AU — Astronomical Unit
Earth's average distance to the Sun, fixed at exactly 149,597,870,700 metres — the natural ruler for solar-system distances.
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Once you start measuring distances in space, kilometres get unmanageable fast. Mars is 200 million km. Saturn is 1.4 billion km. Pluto is 6 billion km. Your brain stops parsing those numbers as anything except "big." So astronomers picked a more human ruler: one Earth-Sun distance. That's an Astronomical Unit, abbreviated AU.
The unit is anchored to a specific integer number of metres — 149,597,870,700 of them, fixed in 2012 — but the practical idea is simpler. Mars is 1.5 AU from the Sun. Jupiter is 5 AU. Pluto orbits between 30 and 50 AU. Voyager 1, the most distant human-made object, is around 165 AU now and still going. Numbers you can actually hold in your head.
AU is everywhere in this app. The /explore screen draws orbital radii in AU. The /plan porkchop axes are calibrated in AU. The HUD on /fly converts AU to kilometres and light-minutes when you need those, but the underlying ruler is always AU. Once you internalise it, the solar system stops feeling abstract — you can roughly map distances by counting hops in AU.
Until 2012 the AU was defined by Earth's orbit. As Earth's orbit slowly evolves under planetary perturbations, the AU was technically changing too. The IAU fixed it in 2012 to a precise integer number of metres — 149,597,870,700 — and the definition has been stable ever since. The AU is no longer 'Earth's orbit'; it's a metric ruler that happens to roughly equal it.
Solar-system distances in AU: Mercury 0.39, Venus 0.72, Earth 1.0, Mars 1.52, Jupiter 5.20, Saturn 9.58, Uranus 19.22, Neptune 30.05, Pluto 39.5 (averages — eccentricity makes them vary). The Voyagers are well past 150 AU and still going. The Oort Cloud reaches out tens of thousands of AU.
Why AU instead of metres? Because numbers in metres are unwieldy at solar-system scale. Saturn's 1.4 billion km is unreadable. 9.58 AU is a number you can hold in your head. Orrery's panels and HUD all use AU for solar-system positions — converted to km, light-minutes, and other units only where the audience needs the conversion.
SEE IN THE APP
- /explore Every planet's semi-major axis is given in AU