Astronomical unit

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Astronomical unit

An astronomical unit (abbreviated as AU, au, a.u., or sometimes ua) is a unit of length

Length

Length is the long dimension of any object. The length of a thing is the distance between its ends, its linear extent as measured from end to end....
based on the mean distance from the Earth

Earth

Earth is the third planet from the Sun. Earth is the largest of the terrestrial planets in the Solar System in diameter, mass and density. It is also referred to as the World and Wiktionary:Terra.Note that by International Astronomical Union convention, the term "Terra" is used for naming extensive land masses, rather...
to the Sun

Sun

The Sun , a G V star, is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 98.6% of the Solar System's mass....
. The precise value of the AU is currently accepted as 149,597,870,691 ±

Plus-minus sign

The plus-minus sign is a mathematical symbol commonly used to indicate the accuracy and precision of an approximation, or as a convenient notation for a value that can be of either sign....
6 metres (nearly 150 million kilometre

Kilometre

The kilometre , symbol km is a Units of measurement of length in the metric system, equal to one thousand metres.Slang terms for kilometre include click and kay ....
s or 93 million mile

Mile

A mile is a Units of measurement of length, usually used to measure distance, in a number of different systems. In contemporary English contexts, mile most commonly refers to the statute mile of 5,280 Feet or the nautical mile of 1,852 meters ....
s).

The symbol ua is recommended by the Bureau International des Poids et Mesures

International Bureau of Weights and Measures

File:Metric seal.svgThe International Bureau of Weights and Measures , is an international standards organization, one of three such organizations established to maintain the International System of Units under the terms of the Metre Convention ....
but in anglophone

Anglophone

An Anglophone is someone who speaks the English language. As an adjective, it refers to belonging to an English-speaking population especially in a country where two or more languages are spoken....
countries the reverse - au - is more common.

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Encyclopedia

An astronomical unit (abbreviated as AU, au, a.u., or sometimes ua) is a unit of length

Length

Length is the long dimension of any object. The length of a thing is the distance between its ends, its linear extent as measured from end to end....
based on the mean distance from the Earth

Earth

Sun

Plus-minus sign

Kilometre

The kilometre , symbol km is a Units of measurement of length in the metric system, equal to one thousand metres.Slang terms for kilometre include click and kay ....
s or 93 million mile

Mile

International Bureau of Weights and Measures

Anglophone

International Astronomical Union

The International Astronomical Union is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy....
recommends au and international standard

International standard

International standards are standards developed by international standards organisations. International standards are available for consideration and use, worldwide....
ISO 31-1

ISO 31-1

ISO 31-1 is the part of international standard ISO 31 that defines names and symbols for physical quantity and physical units related to space and time....
uses AU. In general, capital letters are only used for the symbols of units which are named after individual scientists, while au or a.u. can also mean atomic unit or even arbitrary unit; however, the use of AU to refer to the astronomical unit is widespread. It is AU, not au or ua, according to the Merriam-Webster's Collegiate Dictionary, the American Heritage College Dictionary, and the Oxford Dictionary of English.

Originally, the AU was defined as the length

Length

Length is the long dimension of any object. The length of a thing is the distance between its ends, its linear extent as measured from end to end....
of the semi-major axis

Semi-major axis

In geometry, the semi-major axis is used to describe the dimensions of ellipses and hyperbolae....
of the Earth's elliptical orbit around the Sun

Sun

The Sun , a G V star, is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 98.6% of the Solar System's mass....
. In 1976, the International Astronomical Union revised the definition of the AU for greater precision, defining it as the distance from the centre of the Sun at which a particle

Test particle

In Theoretical physics, a test particle is an idealized model of an object whose physical properties are assumed to be negligible except for the property being studied, which is considered to be insufficient to alter the behavior of the rest of the system....
of negligible mass

Mass

In physical science, mass refers to the degree of acceleration a body acquires when subject to a force: bodies with greater mass are accelerated less by the same force....
, in an unperturbed circular orbit, would have an orbital period

