Astronomy measures distances that the metre cannot express comfortably. The nearest star to the Sun is about 40 trillion kilometres away. Written in metres that is a number with seventeen digits, which is unreadable and impossible to compare at a glance. Astronomers therefore use three larger units, each suited to a different scale: the astronomical unit for the Solar System, the light year for nearby stars, and the parsec for everything beyond.
These are not a measurement system in the sense that SI or Imperial are. They have no base unit, no prefixes and no internal structure. They are individual units, each defined in metres, kept because they make the numbers of a particular scale legible. All three are recognised for use alongside SI.
The problem is range. Within the Solar System, distances span from a few hundred thousand kilometres to a few billion. Between stars, they span light years. Across the observable universe, they span billions of parsecs. A single unit cannot serve all three without producing numbers that are either absurdly large or absurdly small.
There is a second reason, and it is historical. For most of astronomy's history the absolute scale of the Solar System was unknown. Astronomers could measure the ratios of planetary distances with great precision from orbital periods, but not the distances themselves. Expressing everything in terms of the Earth's own distance from the Sun let them work accurately without knowing that distance in kilometres. The astronomical unit began as that reference, and only later became a fixed number of metres.
The astronomical unit, symbol au, is roughly the average distance from the Earth to the Sun. It was originally defined by the Earth's orbit, which made it awkward: the orbit is an ellipse, and the definition depended on the Sun's mass, which is not known exactly.
The International Astronomical Union settled the matter in 2012 by fixing the astronomical unit at exactly 149,597,870,700 metres. It is now a defined conversion factor rather than a measured quantity, in the same way the metre is defined by the speed of light. The value chosen matches the previous measured value closely enough that no practical work was disturbed.
The unit is used almost exclusively inside the Solar System and for exoplanet orbits. Mercury orbits at about 0.39 au, Earth by definition at about 1 au, Jupiter at 5.2 au and Neptune at 30 au. The Kuiper belt lies between roughly 30 and 50 au. Expressed this way the architecture of the Solar System is immediately readable, which it is not in kilometres.
A light year is the distance light travels in one year through a vacuum. It is a unit of distance, not of time, despite the name, and the confusion is common enough to be worth stating plainly.
Its value follows from two fixed numbers. The speed of light is exactly 299,792,458 metres per second. The Julian year used for the definition is exactly 365.25 days of 86,400 seconds. Multiplying gives exactly 9,460,730,472,580,800 metres, a little under 9.5 trillion kilometres. Because both inputs are defined rather than measured, the light year is exact.
The light year is the unit of popular astronomy. It appears in journalism and general writing far more often than in research papers, because it carries an intuitive meaning that the parsec does not: seeing an object 100 light years away means seeing it as it was 100 years ago. Proxima Centauri is 4.25 light years away, the centre of our galaxy about 26,000, and the Andromeda galaxy about 2.5 million.
The parsec is the unit professional astronomers actually use, and it is the only one of the three defined by a measurement technique rather than by a physical quantity.
The name is a contraction of "parallax second". As the Earth orbits the Sun, a nearby star appears to shift slightly against the distant background. Measuring that shift from opposite sides of the orbit gives the star's distance by simple trigonometry. A star that shifts by one arcsecond, which is 1/3600 of a degree, when the baseline is one astronomical unit, lies at a distance of one parsec.
Since 2015 the parsec has been defined exactly as 648,000/π astronomical units, which is about 30.857 trillion kilometres or 3.2616 light years. The definition is useful because it makes the arithmetic trivial: a star's distance in parsecs is simply one divided by its parallax angle in arcseconds. A parallax of 0.1 arcseconds means a distance of 10 parsecs. Multiples follow the usual SI prefixes, giving the kiloparsec for galactic scales and the megaparsec for distances between galaxies.
| Unit | Symbol | Value in metres | Typical use | Convert |
|---|---|---|---|---|
| Astronomical unit | au | 1.495978707 × 1011 | Solar System | au to kilometers |
| Light year | ly | 9.4607304726 × 1015 | Nearby stars | light years to kilometers |
| Parsec | pc | 3.0856775815 × 1016 | Professional astronomy | parsecs to light years |
Kepler's laws gave astronomers the relative sizes of the planetary orbits by the early seventeenth century, but not their absolute scale. The Solar System was a map without a legend. Fixing the scale required measuring one distance directly, and the transit of Venus offered the way to do it.
When Venus crosses the face of the Sun, observers at different latitudes see it trace slightly different paths, and the difference in timing yields the distance by parallax. Edmond Halley proposed the method in 1716, knowing he would not live to use it. International expeditions were mounted for the transits of 1761 and 1769, sending observers to Siberia, South Africa, Tahiti and Hudson Bay in what became one of the first coordinated global scientific efforts. James Cook's first voyage was organised partly for this purpose. The results put the astronomical unit within a few per cent of its modern value.
Radar ended the guesswork. Bouncing a radio signal off Venus in the 1960s and timing the echo gave the distance directly, to a precision of a few hundred metres. Spacecraft telemetry improved it further, and the 2012 decision to fix the value by definition simply acknowledged that the measurement had become better than the definition it served.
Because the speed of light is fixed, any astronomical distance can be stated as the time light takes to cross it. Astronomers use this constantly. The Sun is eight light minutes away, Jupiter about 43 light minutes, and the edge of the Solar System around 18 light hours. Radio commands to a spacecraft at Mars take between four and twenty-four minutes to arrive, which is why probes are programmed to act on their own during landings.
The same idea applies at larger scales with a consequence worth stating: looking out is looking back. An object 100 million light years away is seen as it was 100 million years ago, and there is no way to observe its present state. Telescopes are therefore instruments of history as much as of geography, and the deepest images ever taken show galaxies as they were when the universe was a few hundred million years old.
The choice is a matter of keeping the number between about one and a few thousand. Distances within the Solar System are given in astronomical units. Distances to stars within a few thousand light years are given in parsecs by professionals and light years by everyone else. Distances across the galaxy are given in kiloparsecs, and distances between galaxies in megaparsecs.
Kilometres survive for one job: distances to the Moon and to spacecraft in Earth orbit, where an astronomical unit would be an inconveniently large unit. The Moon is 384,400 kilometres away, which is 0.0026 au, and the second figure conveys nothing useful.
Parallax remains the foundation. It is the only direct method, requiring no assumption about the object being observed, and everything else is calibrated against it. Ground-based telescopes are limited by the atmosphere to a few hundred parsecs. Space changes that. The Hipparcos satellite mapped about 100,000 stars in the 1990s, and the Gaia mission has since measured parallaxes for over a billion stars with a precision of tens of microarcseconds, reaching much of the galaxy.
Beyond parallax, astronomers use standard candles: objects of known intrinsic brightness whose distance can be inferred from how faint they appear. Cepheid variable stars pulse at a rate that reveals their true luminosity, and type Ia supernovae explode at a consistent brightness bright enough to be seen across billions of parsecs. Each rung of this distance ladder is calibrated against the one below it, which is why an improvement in parallax measurements propagates all the way out to cosmological distances.
All three units are exact in metres, so conversions between them are exact too. One parsec is 3.2616 light years and 206,265 astronomical units. One light year is 63,241 astronomical units. The figures are large, so scientific notation is usually clearer than a long string of digits.
You can browse every length unit on this site from the length unit archive, or compare these against the metric units of the SI system.