Theastronomical unit (symbol:au[1][2][3][4] orAU) is aunit of length defined to be exactly equal to149597870700 m.[5] Historically, the astronomical unit was conceived as the average Earth-Sun distance (the average of Earth'saphelion andperihelion), before its modern redefinition in 2012.
The astronomical unit is used primarily for measuring distances within theSolar System or around other stars. It is also a fundamental component in the definition of another unit of astronomical length, theparsec.[6] One au is approximately equivalent to 499light-seconds.
A variety of unit symbols and abbreviations have been in use for the astronomical unit. In a 1976 resolution, theInternational Astronomical Union (IAU) had used the symbolA to denote a length equal to the astronomical unit.[7] In the astronomical literature, the symbol AU is common. In 2006, theInternational Bureau of Weights and Measures (BIPM) had recommended ua as the symbol for the unit, from the French "unité astronomique".[8] In the non-normative Annex C toISO 80000-3:2006 (later withdrawn), the symbol of the astronomical unit was also ua.
In 2012, the IAU, noting "that various symbols are presently in use for the astronomical unit", recommended the use of the symbol "au".[1] Thescientific journals published by theAmerican Astronomical Society and theRoyal Astronomical Society subsequently adopted this symbol.[3][9] In the 2014 revision and 2019 edition of the SI Brochure, the BIPM used the unit symbol "au".[10][11] ISO 80000-3:2019, which replaces ISO 80000-3:2006, does not mention the astronomical unit.[12][13]
Earth's orbit around the Sun is anellipse. Thesemi-major axis of thiselliptic orbit is defined to be half of the straightline segment that joins theperihelion and aphelion. The centre of the Sun lies on this straight line segment, but not at its midpoint. Because ellipses are well-understood shapes, measuring the points of its extremes defined the exact shape mathematically, and made possible calculations for the entire orbit as well as predictions based on observation. In addition, it mapped out exactly the largest straight-line distance that Earth traverses over the course of a year, defining times and places for observing the largestparallax (apparent shifts of position) in nearby stars. Knowing Earth's shift and a star's shift enabled the star's distance to be calculated. But all measurements are subject to some degree of error or uncertainty, and the uncertainties in the length of the astronomical unit only increased uncertainties in the stellar distances. Improvements in precision have always been a key to improving astronomical understanding. Throughout the twentieth century, measurements became increasingly precise and sophisticated, and ever more dependent on accurate observation of the effects described byEinstein'stheory of relativity and upon the mathematical tools it used.
Improving measurements were continually checked and cross-checked by means of improved understanding of the laws ofcelestial mechanics, which govern the motions of objects in space. The expected positions and distances of objects at an established time are calculated (in au) from these laws, and assembled into a collection of data called anephemeris.NASA'sJet Propulsion Laboratory HORIZONS System provides one of several ephemeris computation services.[14]
In 1976, to establish a more precise measure for the astronomical unit, the IAU formallyadopted a new definition. Although directly based on the then-best available observational measurements, the definition was recast in terms of the then-best mathematical derivations from celestial mechanics and planetary ephemerides. It stated that "the astronomical unit of length is that length (A) for which theGaussian gravitational constant (k) takes the value0.01720209895 when the units of measurement are the astronomical units of length, mass and time".[7][15][16] Equivalently, by this definition, one au is "the radius of an unperturbed circular Newtonian orbit about the sun of a particle having infinitesimal mass, moving with anangular frequency of0.01720209895 radians per day";[17] or alternatively that length for which theheliocentric gravitational constant (the productGM☉) is equal to (0.01720209895)2 au3/d2, when the length is used to describe the positions of objects in the Solar System.
