# Astronomical unit <!-- SOLSIM:BEGIN g31 — Solar System explorer state (hand-built on wt-core, specs/solar/); do not hand-edit inside --> **Microsim — three.js (Wikitube framework):** *Distances at true scale (Solar System explorer)* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/solar/Solar_System.html?view=scale&embed=1" data-title="Distances at true scale (Solar System explorer)"></div> *The Solar System explorer locked on this article's state (`?view=scale`); every object and population of the [[PORTAL_Solar_System|Solar System portal]] has its own state in the same scene.* <!-- SOLSIM:END --> *Try: press l to label the planets, then scroll out slowly from the Sun past each orbit, keeping in mind the light-travel times the view quotes, 8.3 minutes to Earth, 43 minutes to Jupiter and 4.2 hours to Neptune; drag to look along the plane of the orbits; pause with space and compare how much wider each gap is than the one inside it.* The **astronomical unit** (symbol **au**, also written AU) is a unit of length defined as exactly 149,597,870,700 metres.[^iau2012] It began as the mean distance between [[Earth]] and the [[Sun]], the scale of the [[PORTAL_Solar_System|Solar System]], and for most of its history its value had to be measured; in 2012 the International Astronomical Union fixed it by definition, so that it is now a conventional unit tied to the metre rather than a property of Earth's [[Orbit|orbit]].[^iau2012][^brumfiel2012] Light crosses one astronomical unit in about 499 seconds, a little over 8 minutes 19 seconds (derived).[^bipm2019] The unit is used mainly for distances within the Solar System and within other planetary systems, and it underlies the parsec, the distance at which one astronomical unit subtends one arcsecond.[^luque2019] Measuring it took more than two thousand years, from Greek geometry through the transits of [[Venus]] to radar and spacecraft tracking.[^hughes2001] The explorer at the top of this page shows the Solar System with distances at true scale, so the astronomical unit is the spacing between the Sun and Earth's orbit and the ruler for everything beyond it. ## History of symbol usage The unit has had several symbols. The IAU's 1976 system of astronomical constants used A for the length of the astronomical unit, while the astronomical literature generally wrote AU.[^iau1976] In 2006 the International Bureau of Weights and Measures (BIPM) recommended ua, from the French *unité astronomique*.[^bipm2006] The IAU settled the question in 2012 by recommending au, noting the variety of symbols in use, and the journals of the American Astronomical Society and the Royal Astronomical Society adopted it.[^iau2012][^aas-style][^mnras-style] The BIPM followed in the 2014 revision of its SI Brochure and in the 2019 edition, which lists the astronomical unit, symbol au, among the non-SI units accepted for use with the SI.[^bipm2014][^bipm2019] The current international standard for quantities of space and time, ISO 80000-3:2019, does not mention it.[^iso2019] ## Development of unit definition For most of the unit's history its size in metres was the thing to be measured. [[Kepler's_laws_of_planetary_motion|Kepler's third law]] gives the relative sizes of all planetary orbits from their periods alone, so a single absolute distance, to any planet at any time, fixes the scale of the whole system. The difficulty lay in getting that one distance. In 1976 the IAU defined the astronomical unit through celestial mechanics: it is the length for which the Gaussian gravitational constant *k* takes the value 0.01720209895 when lengths, masses and times are measured in astronomical units, solar masses and days.[^iau1976][^hussmann2009] Put another way, it is the radius of a circular Newtonian orbit around the Sun in which a massless particle would move at 0.01720209895 radians per day, and the Sun's gravitational parameter GM☉ is k² au³/d². This made the unit a derived quantity, fixed by the dynamics of the planets rather than by a ruler. Planetary ephemerides, the tables of predicted positions such as those from JPL's Horizons system, were computed in these units.[^jpl-horizons] Radar ranging to the planets and radio tracking of spacecraft then measured distances as light travel times. With the metre defined since 1983 as the distance light travels in 1/299,792,458 of a second, a light time converts directly into metres.