Orbital period

The orbital Periodicity is the time taken for a given object to make one complete orbit about another object.When mentioned without further qualification in astronomy this refers to the sidereal period of an astronomical object, which is calculated with respect to the stars....
of 365.2568983 days (one Gaussian year

Gaussian year

A Gaussian year is defined as 365.2568983 days. It was adopted by Carl Friedrich Gauss as the length of the sidereal year in his studies of the dynamics of the solar system....
). This definition gives a value that is slightly less than the mean Earth-Sun distance. An alternative way of stating the definition is that an AU is the distance for which the heliocentric gravitational constant

Standard gravitational parameter

In astrodynamics, the standard gravitational parameter of a celestial body is the product of the gravitational constant and the mass :The units of the standard gravitational parameter are km3s-2...
(the product GM_?) is equal to (0.017 202 098 95)² AU³/d².

History

Aristarchus of Samos

Aristarchus or Aristarch was a Greeks astronomer and mathematician, born on the island of Samos Island, in Greece. He was the first Greek, and the first man in general, to present an explicit argument for a Heliocentrism of the solar system, placing the Sun, not the Earth, at the center of the known universe....
estimated the distance to the Sun to be about 20 times the distance to the moon, whereas the true ratio is about 390. His estimate was based on the angle between the half moon

Moon

The Moon is Earth's only natural satellite and the List of natural satellites by diameter satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is km, about thirty times the diameter of the Earth....
and the sun, which he estimated as 87°.uc

According to Eusebius of Caesarea

Eusebius of Caesarea

Eusebius of Caesarea became the bishop of Caesarea Maritima c 314. He is often referred to as the Father of Church History because of his work in recording the history of the early Christianity church, especially Chronicon and Church_History_....
in the Praeparatio Evangelica
Preparation for the Gospel

???pa?as?e?? ??a??e???? , commonly known by its Latin title Praeparatio evangelica, was a work by Eusebius which attempts to prove the excellence of Christianity over every pagan religion and philosophy....
, Eratosthenes

Eratosthenes

Eratosthenes of Cyrene was a Greeks mathematician, poet, sportsperson, geographer and astronomer. He made several discoveries and inventions including a system of latitude and longitude....
found the distance to the sun to be "stad??? µ???ada? tet?a??s?a? ?a? ??t???sµ???a?" (literally "of stadia myriad

Myriad

Myriad is a classical Greek language name for the number 104 = 10000 . In modern English language the word refers to an unspecified large quantity....
s 400 and 80000"). This has been translated either as 4,080,000 stadia (1903 translation by Edwin Hamilton Gifford), or as 804,000,000 stadia (edition of Édouard des Places, dated 1974-1991). Using the Greek stadium of 185 to 190 metres, the former translation comes to a far too low 755,000 km whereas the second translation comes to 148.7 to 152.8 million km (accurate within 2%).

At the time the AU was introduced, its actual value was very poorly known, but planetary distances in terms of AU could be determined from heliocentric geometry and Kepler's laws of planetary motion

Kepler's laws of planetary motion

In astronomy, Kepler's three laws of planetary motion are*"The orbit of every planet is an ellipse with the sun at a Focus ."*"A line joining a planet and the sun sweeps out equal areas during equal intervals of time."...
. The value of the AU was first estimated with reasonable accuracy by Jean Richer

Jean Richer

Jean Richer was a French astronomer and assistant of Giovanni Domenico Cassini.Between 1671 and 1673 he traveled to Cayenne at the request of the French Academy of Sciences to observe Mars during its perigee....
and Giovanni Domenico Cassini

Giovanni Domenico Cassini

This article is about the Italian-born astronomer. For his French-born great-grandson, see Dominique, comte de Cassini.Giovanni Domenico Cassini was an Italy/France mathematician, astronomer, engineer, and astrologer....
in 1672. By measuring the parallax

Parallax

Parallax is an apparent displacement or difference of orientation of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines....
of Mars

MARS

In cryptography, MARS is a block cipher that was IBM's submission to the Advanced Encryption Standard process. MARS was selected as an AES finalist in August 1999, after the AES2 conference in March 1999, where it was voted as the fifth and last finalist algorithm....
from two locations on the Earth, they arrived at a figure of about 140 million kilometers.