Subsequent explorations of the Solar System byspace probes made it possible to obtain precise measurements of the relative positions of theinner planets and other objects by means ofradar andtelemetry. As with all radar measurements, these rely on measuring the time taken forphotons to be reflected from an object. Because all photons move at thespeed of light in vacuum, a fundamental constant of the universe, the distance of an object from the probe is calculated as the product of the speed of light and the measured time. However, for precision the calculations require adjustment for things such as the motions of the probe and object while the photons are transiting. In addition, the measurement of the time itself must be translated to a standard scale that accounts forrelativistic time dilation. Comparison of the ephemeris positions with time measurements expressed inBarycentric Dynamical Time (TDB) leads to a value for the speed of light in astronomical units per day (of86400 s). By 2009, the IAU had updated its standard measures to reflect improvements, and calculated the speed of light at173.1446326847(69) au/d (TDB).[18]
In 1983, the CIPM modified theInternational System of Units (SI) to make the metre defined as the distance travelled in a vacuum by light in 1 / 299792458 s. This replaced the previous definition, valid between 1960 and 1983, which was that the metre equalled a certain number of wavelengths of a certain emission line of krypton-86. (The reason for the change was an improved method of measuring the speed of light.) The speed of light could then be expressed exactly asc0 =299792458 m/s, a standard also adopted by theIERS numerical standards.[19] From this definition and the 2009 IAU standard, the time for light to traverse an astronomical unit is found to beτA =499.0047838061±0.00000001 s, which is slightly more than 8 minutes 19 seconds. By multiplication, the best IAU 2009 estimate wasA =c0τA =149597870700±3 m,[20] based on a comparison of Jet Propulsion Laboratory andIAA–RAS ephemerides.[21][22][23]
In 2006, the BIPM reported a value of the astronomical unit as1.49597870691(6)×1011 m.[8] In the 2014 revision of the SI Brochure, the BIPM recognised the IAU's 2012 redefinition of the astronomical unit as149597870700 m.[10]
This estimate was still derived from observation and measurements subject to error, and based on techniques that did not yet standardize all relativistic effects, and thus were not constant for all observers. In 2012, finding that the equalization of relativity alone would make the definition overly complex, the IAU simply used the 2009 estimate to redefine the astronomical unit as a conventional unit of length directly tied to the metre (exactly149597870700 m).[20][24] The new definition recognizes as a consequence that the astronomical unit has reduced importance, limited in use to a convenience in some applications.[20]
This definition makes the speed of light, defined as exactly299792458 m/s, equal to exactly299792458 × 86400 ÷ 149597870700 or about173.144632674240 au/d, some 60 parts pertrillion less than the 2009 estimate.
With the definitions used before 2012, the astronomical unit was dependent on theheliocentric gravitational constant, that is the product of thegravitational constant,G, and thesolar mass,M☉. NeitherG norM☉ can be measured to high accuracy separately, but the value of their product is known very precisely from observing the relative positions of planets (Kepler's third law expressed in terms of Newtonian gravitation). Only the product is required to calculate planetary positions for an ephemeris, so ephemerides are calculated in astronomical units and not in SI units.
The calculation of ephemerides also requires a consideration of the effects ofgeneral relativity. In particular, time intervals measured on Earth's surface (Terrestrial Time, TT) are not constant when compared with the motions of the planets: the terrestrial second (TT) appears to be longer near January and shorter near July when compared with the "planetary second" (conventionally measured in TDB). This is because the distance between Earth and the Sun is not fixed (it varies between0.9832898912 and1.0167103335 au) and, when Earth is closer to the Sun (perihelion), the Sun's gravitational field is stronger and Earth is moving faster along its orbital path. As the metre is defined in terms of the second and the speed of light is constant for all observers, the terrestrial metre appears to change in length compared with the "planetary metre" on a periodic basis.
The metre is defined to be a unit ofproper length. Indeed, theInternational Committee for Weights and Measures (CIPM) notes that "its definition applies only within a spatial extent sufficiently small that the effects of the non-uniformity of the gravitational field can be ignored".[25] As such, a distance within the Solar System without specifying theframe of reference for the measurement is problematic. The 1976 definition of the astronomical unit was incomplete because it did not specify the frame of reference in which to apply the measurement, but proved practical for the calculation of ephemerides: a fuller definition that is consistent with general relativity was proposed,[26] and "vigorous debate" ensued[27] until August 2012 when the IAU adopted the current definition of 1 astronomical unit =149597870700metres.
