astronomical system of units

{{Main|Day}}

The astronomical unit of time is the day, defined as {{val|86400}} seconds. 365.25 days make up one Julian year.{{Citation

| editor-last=Müller | editor-first=Edith A.

| editor2-last=Jappel | editor2-first=A.

| year=1977

| title=Proceedings of the Sixteenth General Assembly Grenoble 1976

| series=Transactions of the International Astronomical Union

| section=Report of Joint Meeting of Commissions 4, 8, 31 on the New System of Astronomical Constants

| volume=16B

| pages=52-67

| url=https://archive.org/details/proceedingsofsix0016inte/page/52/

| publisher=D. Reidel

| place=Dordrecht, Holland

| isbn=90-277-0836-3

}}. 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.

{{Main|Solar mass}}

The astronomical unit of mass is the solar mass. The symbol {{Solar mass}} is often used to refer to this unit.

The solar mass ({{Solar mass}}), {{val|1.98892|e=30|ul=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 {{val|333000}} times the mass of the Earth or 1 048 times the mass of Jupiter.

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 {{nowrap|1=G{{Solar mass}} = {{val|1.32712442099|(10)|e=20|u=m³⋅s⁻²}}}}.{{cite web |title=Table 1.1: IERS numerical standards |work=IERS technical note no. 36: General definitions and numerical standards |editor=Gérard Petit and Brian Luzum |url=http://tai.bipm.org/iers/conv2010/chapter1/tn36_c1.pdf |publisher=International Earth Rotation and Reference Systems Service |date=2010 }} For complete document see {{cite book |title=IERS Conventions (2010): IERS technical note no. 36 |editor=Gérard Petit and Brian Luzum |isbn=978-3-89888-989-6 |url=http://www.iers.org/nn_11216/IERS/EN/Publications/TechnicalNotes/tn36.html |publisher=International Earth Rotation and Reference Systems Service |date=2010 |access-date=2012-01-17 |archive-date=2019-06-30 |archive-url=https://web.archive.org/web/20190630104818/https://www.iers.org/nn_11216/IERS/EN/Publications/TechnicalNotes/tn36.html |url-status=dead }}

{{Main|Jupiter mass}}

Jupiter mass ({{Jupiter mass}} or M_JUP), is the unit of mass equal to the total mass of the planet Jupiter, {{val|1.898|e=27|ul=kg}}. Jupiter mass is used to describe masses of the gas giants, such as the outer planets and extrasolar planets. It is also used in describing brown dwarfs and Neptune-mass planets.

{{Main|Earth mass}}

Earth mass ({{Earth mass|sym=y}}) is the unit of mass equal to that of the Earth. 1 {{Earth mass|sym=y}} = {{val|5.9742|e=24|ul=kg}}. 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 {{val|0.00315}} times a Jupiter mass.

{{Main|Astronomical unit}}

The astronomical unit of length is now defined as exactly 149 597 870 700 meters.{{citation |contribution=RESOLUTION B2 on the re-definition of the astronomical unit of length |title=RESOLUTION B2 | editor-last = International Astronomical Union |publisher=International Astronomical Union |place=Beijing, China |date=31 August 2012 | contribution-url = http://www.iau.org/static/resolutions/IAU2012_English.pdf |quote=The XXVIII General Assembly of International Astronomical Union … recommends … 1. that the astronomical unit be re-defined to be a conventional unit of length equal to 149 597 870 700 m exactly}} 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 {{val|0.01720209895}} when the units of measurement are the astronomical units of length, mass and time. The dimensions of k² 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 {{val|0.01720209895}} radians per day.{{SIbrochure8th|page=126}}.

The speed of light in IAU is the defined value c₀ = {{val|299792458|u=m/s}} of the SI units. In terms of this speed, the old definition of the astronomical unit of length had the accepted value: 1 au = c₀τ_A = ({{val|149597870700|3}}) 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.

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

• Astronomical constant
• Standard gravitational parameter
• Planetary mass
• Natural units

{{reflist}}

• [http://www.iau.org/public_press/themes/measuring/ The IAU and astronomical units]
• "[http://asa.usno.navy.mil/static/files/2014/Astronomical_Constants_2014.pdf 2014 Selected Astronomical Constants] {{Webarchive|url=https://web.archive.org/web/20131110215339/http://asa.usno.navy.mil/static/files/2014/Astronomical_Constants_2014.pdf |date=2013-11-10 }}" in {{citation | title = The Astronomical Almanac Online | url = http://asa.usno.navy.mil/ | publisher = USNO–UKHO | access-date = 2009-05-05 | archive-date = 2016-12-24 | archive-url = https://web.archive.org/web/20161224174302/http://asa.usno.navy.mil/static/files/2016/Astronomical_Constants_2016.pdf | url-status = dead }}.

{{Units of length used in Astronomy}}

{{Systems of measurement}}

{{Portal bar|Mathematics|Astronomy|Stars|Spaceflight|Outer space|Solar System|Science}}

{{Authority control}}

*

Category:Systems of units