The astronomical unit (AU) equals exactly 149,597,870,700 m since the 2012 IAU resolution. It's the Solar System's preferred yardstick: planetary orbits, probes, comets, and exoplanet systems.
The astronomical unit is the Solar System's standard yardstick. To the naked eye, the Sun and Moon look roughly the same size; that's a beautiful coincidence, but misleading — the Sun is 400 times wider and 400 times farther. That 'farther' is exactly 1 AU: 149,597,870,700 metres.
Earth does not follow a perfect circle around the Sun but an ellipse of eccentricity 0.0167. The real distance therefore varies: 147.1 × 10⁶ km at perihelion (around 3 January) and 152.1 × 10⁶ km at aphelion (around 4 July). The mean over one orbit is roughly 1.00001 AU. To avoid any ambiguity, the IAU ruled in 2012: the AU is no longer defined as 'an average' (fuzzy) but as a fixed conventional length, exactly 149,597,870,700 m, full stop.
This unit is especially natural for the Solar System. Mercury orbits at 0.39 AU, Venus at 0.72, Earth at 1 by definition, Mars at 1.52, Jupiter at 5.20, Saturn at 9.58, Uranus at 19.19, Neptune at 30.07. Pluto's orbit ranges from 29.66 to 49.31 AU. The Voyager 1 and 2 probes, launched in 1977, now sit at 166 AU and 138 AU respectively (April 2026) — well beyond the heliopause but still far from the edge of the Oort Cloud (50,000 to 100,000 AU).
The 2012 IAU resolution (28th General Assembly, Beijing) also introduced the official symbols 'au' (lowercase) in English and 'ua' in French. The AU is among units 'accepted for use with the SI' according to BIPM, but is not part of the SI proper.
Exact definition (IAU 2012):
1 AU = 149,597,870,700 m (exactly)
This value is now a defined constant, no longer a measurement. All precision henceforth comes from the distances themselves.
Key conversions:
• 1 AU ≈ 149.6 × 10⁶ km ≈ 9.295 × 10⁷ miles • 1 AU ≈ 499.00 light-seconds ≈ 8.317 light-minutes • 1 AU ≈ 1.581 × 10⁻⁵ ly • 1 AU ≈ 4.848 × 10⁻⁶ pc • 1 ly ≈ 63,241 AU • 1 pc ≈ 206,265 AU
Important: 1 pc = 206,264.806 AU exactly (it's the parsec's definition, based on 1 radian = 206,264.806 arcsec).
Planetary orbits (semi-major axis):
• Mercury 0.387 AU · Venus 0.723 AU · Earth 1.000 AU · Mars 1.524 AU • Main asteroid belt ≈ 2.2-3.2 AU · Jupiter 5.204 AU · Saturn 9.582 AU • Uranus 19.20 AU · Neptune 30.05 AU · Pluto 39.48 AU • Kuiper Belt ≈ 30-50 AU · heliopause ≈ 120 AU · Oort Cloud 10³-10⁵ AU
Notable exoplanets: TRAPPIST-1e at 0.029 AU from its red dwarf, Proxima b at 0.0485 AU, HD 209458 b ('Osiris') at 0.047 AU.
Within the Solar System, the AU ranges from fractions to thousands.
Inner Solar System (< 2 AU). Terrestrial planets: Mercury (0.39 AU), Venus (0.72), Earth (1), Mars (1.52). The realm of crewed missions (6-9 months to Mars), fast probes, classic Hohmann transfers.
Main belt and giant planets (2 to 30 AU). The asteroid belt (≈ 2.2-3.2 AU, including Ceres at 2.77), Jupiter (5.2), Saturn (9.6), Uranus (19.2), Neptune (30). Flagship missions: Galileo (1989-2003, Jupiter), Cassini-Huygens (1997-2017, Saturn). Transfers take 6-10 years and demand gravity assists.
Outer Solar System (30 to 100 AU). Kuiper Belt (Pluto 39.5 AU, Haumea 43, Makemake 45, Eris 68), scattered disk, detached objects (Sedna 76 AU at perihelion, up to 1,000 AU at aphelion). New Horizons flew past Pluto in 2015 at 34 AU, then Arrokoth (2014 MU69) in 2019 at 44 AU. Voyager 1 crossed the heliopause in August 2012 at 121 AU — the boundary between solar wind and interstellar medium.
Nearby interstellar medium (> 100 AU). Voyager 1 (166 AU in April 2026) and Voyager 2 (138 AU) now drift through interstellar space. The Oort Cloud, a hypothetical cometary reservoir, would start around 2,000 AU and extend to 100,000 AU (≈ 1.6 ly), nearly half the distance to Alpha Centauri.
Exoplanets. Exoplanet systems naturally use AU: hot Jupiters orbit at 0.02-0.1 AU, red dwarf habitable zones typically at 0.05-0.3 AU, Sun-like star habitable zones at 0.8-2 AU.