The astronomical unit is typically used forstellar system scale distances, such as the size of a protostellar disk or theheliocentric distance of an asteroid, whereas other units are used forother distances in astronomy. The astronomical unit is too small to be convenient for interstellar distances, where theparsec andlight-year are widely used. The parsec (parallaxarcsecond) is defined in terms of the astronomical unit, being the distance of an object with a parallax of1″. The light-year is often used in popular works, but is not an approved non-SI unit and is rarely used by professional astronomers.[28]
Many early measurements of the Earth-Sun distance are wildly incorrect, primarily because the measurements rely on the ratio of the size of the Earth to the distance of the Sun. Since this ratio is 1/12000, small errors in the size of the Earth lead to large errors in the Earth-Sun distance.[29]
Around 280 BC,Aristarchus carefully measured the Moon-Earth-Sun angle when the Moon is in its firstquarter and used this to estimate the distance to the Sun. The exact timing and angle measurement are essential.[29] He estimated the angle at as87° (the true value being close to89.853°) and reported inOn the Sizes and Distances of the Sun and Moon the distance to the Sun is 18 to 20 times thedistance to the Moon, whereas the true ratio is about389.174. Depending on the distance that Aristarchus used for the distance to the Moon, his calculated distance to the Sun would fall between380 and1520 Earth radii.[30]
Hipparchus gave an estimate of the distance of Earth from the Sun, quoted byPappus as equal to 490 Earth radii. According to the conjectural reconstructions ofNoel Swerdlow andG. J. Toomer, this was derived from his assumption of a "least perceptible" solar parallax of7′.[31]
In the 2nd century CE,Ptolemy estimated the mean distance of the Sun as1210 timesEarth's radius.[34][35] To determine this value, Ptolemy started by measuring the Moon's parallax, finding what amounted to a horizontal lunar parallax of 1° 26′, which was much too large. He then derived a maximum lunar distance of64+1/6 Earth radii. Because of cancelling errors in his parallax figure, his theory of the Moon's orbit, and other factors, this figure was approximately correct.[36][37] He then measured the apparent sizes of the Sun and the Moon and concluded that the apparent diameter of the Sun was equal to the apparent diameter of the Moon at the Moon's greatest distance, and from records of lunar eclipses, he estimated this apparent diameter, as well as the apparent diameter of the shadow cone of Earth traversed by the Moon during a lunar eclipse. Given these data, the distance of the Sun from Earth can be trigonometrically computed to be1210 Earth radii. This gives a ratio of solar to lunar distance of approximately 19, matching Aristarchus's figure. Although Ptolemy's procedure is theoretically workable, it is very sensitive to small changes in the data, so much so that changing a measurement by a few per cent can make the solar distance infinite.[36]
After Greek astronomy was transmitted to the medieval Islamic world, astronomers made some changes to Ptolemy's cosmological model, but did not greatly change his estimate of the Earth–Sun distance. For example, in his introduction to Ptolemaic astronomy,al-Farghānī gave a mean solar distance of1170 Earth radii, whereas in hiszij,al-Battānī used a mean solar distance of1108 Earth radii. Subsequent astronomers, such asal-Bīrūnī, used similar values.[38] Later in Europe,Copernicus andTycho Brahe also used comparable figures (1142 and1150 Earth radii), and so Ptolemy's approximate Earth–Sun distance survived through the 16th century.[39]
By the 17th century the problems with the measurement techniques came to be understood.[29]: 19 Johannes Kepler was the first to realize that Ptolemy's estimate must be significantly too low (according to Kepler, at least by a factor of three) in hisRudolphine Tables (1627).Kepler's laws of planetary motion allowed astronomers to calculate the relative distances of the planets from the Sun, and rekindled interest in measuring the absolute value for Earth (which could then be applied to the other planets). The invention of thetelescope allowed far more accurate measurements of angles than is possible with the naked eye. Flemish astronomerGodefroy Wendelin repeated Aristarchus’ measurements in 1635, and found that Ptolemy's value was too low by a factor of at least eleven.
A somewhat more accurate estimate can be obtained by observing thetransit of Venus.[40] By measuring the transit in two different locations, one can accurately calculate the parallax of Venus and from the relative distance of Earth and Venus from the Sun, thesolar parallaxα (which cannot be measured directly due to the brightness of the Sun[41]).Jeremiah Horrocks had attempted to produce an estimate based on his observation of the1639 transit (published in 1662), giving a solar parallax of15″, similar to Wendelin's figure. The solar parallax is related to the Earth–Sun distance as measured in Earth radii by
The smaller the solar parallax, the greater the distance between the Sun and Earth: a solar parallax of15″ is equivalent to an Earth–Sun distance of13750 Earth radii.
Christiaan Huygens believed that the distance was even greater: by comparing the apparent sizes of Venus andMars, he estimated a value of about24000 Earth radii,[33] equivalent to a solar parallax of8.6″. Although Huygens' estimate is remarkably close to modern values, it is often discounted by historians of astronomy because of the many unproven (and incorrect) assumptions he had to make for his method to work; the accuracy of his value seems to be based more on luck than good measurement, with his various errors cancelling each other out.