[^iers2010] Comparing ephemerides with these times gives the light time for one astronomical unit: the IAU's 2009 best estimate was τ = 499.0047838061 s, or A = cτ = 149,597,870,700 m, based on the JPL and Russian IAA ephemerides.[^pitjeva2009][^capitaine2012] The BIPM had quoted 1.49597870691 × 10¹¹ m in 2006.[^bipm2006] The dynamical definition had become awkward. A consistent version under general relativity would have had to specify a frame of reference and a time scale, and the value was still subject to measurement error.[^huang1995][^dodd2011] In August 2012 the IAU therefore adopted the 2009 value as an exact definition: 1 au = 149,597,870,700 m.[^iau2012][^capitaine2012] The Gaussian constant was dropped as a defining constant, and GM☉ is now measured in SI units like any other quantity.[^iau2012] ## Usage and significance Before 2012 the unit's value depended on the Sun's gravitational parameter, the product of the gravitational constant G and the Sun's mass. Neither factor is known to high accuracy on its own, but their product is known very precisely from the planets' motions, and only the product enters an ephemeris; that is why ephemerides were kept in astronomical units rather than metres.[^dodd2011] General relativity complicates distances at this scale. Clocks on Earth run at a rate that changes through the year as Earth's distance from the Sun varies between about 0.983 and 1.017 au, so the terrestrial second, and with it the metre, differs slightly and periodically from the "planetary" units of Barycentric Dynamical Time used in ephemerides.[^nasa-fs] The International Committee for Weights and Measures notes that the metre is a unit of proper length, valid only over regions small enough for variations in the gravitational field to be ignored.[^bipm2019] In practice the astronomical unit is the working unit for distances of the size of a planetary system: the heliocentric distance of an asteroid or comet, the radius of a protoplanetary disc, the separation of a wide binary. For distances between stars it is too small, and the parsec and the light-year take over; the light-year is common in popular writing but is not an approved unit and is little used by professional astronomers.[^dodd2011b] The unit also suits computation: when the Solar System is simulated as a numerical model, working in astronomical units keeps the numbers near 1 and limits floating-point error. The explorer's scale view is built on the same arithmetic. Its planets sit on orbits placed from JPL orbital elements, and the light times it quotes follow from the distances in astronomical units and the defined speed of light.[^jpl-t1][^bipm2019] ## History Early estimates of the Sun's distance were far too small, because they depended on measuring an angle that is tiny: the Sun is about 12,000 Earth diameters away, so small errors in the geometry produce large errors in the result.[^hughes2001] Around 280 BC Aristarchus of Samos measured the angle between the Sun and the Moon at first quarter as 87° (the true value is about 89.85°) and concluded that the Sun was 18 to 20 times farther than the [[Moon]], against a true ratio of about 390.[^vanhelden1985] Hipparchus gave a distance of 490 Earth radii, which G. J. Toomer reconstructs as coming from an assumed "least perceptible" solar parallax of 7 arcminutes.[^toomer1974] Ptolemy's figure of 1,210 Earth radii, derived from lunar parallax and eclipse geometry, dominated for fourteen centuries; al-Farghānī and al-Battānī used similar values, and Copernicus and Tycho Brahe still worked with about 1,150 Earth radii.[^goldstein1967][^vanhelden1985] In the seventeenth century the scale grew. [[Johannes_Kepler|Kepler]] argued in the *Rudolphine Tables* (1627) that Ptolemy's value was at least three times too small, and Jeremiah Horrocks, from the 1639 transit of Venus, found a solar parallax of about 15 arcseconds.[^hughes2001] [[Christiaan_Huygens|Christiaan Huygens]] estimated about 24,000 Earth radii by assuming that Venus and [[Mars]] are of similar size, a result close to the truth mostly by luck.[^goldstein1985] In 1672 Jean Richer in Cayenne and Giovanni Domenico Cassini in Paris measured the parallax of Mars at a close approach and obtained 9.5 arcseconds, about 21,700 Earth radii (derived).