A somewhat more accurate estimate can be obtained by observing the transit of Venus

Transit of Venus

A transit of Venus across the Sun takes place when the planet Venus passes directly between the Sun and Earth, obscuring a small portion of the solar disk....
. Jeremiah Horrocks

Jeremiah Horrocks

Jeremiah Horrocks , sometimes given as Jeremiah Horrox , was an England astronomer who was the only person to predict, and one of only two people to observe and record, the transit of Venus of 1639....
had attempted to produce an estimate based on his observation of the 1639 transit (published in 1662), producing a figure of 95 million kilometres. A better method was devised by James Gregory

James Gregory (astronomer and mathematician)

James Gregory , was a Scotland mathematician and astronomer. It has been said that "Of the British mathematicians of the seventeenth century, Gregory was only excelled by Isaac Newton."...
and published in his Optica Promata. It was strongly advocated by Edmond Halley

Edmond Halley

Edmond Halley Royal Society was an English astronomer, geophysicist, mathematician, meteorologist, and physicist.Biography and career ...
and was applied to the transits of Venus observed in 1761 and 1769, and then again in 1874 and 1882.

Another method involved determining the constant of aberration

Aberration of light

The aberration of light is an astronomical phenomenon which produces an improper motion of celestial objects about their real locations. It was discovered and later explained by the third Astronomer Royal, James Bradley, in 1725, who attributed it to the finite speed of light and the motion of Earth in its orbit around the Sun....
, and Simon Newcomb

Simon Newcomb

Simon Newcomb was a Canadaian-U.S. astronomer and mathematician. Though he had little conventional schooling, he made important contributions to timekeeping as well as writing on economics, statistics and authoring a science fiction novel....
gave great weight to this method when deriving his widely accepted value of 8.80? for the solar parallax (close to the modern value of 8.794148?).

The discovery of the near-Earth asteroid 433 Eros

433 Eros

433 Eros is the first discovered Near-Earth asteroid, named after the Greek mythology of love, Eros . It is an S-type asteroid approximately 34.4?11.2?11.2 km in size, the second-largest near-Earth asteroid after 1036 Ganymed, belonging to the Amor asteroid....
and its passage near the Earth in 1900–1901 allowed a considerable improvement in parallax measurement. More recently very precise measurements have been carried out by radar

Radar

Radar is a system that uses electromagnetic radiation waves to identify the range, altitude, direction, or speed of both moving and fixed objects such as aircraft, ships, motor vehicles, weather formations, and terrain....
and by telemetry

Telemetry

Telemetry is a technology that allows the remote measurement and reporting of information of interest to the system designer or operator. The word is derived from Greek language roots tele = remote, and metron = measure....
from space probe

Space probe

A robotic spacecraft is a spacecraft with no humans on board, that is usually under telerobotic control. A robotic spacecraft designed to make scientific research measurements is often called a space probe....
s.

While the value of the astronomical unit is now known to great precision, the value of the mass of the Sun is not, because of uncertainty in the value of the gravitational constant

Gravitational constant

The gravitational constant, denoted G, is an empirical physical constant involved in the calculation of the gravitation between objects with mass....
. Because the gravitational constant is known to only five or six significant digits while the positions of the planets are known to 11 or 12 digits, calculations in celestial mechanics are typically performed in solar masses and astronomical units rather than in kilograms and kilometres. This approach makes all results dependent on the gravitational constant. A conversion to SI

Si, si, or SI may refer to :...
units would separate the results from the gravitational constant, at the cost of introducing additional uncertainty by assigning a specific value to that unknown constant.

Examples

The distances are approximate mean distances. It has to be taken into consideration that the distances between celestial bodies

Astronomical object

s are significant entity, associations or structures which current science has confirmed to exist in outer space. This does not necessarily mean that more current science will not disprove their existence....
change in time

Time

Time is a component of the measurement used to sequence events, to compare the durations of events and the intervals between them, and to quantify the motions of objects....
due to their orbit

ORBit

ORBit is a Common Object Request Broker Architecture 2.4 compliant Object Request Broker . It features mature C , C++ and Python bindings, and less developed bindings for Perl, Lisp , Pascal , Ruby , and Tcl....
s and other factors.

