How this instrument works
Before 2012, the astronomical unit wasn't a fixed length at all — it was derived from the Gaussian gravitational constant, a number Carl Friedrich Gauss introduced in 1809 to simplify orbital mechanics. Astronomers defined 1 au as the radius a massless body would need to orbit the Sun in one Gaussian year (2π ⁄ k days, with k = 0.01720209895) under Kepler's third law. That definition worked well for a solar system studied only through telescopes, but it tied the au's length to the Sun's gravitational mass.
The trouble is that the Sun's mass isn't constant. Fusion and the solar wind carry away roughly one ten-trillionth of its mass every year, and by the 2000s, radar ranging and spacecraft tracking had become precise enough to notice the dynamical au creeping by centimetres annually. On 30 August 2012, at its General Assembly in Beijing, the International Astronomical Union adopted Resolution B2, fixing the astronomical unit at exactly 149,597,870,700 metres — chosen to match the old dynamical value so no planetary table needed rewriting, but now a defined constant rather than a measured quantity.
That single change decoupled distance from mass. Astronomers can now measure the Sun's mass loss directly, using the au as a fixed ruler instead of the other way round, and the definition works identically whether you're using Barycentric Coordinate Time, Terrestrial Time, or any other relativistic time scale in the calculation. Nothing about Earth's real orbit — its 147.1-million-km closest approach in January or its 152.1-million-km furthest point in July — is measured every time someone converts au to kilometres; the factor is simply looked up.
- Enter your distance in the Astronomical units (au) field. It starts at 1, Earth's own average distance from the Sun.
- The Kilometres (km) field recalculates instantly, multiplying by the fixed IAU constant 149,597,870.7.
- Working backwards from a kilometre figure? Divide it by 149,597,870.7 to recover astronomical units.
- For a fast mental estimate, round 1 au to 150 million km, then swap in the exact factor once precision matters.
Worked example — Earth's own orbit, 1 au exactly
Enter 1 into Astronomical units (au) and Kilometres (km) reads 149597870.7 — the very figure the IAU fixed in Beijing rather than something astronomers went out and measured that year. It was chosen deliberately to match the pre-2012 dynamical value, so switching to the new fixed definition changed no planet's catalogued distance by any meaningful amount.
Scale that up and the calculator keeps pace with no extra uncertainty: Mars sits around 1.52 au out, so roughly 227.4 million km; Jupiter's 5.20 au works out near 778.0 million km. Every one of those figures rests on the same 149,597,870.7 multiplier, which is why mission planners at NASA's Deep Space Network quote spacecraft ranges in au for readability, then convert to kilometres — or to light-time in seconds — the moment trajectory or communications-delay maths is involved.
Questions
Is the astronomical unit still tied to Earth's actual orbit?
No. Since IAU Resolution B2 in 2012, the au is a fixed constant — exactly 149,597,870,700 metres — independent of Earth's real, slightly elliptical path around the Sun. Earth's actual average distance is close to that figure by design, since the fixed value was chosen to match the pre-2012 dynamical measurement, but the two are no longer the same thing by definition.
Why did the IAU redefine the astronomical unit in 2012?
The old definition, built on the Gaussian gravitational constant and Kepler's third law, tied the au's length to the Sun's gravitational mass. Because the Sun sheds mass continuously through fusion and the solar wind, radar ranging and spacecraft tracking had grown precise enough to detect the au drifting by centimetres a year. Fixing the value at 149,597,870,700 m stopped that drift and let solar mass loss become something astronomers measure instead of something that quietly changed the ruler.
How was the astronomical unit defined before 2012?
Via the Gaussian gravitational constant, k = 0.01720209895, introduced by Carl Friedrich Gauss in 1809. One au was the radius at which a massless body, orbiting the Sun under Kepler's third law, would complete one revolution in a Gaussian year of 2π ⁄ k days — about 365.2569 days. That construction made the au's length a function of the Sun's assumed mass rather than a fixed distance.
What is Earth's actual average distance from the Sun?
About 149.60 million km, very close to 1 au by design, though Earth's orbit is elliptical rather than circular. Perihelion, the closest approach in early January, runs around 147.1 million km; aphelion, the furthest point in early July, reaches about 152.1 million km — a difference of roughly 5 million km across the year.
How many astronomical units away are the other planets?
Roughly: Mercury 0.39 au, Venus 0.72 au, Mars 1.52 au, Jupiter 5.20 au, Saturn 9.58 au, Uranus 19.2 au, and Neptune 30.1 au, all measured from the Sun on average. These are approximate mean distances — every planetary orbit is an ellipse, so the true distance varies across its year.
Why do astronomers use au instead of kilometres for solar-system distances?
Readability. Jupiter's average distance is 778,000,000 km or 5.20 au — the second figure is far easier to compare against Earth's own orbit at a glance. Kilometres return once engineers need absolute values, such as calculating radio signal delay to a spacecraft or plotting a trajectory that has to clear a specific altitude above a planet's surface.