[^hughes2001] Edmond Halley proposed in 1716 that timing a transit of Venus from widely separated places would give the solar parallax precisely.[^halley1716] The transits of 1761 and 1769 sent expeditions around the world; Jérôme Lalande's reduction of them gave about 8.6 arcseconds.[^pogge2004][^hughes2001] Simon Newcomb's value of 8.80 arcseconds, adopted at an international conference in Paris in 1896, served until the IAU's 1964 constants.[^newcomb1871][^iau1964] Observations of the near-Earth asteroid Eros refined it: Arthur Hinks obtained 8.807 arcseconds from the 1900 opposition, and Harold Spencer Jones 8.790 arcseconds from that of 1931.[^hinks1909][^spencerjones1941] | Estimate | Solar parallax | Earth radii (derived) | |---|---|---| | Horrocks, 1639 | 15″ | 13,750 | | Richer and Cassini, 1672 | 9.5″ | 21,700 | | Lalande, 1771 | 8.6″ | 24,000 | | Newcomb, 1895 | 8.80″ | 23,440 | | Hinks, 1909 | 8.807″ | 23,420 | | Spencer Jones, 1941 | 8.790″ | 23,470 | | From the 2012 definition | 8.794143″ | 23,455 | The Earth radii follow from 206,264.806″ divided by the parallax; the last row uses the IERS value of Earth's equatorial radius, 6,378.1366 km, and the defined astronomical unit.[^iers2010][^iau2012] ## Developments The astronomical unit can be written in terms of other constants: under the old definition, A³ = GM☉ D² / k², where D is the length of a day.[^iau2012] Because the Sun loses mass through its radiation and the solar wind, GM☉ decreases slowly and the planets' orbits widen; on the pre-2012 definition the unit itself would have drifted, which was one argument for fixing it.[^noerdlinger2008][^newscientist2008] With the speed of light fixed in SI units and *k* fixed in astronomical units, measuring the light time for one astronomical unit was equivalent to measuring GM☉ in SI units, so ephemerides can be built entirely in SI units, as is now usual.[^capitaine2012] A 2004 analysis of radiometric data by George Krasinsky and Victor Brumberg reported a secular increase in the astronomical unit of about 15 metres per century, far more than the Sun's mass loss would explain.[^krasinsky2004][^anderson2009] Other groups have not confirmed it, and since 2010 planetary ephemerides no longer estimate the astronomical unit at all.[^fienga2011] ## Examples The table gives some distances in astronomical units, including a few lengths far too short or too long to be expressed in au in practice. | Object or length | Distance (au) | Note | |---|---|---| | Light-second | 0.002 | derived from the defined speed of light[^bipm2019] | | Earth–Moon distance | 0.00257 | mean, 384,400 km[^nasa-fs] | | Radius of the Sun | 0.00465 | 695,700 km[^nasa-sun] | | Light-minute | 0.120 | derived[^bipm2019] | | [[Mercury_(planet)|Mercury]] | 0.387 | mean distance from the Sun[^nasa-fs] | | Venus | 0.723 | mean distance[^nasa-fs] | | Earth | 1.000 | mean distance; 0.983 at perihelion, 1.017 at aphelion[^nasa-fs] | | Mars | 1.52 | mean distance[^nasa-fs] | | [[Jupiter]] | 5.20 | mean distance[^nasa-fs] | | Light-hour | 7.21 | derived[^bipm2019] | | [[Saturn]] | 9.54 | mean distance[^nasa-fs] | | [[Uranus]] | 19.2 | mean distance[^nasa-fs] | | [[Neptune]] | 30.1 | mean distance[^nasa-fs] | | [[Kuiper_belt|Kuiper belt]], inner edge | about 30 | [^stern1997] | | [[Eris_(dwarf_planet)|Eris]] | 67.9 | semi-major axis[^jpl-sbdb] | | Voyager 2 | 144.0 | from the Sun, 18 September 2026[^jpl-horizons] | | [[Voyager_1|Voyager 1]] | 171.8 | from the Sun, 18 September 2026[^jpl-horizons] | | Light-day | 173.1 | derived[^bipm2019] | | [[Sedna_(dwarf_planet)|Sedna]], aphelion | about 1,010 | osculating[^jpl-sbdb] | | Light-year | 63,241 | Julian year of 365.25 days, derived[^bipm2019] | | Parsec | 206,265 | 648,000/π au[^luque2019] | On 18 September 2026 light needed about 0.99 days to cover Voyager 1's distance from the Sun; receding at about 3.6 au a year, the spacecraft will be one light-day from the Sun roughly four to five months later (derived).