The Earth
Earth

Earth is the third planet from the Sun. Earth is the largest of the terrestrial planets in the Solar System in diameter, mass and density. It is also referred to as the World and Wiktionary:Terra.Note that by International Astronomical Union convention, the term "Terra" is used for naming extensive land masses, rather...
is 1.00 ± 0.02 AU from the Sun
Sun

The Sun , a G V star, is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 98.6% of the Solar System's mass....
.
The Moon
Moon

The Moon is Earth's only natural satellite and the List of natural satellites by diameter satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is km, about thirty times the diameter of the Earth....
is 0.0026 ± 0.0001 AU from the Earth.
Mars
MARS

In cryptography, MARS is a block cipher that was IBM's submission to the Advanced Encryption Standard process. MARS was selected as an AES finalist in August 1999, after the AES2 conference in March 1999, where it was voted as the fifth and last finalist algorithm....
is 1.52 ± 0.14 AU from the Sun.
Jupiter
Jupiter

Jupiter is the fifth planet from the Sun and the Solar system by size planet within the Solar System. It is two and a half times as massive as all of the other planets in our Solar System combined....
is 5.20 ± 0.05 AU from the Sun.
Pluto
Pluto

Pluto , Minor planet names Pluto, is the second-largest known dwarf planet in the Solar System and the tenth-largest body observed directly orbiting the Sun....
is 39.5 ± 9.8 AU from the Sun.
The Kuiper Belt
Kuiper belt

The Kuiper belt , sometimes called the Edgeworth-Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 55 Astronomical unit from the Sun....
begins at roughly 35 AU
Beginning of Scattered disk at 45 AU (10 AU overlap with Kuiper Belt)
Ending of Kuiper Belt at 50-55 AU
90377 Sedna
90377 Sedna

90377 Sedna is a trans-Neptunian object and a likely dwarf planet, discovered by Michael E. Brown , Chad Trujillo and David L. Rabinowitz on November 14, 2003....
's orbit ranges between 76 and 942 AU from the Sun; Sedna is currently about 90 AU from the Sun.
94 AU: Termination shock
Heliosphere

The heliosphere is a bubble in outer space "blown" into the interstellar medium by the solar wind. Although electrically neutral atoms from interstellar space can penetrate this bubble, virtually all of the material in the heliosphere emanates from the Sun itself....
between Solar wind
Solar wind

The solar wind is a Electric current—a Plasma —ejected from the stellar atmosphere of the sun. It consists mostly of electrons and protons with energies of about 1 electron volt....
s/Interstellar wind
Stellar wind

A stellar wind is a flow of neutral or charged gas ejected from the celestial body atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric....
s/Interstellar medium
Interstellar medium

In astronomy, the interstellar medium is the gas and cosmic dust that pervade interstellar space: the matter that exists between the stars within a galaxy....
.
100 AU: Heliosheath
108 AU: As of November 16, 2008, Voyager 1
Voyager 1

The spacecraft is a 722-kilogram Robotic spacecraft space probe of the outer Solar System and beyond, launched September 5, 1977. It remains operational, currently pursuing its extended mission to locate and study the boundaries of the Solar System, including the Kuiper belt and beyond....
is the furthest of any human-made objects from the Sun.
100-150 AU: Ending of Scattered Disc
Scattered disc

The scattered disc is a distant region of the Solar System that is sparsely populated by icy minor planets known as scattered disc objects ; a subset of the broader family of trans-Neptunian objects ....
500-3000 AU: Beginning of Hills cloud
Hills cloud

The Hills Cloud, also called the Inner Oort Cloud and Inner Cloud is, in astronomy, a vast hypothetical spherical body interior to the Oort Cloud, whose outer border would be located at around 2 to 3 Astronomical Units from the Sun, and whose inner border, less well defined, is hypothetically located at AU, well beyond planet a...
/"Inner Oort Cloud
Oort cloud