Transits of Venus across the face of the Sun were, for a long time, the best method of measuring the astronomical unit, despite the difficulties (here, the so-called "black drop effect") and the rarity of observations.
Jean Richer andGiovanni Domenico Cassini measured the parallax of Mars between Paris andCayenne inFrench Guiana when Mars was at its closest to Earth in 1672. They arrived at a figure for the solar parallax of9.5″, equivalent to an Earth–Sun distance of about22000 Earth radii. They were also the first astronomers to have access to an accurate and reliable value for the radius of Earth, which had been measured by their colleagueJean Picard in 1669 as3269000toises. This same year saw another estimate for the astronomical unit byJohn Flamsteed, which accomplished it alone by measuring themartiandiurnal parallax.[42] Another colleague,Ole Rømer, discovered the finite speed of light in 1676: the speed was so great that it was usually quoted as the time required for light to travel from the Sun to the Earth, or "light time per unit distance", a convention that is still followed by astronomers today.
A better method for observing Venus transits was devised byJames Gregory and published in hisOptica Promata (1663). It was strongly advocated byEdmond Halley[43] and was applied to the transits of Venus observed in 1761 and 1769, and then again in 1874 and 1882. Transits of Venus occur in pairs, but less than one pair every century, and observing the transits in 1761 and 1769 was an unprecedented international scientific operation including observations by James Cook and Charles Green from Tahiti. Despite theSeven Years' War, dozens of astronomers were dispatched to observing points around the world at great expense and personal danger: several of them died in the endeavour.[44] The various results were collated byJérôme Lalande to give a figure for the solar parallax of8.6″.Karl Rudolph Powalky had made an estimate of8.83″ in 1864.[45]
Date
Method
A/Gm
Uncertainty
1895
aberration
149.25
0.12
1941
parallax
149.674
0.016
1964
radar
149.5981
0.001
1976
telemetry
149.597870
0.000001
2009
telemetry
149.597870700
0.000000003
Another method involved determining the constant ofaberration.Simon Newcomb gave great weight to this method when deriving his widely accepted value of8.80″ for the solar parallax (close to the modern value of8.794143″), although Newcomb also used data from the transits of Venus. Newcomb also collaborated withA. A. Michelson to measure the speed of light with Earth-based equipment; combined with the constant of aberration (which is related to the light time per unit distance), this gave the first direct measurement of the Earth–Sun distance in metres. Newcomb's value for the solar parallax (and for the constant of aberration and the Gaussian gravitational constant) were incorporated into the first international system ofastronomical constants in 1896,[46] which remained in place for the calculation of ephemerides until 1964.[47] The name "astronomical unit" appears first to have been used in 1903.[48][failed verification]
The discovery of thenear-Earth asteroid433 Eros and its passage near Earth in 1900–1901 allowed a considerable improvement in parallax measurement.[49] Another international project to measure the parallax of 433 Eros was undertaken in 1930–1931.[41][50]
Direct radar measurements of the distances to Venus and Mars became available in the early 1960s. Along with improved measurements of the speed of light, these showed that Newcomb's values for the solar parallax and the constant of aberration were inconsistent with one another.[51]
Theastronomical unit is used as the baseline of the triangle to measurestellar parallaxes (distances in the image are not to scale)
The unit distanceA (the value of the astronomical unit in metres) can be expressed in terms of other astronomical constants:
whereG is theNewtonian constant of gravitation,M☉ is the solar mass,k is the numerical value of Gaussian gravitational constant andD is the time period of one day.[1]The Sun is constantly losing mass by radiating away energy,[52] so the orbits of the planets are steadily expanding outward from the Sun. This has led to calls to abandon the astronomical unit as a unit of measurement.[53]
As the speed of light has an exact defined value in SI units and the Gaussian gravitational constantk is fixed in theastronomical system of units, measuring the light time per unit distance is exactly equivalent to measuring the productG×M☉ in SI units. Hence, it is possible to construct ephemerides entirely in SI units, which is increasingly becoming the norm.
A 2004 analysis of radiometric measurements in the inner Solar System suggested that thesecular increase in the unit distance was much larger than can be accounted for by solar radiation, +15±4 metres per century.[54][55]
The measurements of the secular variations of the astronomical unit are not confirmed by other authors and are quite controversial.Furthermore, since 2010, the astronomical unit has not been estimated by the planetary ephemerides.[56]
The following table contains some distances given in astronomical units. It includes some examples with distances that are normally not given in astronomical units, because they are either too short or far too long. Distances normally change over time. Examples are listed by increasing distance.
