Units · Solar System scale
Astronomical Unit
Exactly 149,597,870,700 metres (roughly 150 million km) — the mean Earth–Sun distance. The standard unit for distances within the Solar System, abbreviated AU (or au).
In other units
1 AU = 149,597,870.7 km
≈ 8.317 light-minutes · ≈ 1/63,241 light-year · ≈ 1/206,265 parsec
Solar System distances in AU
The AU gives the Solar System human-comprehensible proportions. Expressing planetary distances in kilometres produces unwieldy numbers; in AU the structure becomes intuitive. The gas giants lie between 5 and 30 AU — five to thirty times Earth's distance from the Sun. Voyager 1, humanity's most distant spacecraft, is currently at roughly 163 AU — over a hundred times farther from the Sun than Earth.
Convert between AU, light-years, parsecs, and kilometres with our Distance Converter tool. For context on the units above and beyond AU, see light-year and parsec in the dictionary.
Frequently asked questions
- Historically, the AU was measured using the transit of Venus — when Venus passes across the face of the Sun as seen from Earth. Observers at widely separated locations on Earth time the transit, and the small difference in observed path lengths allows the Earth–Venus distance (and hence Earth–Sun distance) to be calculated via parallax. Edmond Halley proposed the method in 1716; it was carried out during the 1761 and 1769 transits. Later methods used asteroid 433 Eros (radar ranging) and eventually direct radar measurements of Venus in the 1960s. Since 2012 the IAU defines the AU as exactly 149,597,870,700 m — a fixed constant rather than a measured average.
- Expressing planetary distances in AU reveals the Solar System's structure clearly: Mercury 0.39 AU, Venus 0.72 AU, Earth 1.00 AU, Mars 1.52 AU, Jupiter 5.20 AU, Saturn 9.58 AU, Uranus 19.2 AU, Neptune 30.1 AU. The asteroid belt lies between 2.2 and 3.2 AU; the Kuiper Belt from 30 to 50 AU; the Oort Cloud hypothetically extends from ~2,000 to 100,000 AU. Our Solar System data page lists full orbital parameters for all planets.
- 1 AU = 149,597,870.7 km. 1 light-year ≈ 63,241 AU ≈ 9.461 × 10¹² km. 1 parsec ≈ 206,265 AU ≈ 3.086 × 10¹³ km ≈ 3.26 light-years. The AU is practical for Solar System distances; light-years are used in popular astronomy for interstellar distances; parsecs are the professional standard beyond the Solar System. Convert between all three instantly with our Distance Converter tool.
- The AU is the natural unit for describing where a planet sits relative to its star — particularly for the habitable zone, the range of orbital distances where liquid water could exist on a rocky planet's surface. Earth sits at 1 AU, comfortably within the Sun's habitable zone (roughly 0.95–1.70 AU). When astronomers discover an exoplanet, reporting its orbital radius in AU immediately contextualises whether it could be habitable. For example, the TRAPPIST-1 system has seven rocky planets between 0.01 and 0.06 AU from their red dwarf host. See our guide on how we find exoplanets.
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