[^jpl-horizons] ## See also - [[Solar_System_model]] - [[Kepler's_laws_of_planetary_motion]] - [[Planetary_system]] - Parsec · Light-year · Orders of magnitude (length) (plain text: not yet on Wikitube) ## References [^iau2012]: International Astronomical Union (31 August 2012). "Resolution B2: On the re-definition of the astronomical unit of length". XXVIII General Assembly, Beijing. https://iauarchive.eso.org/static/resolutions/IAU2012_English.pdf [^brumfiel2012]: Brumfiel, G. (14 September 2012). "The astronomical unit gets fixed: Earth–Sun distance changes from slippery equation to single number". *Nature*. https://doi.org/10.1038/nature.2012.11416 [^bipm2006]: Bureau International des Poids et Mesures (2006). *The International System of Units (SI)*, 8th ed., p. 126. http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf [^bipm2014]: Bureau International des Poids et Mesures (2014). *The International System of Units (SI)*, 8th ed., 2014 update, Table 6. http://www.bipm.org/en/publications/si-brochure/table6.html [^bipm2019]: Bureau International des Poids et Mesures (2019). *The International System of Units (SI)*, 9th ed., p. 145 (the speed of light fixed at 299,792,458 m/s; the astronomical unit, au, accepted for use with the SI). https://www.bipm.org/utils/common/pdf/si-brochure/SI-Brochure-9-EN.pdf [^luque2019]: Luque, B.; Ballesteros, F. J. (2019). "To the Sun and beyond". *Nature Physics* 15: 1302. https://doi.org/10.1038/s41567-019-0685-3 [^hughes2001]: Hughes, D. W. (2001). "Six stages in the history of the astronomical unit". *Journal of Astronomical History and Heritage* 4: 15–28. Bibcode 2001JAHH....4...15H. [^iau1976]: IAU Commission 4 (Ephemerides) (1976). "IAU (1976) System of Astronomical Constants", item 12: Unit distance. https://iauarchive.eso.org/static/resolutions/IAU1976_French.pdf [^aas-style]: American Astronomical Society. "Manuscript preparation: AJ & ApJ author instructions". http://aas.org/authors/manuscript-preparation-aj-apj-author-instructions#_Toc2.2 [^mnras-style]: Royal Astronomical Society. "Monthly Notices of the Royal Astronomical Society: Instructions to authors". https://academic.oup.com/mnras/pages/General_Instructions [^iso2019]: International Organization for Standardization (2019). "ISO 80000-3:2019 Quantities and units — Part 3: Space and time". https://www.iso.org/standard/64974.html [^hussmann2009]: Hussmann, H.; Sohl, F.; Oberst, J. (2009). "Basic data of planetary bodies". In *Landolt–Börnstein, New Series VI/4B: Solar System*. Springer, p. 4. ISBN 978-3-540-88054-7. [^jpl-horizons]: JPL Solar System Dynamics. Horizons System: heliocentric vectors of Voyager 1 (−31) and Voyager 2 (−32) for 2026-09-18 00:00 TDB. https://ssd.jpl.nasa.gov/horizons/ (queried 2026-09-18). [^iers2010]: Petit, G.; Luzum, B., eds. (2010). *IERS Conventions (2010)*, IERS Technical Note 36, Table 1.1: IERS numerical standards. https://www.iers.org/IERS/EN/Publications/TechnicalNotes/tn36.html [^pitjeva2009]: Pitjeva, E. V.; Standish, E. M. (2009). "Proposals for the masses of the three largest asteroids, the Moon–Earth mass ratio and the astronomical unit". *Celestial Mechanics and Dynamical Astronomy* 103: 365–372. https://doi.org/10.1007/s10569-009-9203-8 [^capitaine2012]: Capitaine, N.; Klioner, S.; McCarthy, D. (2012). "The re-definition of the astronomical unit of length: Reasons and consequences". IAU XXVIII General Assembly, Joint Discussion 7, p. 40. http://referencesystems.info/uploads/3/0/3/0/3030024/jd7_5-06.pdf [^huang1995]: Huang, T.-Y.; Han, C.-H.; Yi, Z.-H.; Xu, B.-X. (1995). "What is the astronomical unit of length?". *Astronomy and Astrophysics* 298: 629–633. Bibcode 1995A&A...298..629H. [^dodd2011]: Dodd, R. (2011). *Using SI Units in Astronomy*. Cambridge University Press, p. 76. ISBN 978-0-521-76917-4. [^dodd2011b]: Dodd, R. (2011). *Using SI Units in Astronomy*. Cambridge University Press, p. 82. ISBN 978-0-521-76917-4. [^nasa-fs]: NASA NSSDCA. "Planetary Fact Sheet" (mean distances, perihelion and aphelion of Earth, Moon's mean distance). https://nssdc.gsfc.nasa.gov/planetary/factsheet/ (fetched 2026-09-18). [^nasa-sun]: NASA NSSDCA. "Sun Fact Sheet". https://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html (fetched 2026-09-18). [^jpl-t1]: JPL Solar System Dynamics. "Approximate Positions of the Planets", Table 1. https://ssd.jpl.nasa.gov/planets/approx_pos.html [^jpl-sbdb]: JPL Solar System Dynamics. Small-Body Database Lookup: 136199 Eris, 90377 Sedna. https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html (elements fetched 