Historically, measuring the AU was one of astronomy's great challenges.
Planetary parallaxes (17th-18th centuries). Kepler established the ratios of planetary distances (third law: T² ∝ a³) as early as 1619, but without absolute values. Cassini measured Mars's parallax in 1672 and obtained ≈ 140 × 10⁶ km — already close. The Venus transits of 1761 and 1769 rallied dozens of international expeditions (Cook to Tahiti, Le Gentil to India, Chappe to California) and yielded an AU of ≈ 153 × 10⁶ km.
Radar and in-system measurements (20th century). The first radars pinged Venus in 1961 (JPL/Goldstone). The measured round-trip time, combined with celestial mechanics, pinned down the AU to the metre. Tracking of spacecraft (Mariner, Pioneer, Voyager, Magellan, Cassini) refined the value further.
Conventional definition (2012). The problem: the historical AU depended on the solar mass via Gauss's constant k. But the Sun loses 4.3 × 10⁹ kg per second through fusion and solar wind — mass varies! The IAU therefore fixed the AU at a whole-metre constant (149,597,870,700 m, close to the previous radar value) and redefined k separately. The AU is now a pure length, decoupled from solar mass.
What about amateurs? You can replay Cassini's historical measurement by triangulating Mars from two distant Earth sites during an opposition (or more modestly, by reading the JPL Horizons ephemeris). Our solar system tool shows live heliocentric distances.
Several units and concepts orbit the AU.
AU vs solar radius (R☉). Careful: 1 R☉ ≈ 696,340 km = 0.00465 AU. Red supergiants like Betelgeuse are about 700-900 R☉, roughly 3-4 AU — literally larger than Mars's orbit. Don't confuse stellar radius with stellar distance.
AU vs light-year. 1 ly = 63,241 AU. The AU works out to the edge of the Solar System (< 10⁵ AU); beyond, you switch to the light-year or parsec. Saying 'Proxima Centauri is 267,000 AU away' is correct but impractical.
AU vs perihelion/aphelion. The AU is NOT the instantaneous Earth-Sun distance. Earth's perihelion is 0.983 AU, aphelion 1.017 AU. Earth crosses the 1-AU mark twice a year, around early April and early October. Earth's semi-major axis is 1.00001 AU — but the modern AU is a fixed constant, decoupled from the actual orbit.
AU vs solar gravitational constant. Before 2012, the AU's definition involved Gauss's constant k² = GM☉/AU³. Since 2012, the AU is pure length and k is redefined. Older references: the same number may conceal two definitions.
AU vs mean Earth-Sun radius. Since 2012, the equivalence is no longer strict — the value was fixed slightly above the actual orbital semi-major axis (which keeps varying marginally due to planetary perturbations).
Because the historical definition tied the AU to the Sun's mass via Gauss's constant (k² = GM☉/AU³). But the Sun loses about 4.3 × 10⁹ kg per second (nuclear fusion + solar wind), making that mass technically variable. By fixing the AU at exactly 149,597,870,700 m, the IAU decoupled length from solar mass and simplified planetary ephemerides. It's a cousin of the 1983 metre redefinition (now tied to c rather than a platinum-iridium artefact). The AU thus becomes a pure yardstick, perfectly stable.
Mars lies between 0.52 AU (opposition) and 2.52 AU (conjunction) from Earth. Crewed missions target Hohmann transfers of 6-9 months when Mars is favourable (roughly every 26 months). Perseverance (2020) took 203 days to cover ≈ 472 million km. A crewed mission must handle the outbound leg (6-9 months), a stay (≈ 1.5 years to await a return window) and the return (6-9 months): total 2.5-3 years. The mean Earth-Mars distance is about 1.52 AU — but that number means little, since both planets keep moving.
Because kilometre figures become unmanageable. 'Jupiter orbits at 5.2 AU' is clear; '7.78 × 10⁸ km', much less so. The AU also carries direct physical meaning: Mercury at 0.39 AU immediately suggests extreme thermal conditions, while Neptune at 30 AU conjures an icy realm. For exoplanetary systems, the AU enables direct comparison with our own orbital architecture. And historically, from Kepler to Newton, all celestial mechanics was built on AU ratios long before anyone could assign an absolute value.
In April 2026, Voyager 1 is about 166 AU from the Sun (more than 24.8 billion km), Voyager 2 at 138 AU. Both have crossed the heliopause (Voyager 1 in August 2012 at 121 AU, Voyager 2 in November 2018 at 119 AU) and now drift through interstellar space. Communication still goes through the Deep Space Network, with a one-way light time of 23 h for Voyager 1. At current speeds (≈ 17 km/s for Voyager 1), the probes will reach the presumed edge of the Oort Cloud in about 300 years and 'leave' it in 30,000 years — a humbling perspective on our space conquest.