The Oort cloud is a hypothetical spherical cloud of comets which may lie roughly 50 000 astronomical unit, or nearly a light-year, from the Sun....
"
20,000 AU: Ending of Hills Cloud/"Inner Oort Cloud", beginning of "Outer Oort Cloud"
50,000 AU: possible closest estimate of the "Outer Oort Cloud" limits (0.8 ly
Light-year

A light-year or light year is a Units of measurement of length, equal to just under ten orders_of_magnitude_%28numbers%29#1012 kilometres....
)
100,000 AU: possible farthest estimate of the "Outer Oort Cloud" limits (1.6 ly).
125,000 AU: maximum extent of influence of the Sun's gravitational field
Gravitational field

A gravitational field is a scientific model used within physics to explain how gravitation exists in the universe. In its original concept, gravity was a force between point masses....
(Hill/Roche sphere
Hill sphere

A Hill sphere is, roughly, the volume around an astronomical body where it dominates in attraction of satellites to that body, rather than to a larger body which it orbits....
). beyond this is true interstellar space
Interstellar space

Interstellar space may mean:* In astronomy: all the space within a galaxy not occupied by star or their planetary systems. The interstellar medium resides ? by definition ? in interstellar space....
. This distance is roughly 1.8-2.0 light-years.
Proxima Centauri
Proxima Centauri

Proxima Centauri is a red dwarf star approximately 4.2 light-years distant in the constellation of Centaurus. It was discovered in 1915 by Robert Innes, the Director of the Union Observatory in South Africa....
(the nearest star
Star

A star is a massive, luminous ball of Plasma that is held together by its own gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth....
to Earth
Earth

Earth is the third planet from the Sun. Earth is the largest of the terrestrial planets in the Solar System in diameter, mass and density. It is also referred to as the World and Wiktionary:Terra.Note that by International Astronomical Union convention, the term "Terra" is used for naming extensive land masses, rather...
, excluding our own Sun) is ~268 000 AU away from the Sun.
The mean diameter of Betelgeuse
Betelgeuse

Betelgeuse is a semiregular variable star located approximately 600 light-years away from Earth. It is the second brightest star in the constellation Orion and the ninth list of brightest stars in the night sky....
is 2.57 AU.
The distance from the Sun to the centre of the Milky Way
Milky Way

The Milky Way, sometimes called simply the Galaxy, is the galaxy in which the Solar System is located. It is a barred spiral galaxy that is part of the Local Group of galaxies....
is approximately 1.7 AU.

Some conversion factors:

1 AU = 149,597,870.691 ± 0.030 km ˜ 92,955,807 mi ˜ 8.317 light minutes
Light-year

A light-year or light year is a Units of measurement of length, equal to just under ten orders_of_magnitude_%28numbers%29#1012 kilometres....
˜ 499 light-seconds
1 light-second ˜ 0.002 AU
1 gigametre ˜ 0.007 AU
1 light-minute ˜ 0.120 AU
1 microparsec ˜ 0.206 AU
1 terametre ˜ 6.685 AU
1 light-hour ˜ 7.214 AU
1 light-day ˜ 173.263 AU
1 milliparsec ˜ 206.265 AU
1 light-week ˜ 1,212.84 AU
1 light-month ˜ 5,197.9 AU
1 light-year
Light-year

A light-year or light year is a Units of measurement of length, equal to just under ten orders_of_magnitude_%28numbers%29#1012 kilometres....
˜ 63.241x10³ AU
1 parsec
Parsec

The parsec is a units of measurement of astronomical units of length, equal to just under 31 orders_of_magnitude_#1012 kilometres , or about 3.26 light-years....
˜ 206.265x10³ AU

External links

Smithsonian Institution Libraries
(at the NIST web site)
(at the IAU
International Astronomical Union

The International Astronomical Union is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy....
web site)
(a discussion of the relation between the AU and other quantities)
(measuring distances in AU will become increasingly imprecise as the Sun radiates away its energy) New Scientist, 6 February 2008

Astronomical unit

History

Examples

See also

External links