Oneparsec. The parsec is defined in terms of the astronomical unit, is used to measure distances beyond the scope of the Solar System and is about 3.26 light-years: 1 pc = 1 au/tan(1″)
Distance from the Sun to the centre of the Milky Way
—
Note: Figures in this table are generally rounded estimates, often rough estimates, and may considerably differ from other sources. Table also includes other units of length for comparison.
^abcOn the re-definition of the astronomical unit of length(PDF). XXVIII General Assembly of International Astronomical Union. Beijing, China: International Astronomical Union. 31 August 2012. Resolution B2. Archived fromthe original(PDF) on 5 March 2025.... recommends ... 5. that the unique symbol "au" be used for the astronomical unit.
^On the re-definition of the astronomical unit of length(PDF). XXVIII General Assembly of International Astronomical Union. Beijing: International Astronomical Union. 31 August 2012. Resolution B2. Archived fromthe original(PDF) on 5 March 2025.... recommends [adopted] that the astronomical unit be re-defined to be a conventional unit of length equal to exactly149597870700 metres, in agreement with the value adopted in IAU 2009 Resolution B2
^abCommission 4: Ephemerides/Ephémérides (1976).item 12: Unit distance(PDF). XVIth General Assembly of the International Astronomical Union. IAU (1976) System of Astronomical Constants. Grenoble, FR. Commission 4, part III, Recommendation 1, item 12. Archived fromthe original(PDF) on 9 October 2022.{{cite conference}}: CS1 maint: numeric names: authors list (link)
^"Instructions to Authors".Monthly Notices of the Royal Astronomical Society. Oxford University Press. Retrieved5 November 2020.The units of length/distance are Å, nm, µm, mm, cm, m, km, au, light-year, pc.
^Williams Gareth V. (1997)."Astronomical unit". In Shirley, James H.; Fairbridge, Rhodes Whitmore (eds.).Encyclopedia of planetary sciences. Springer. p. 48.ISBN978-0-412-06951-2.
^van Helden, Albert (1985).Measuring the Universe: Cosmic dimensions from Aristarchus to Halley. Chicago: University of Chicago Press. pp. 5–9.ISBN978-0-226-84882-2.
^Lloyd, G. E. R. (1996).Adversaries and Authorities: Investigations into Ancient Greek and Chinese Science. Cambridge University Press. pp. 59–60.ISBN978-0-521-55695-8.
^abGoldstein, S. J. (1985). "Christiaan Huygens' measurement of the distance to the Sun".The Observatory.105: 32.Bibcode:1985Obs...105...32G.
^Goldstein, Bernard R. (1967). "The Arabic version of Ptolemy'splanetary hypotheses".Trans. Am. Philos. Soc.57 (4):9–12.doi:10.2307/1006040.JSTOR1006040.
^van Helden, Albert (1985).Measuring the Universe: Cosmic Dimensions from Aristarchus to Halley. Chicago: University of Chicago Press. pp. 15–27.ISBN978-0-226-84882-2.
^Bell, Trudy E. (Summer 2004)."Quest for the astronomical unit"(PDF).The Bent of Tau Beta Pi. p. 20. Archived fromthe original(PDF) on 24 March 2012. Retrieved16 January 2012 – provides an extended historical discussion of thetransit of Venus method.
^abWeaver, Harold F. (March 1943). The Solar Parallax.Astronomical Society of the Pacific Leaflets (Report). Vol. 4. pp. 144–51.Bibcode:1943ASPL....4..144W.
^Van Helden, A. (2010). Measuring the universe: cosmic dimensions from Aristarchus to Halley. University of Chicago Press. Ch. 12.
^Conférence internationale des étoiles fondamentales, Paris, 18–21 May 1896
^On the system of astronomical constants(PDF). XIIth General Assembly of the International Astronomical Union. Hamburg, Germany: International Astronomical Union. 1964. Archived fromthe original(PDF) on 11 March 2025.
^Spencer Jones, H. (1941). "The solar parallax and the mass of the Moon from observations of Eros at the opposition of 1931".Mem. R. Astron. Soc.66:11–66.ISSN0369-1829.
^Mikhailov, A. A. (1964). "The Constant of Aberration and the Solar Parallax".Sov. Astron.7 (6):737–39.Bibcode:1964SvA.....7..737M.