2026-09-18). [^vanhelden1985]: van Helden, A. (1985). *Measuring the Universe: Cosmic Dimensions from Aristarchus to Halley*. University of Chicago Press, pp. 5–9, 15–27, 29–53. ISBN 978-0-226-84882-2. [^toomer1974]: Toomer, G. J. (1974). "Hipparchus on the distances of the sun and moon". *Archive for History of Exact Sciences* 14: 126–142. https://doi.org/10.1007/BF00329826 [^goldstein1967]: Goldstein, B. R. (1967). "The Arabic version of Ptolemy's Planetary Hypotheses". *Transactions of the American Philosophical Society* 57 (4): 9–12. https://doi.org/10.2307/1006040 [^goldstein1985]: Goldstein, S. J. (1985). "Christiaan Huygens' measurement of the distance to the Sun". *The Observatory* 105: 32. Bibcode 1985Obs...105...32G. [^halley1716]: Halley, E. (1716). "A new method of determining the parallax of the Sun, or his distance from the Earth". *Philosophical Transactions of the Royal Society* 29: 454–464. https://doi.org/10.1098/rstl.1714.0056 [^pogge2004]: Pogge, R. (May 2004). "How far to the Sun? The Venus transits of 1761 & 1769". Ohio State University. http://www.astronomy.ohio-state.edu/~pogge/Ast161/Unit4/venussun.html [^newcomb1871]: Newcomb, S. (1871). "The solar parallax". *Nature* 5: 60–61. https://doi.org/10.1038/005060a0 [^iau1964]: International Astronomical Union (1964). "On the system of astronomical constants". https://iauarchive.eso.org/static/resolutions/IAU1964_French.pdf [^hinks1909]: Hinks, A. R. (1909). "Solar parallax papers No. 7: The general solution from the photographic right ascensions of Eros, at the opposition of 1900". *Monthly Notices of the Royal Astronomical Society* 69: 544–567. https://doi.org/10.1093/mnras/69.7.544 [^spencerjones1941]: Spencer Jones, H. (1941). "The solar parallax and the mass of the Moon from observations of Eros at the opposition of 1931". *Memoirs of the Royal Astronomical Society* 66: 11–66. [^noerdlinger2008]: Noerdlinger, P. D. (2008). "Solar mass loss, the astronomical unit, and the scale of the Solar System". arXiv:0801.3807. [^newscientist2008]: "Astronomical unit may need to be redefined" (6 February 2008). *New Scientist*. https://www.newscientist.com/article/dn13286-astronomical-unit-may-need-to-be-redefined.html [^krasinsky2004]: Krasinsky, G. A.; Brumberg, V. A. (2004). "Secular increase of astronomical unit from analysis of the major planet motions, and its interpretation". *Celestial Mechanics and Dynamical Astronomy* 90: 267–288. https://doi.org/10.1007/s10569-004-0633-z [^anderson2009]: Anderson, J. D.; Nieto, M. M. (2009). "Astrometric Solar-System anomalies". *Proceedings of the International Astronomical Union* 5 (S261): 189–197. https://doi.org/10.1017/S1743921309990378 [^fienga2011]: Fienga, A.; Kuchynka, P.; Manche, H.; Desvignes, G.; Gastineau, M.; Cognard, I.; et al. (2011). "The INPOP10a planetary ephemeris and its applications in fundamental physics". *Celestial Mechanics and Dynamical Astronomy* 111: 363–385. https://doi.org/10.1007/s10569-011-9377-8 [^stern1997]: Stern, S. A.; Colwell, J. E. (1997). "Collisional erosion in the primordial Edgeworth–Kuiper belt and the generation of the 30–50 AU Kuiper gap". *The Astrophysical Journal* 490: 879–882. https://doi.org/10.1086/304912 ## Further reading - van Helden, A. (1985). *Measuring the Universe: Cosmic Dimensions from Aristarchus to Halley*. University of Chicago Press. ISBN 978-0-226-84882-2. - Dodd, R. (2011). *Using SI Units in Astronomy*. Cambridge University Press. ISBN 978-0-521-76917-4. ## External links - Capitaine, N.; Klioner, S.; McCarthy, D. (2012). "The re-definition of the astronomical unit of length: Reasons and consequences". http://referencesystems.info/uploads/3/0/3/0/3030024/jd7_5-06.pdf - JPL Horizons System. https://ssd.jpl.nasa.gov/horizons/ - NASA NSSDCA Planetary Fact Sheet. https://nssdc.gsfc.nasa.gov/planetary/factsheet/ ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Astronomical_unit) : [Wikitube](https://en.wikitube.io/wiki/Astronomical_unit) · pinned revision [1372862298](https://en.wikipedia.org/w/index.php?oldid=1372862298) · 2026-09-18 ## Previous hub tags Hubs: `Life_Physics`. Portals: [[PORTAL_Solar_System]]. --- *Solar System portal child articles, wave 1 · 2026-09-18 · drafted · row SOL-005 · explorer state `?view=scale`.* <!-- hub_tags: Life_Physics · PORTAL_Solar_System -->