with astronomical data.

The astronomical system of units is a three-dimensional system, in which the units of length, mass and time are defined. The associated astronomical constants also fix the various reference frames that are needed to report observations. The system is a conventional system, in which 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: day, defined as 86,400 seconds. 365.25 days constitutes a Julian year. Astronomy uses the "D" symbol to refer to this unit.

• Astronomical mass unit: solar mass. Astronomy often uses the "S" symbol to refer to this unit, although it is also common "M" ». The solar mass (M_Δ), 1,98892 × 10³⁰ kg, is a standard way to express the mass in astronomy, used to describe the masses of other stars and galaxies. It is equal to the mass of the Sun, approximately 333,000 times the mass of the Earth, or 1,048 times the mass of Jupiter.

MΔ Δ =1,98892× × 1030kg{displaystyle M_{odot }=1,98892times 10^{30}{hbox{kg}}}}

• Astronomical unit of length (abbreviation) ua, au, UA or AU). It is a unit of length equal by definition to 149,597,870,700 meters, which is approximately the average distance between the planet Earth and the Sun. This definition is in force since the general assembly of the International Astronomical Union of 31 August 2012, in which the Gaussian definition used since 1976 was left without effect, which was "the radius of a circular Newtonian orbit and free of disturbances around the Sun described by an infinitesimal mass particle that moves on average to 0.01720209895 radians per day".

The symbol ua is recommended by the International Bureau of Weights and Measurements and the international standard ISO 80000, while au is the only one considered valid by the International Astronomical Union, and the most common in the Anglo-speaking countries. It is also common to see the symbol written in capital letters, UA or AU, even though the International Unit System uses capital letters only for the symbols of the units bearing the name of a person.

Other units of mass (not from the UAI)

Although they are not SI units or IAU units, the following units are sometimes used in astronomy.

Mass of Jupiter

The mass of Jupiter (M_J or M_JUP), is the unit of mass equal to the total mass of the planet Jupiter, 1.8986 × 10²⁷kg. Jupiter's mass is used to describe the masses of gas giants such as the outer planets and extrasolar planets. It is also used in the description of brown dwarfs.

MJ=1,8986× × 1027kg{displaystyle M_{J}=1,896times 10^{27}{hbox{ kg}}}}{hbox{ kg}}}}

Mass of Earth

The mass of the Earth (M_⊕) is the unit of mass equal to the mass of the Earth. 1 M_⊕ = 5.9742 × 10²⁴ kg. The mass of the Earth is often used to describe the masses of rocky terrestrial planets. The mass of the Earth is 0.00315 times the mass of Jupiter.

M =5.9742× × 1024kg{displaystyle M_{oplus }=5.9742times 10^{24}{hbox{ kg}}}}

Other astronomical units of distance (not from the UAI)

Distances to distant galaxies are usually not quoted in units of distance at all, but rather in terms of redshift. The reasons for this are that the conversion of redshifts to distances requires knowledge of the Hubble constant, which was not precisely measured until the turn of the century XXI, and that, at cosmological distances, the curvature of space-time allows various definitions of distance to be reached. For example, the distance defined as the amount of time it takes for a beam of light to travel to us is different from the distance defined by the apparent size of an object.

Other distance units used more or less informally are the following:

Other astronomical units

Some complementary units are also used, such as the Jansky, which measures the Gloss'