| Kilometers (km) | Parsecs (pc) |
|---|---|
| 1 Kilometer | 3.24077928944 × 10-14 pc |
| 2 Kilometers | 6.48155857889 × 10-14 pc |
| 3 Kilometers | 9.72233786833 × 10-14 pc |
| 4 Kilometers | 1.29631171578 × 10-13 pc |
| 5 Kilometers | 1.62038964472 × 10-13 pc |
| 10 Kilometers | 3.24077928944 × 10-13 pc |
| 20 Kilometers | 6.48155857889 × 10-13 pc |
| 25 Kilometers | 8.10194822361 × 10-13 pc |
| 50 Kilometers | 1.62038964472 × 10-12 pc |
| 100 Kilometers | 3.24077928944 × 10-12 pc |
| Reference | Kilometers (km) | Parsecs (pc) |
|---|---|---|
| A sheet of A4 paper (long side) | 0.000297 km | 9.62511 × 10-18 pc |
| Average adult human height | 0.0017 km | 5.50932 × 10-17 pc |
| A football pitch (length) | 0.105 km | 3.40282 × 10-15 pc |
| A marathon | 42.195 km | 1.36745 × 10-12 pc |
| Height of Mount Everest | 8.849 km | 2.86777 × 10-13 pc |
The kilometre is one thousand metres, written km. It is the standard unit for road distance, geographic separation and travel across almost the entire world, and after the metre itself it is the most widely used SI length unit.
Road signs, speed limits, vehicle odometers and navigation systems use it in every country except the United States, Liberia and Myanmar, where miles remain in place. The United Kingdom occupies a middle position: road signs and speed limits are in miles, while most other official measurement is metric. Running and cycling events are almost universally set in kilometres, with the marathon a notable exception at 42.195 km, a distance fixed by the route used at the 1908 London Olympics rather than by any round figure.
The unit suits the human scale of travel. A brisk walk covers about five kilometres in an hour. Urban journeys are typically a few kilometres, and national distances run to hundreds or thousands. Below one kilometre metres are more natural, and above a few thousand many people switch to comparing flight times instead.
Pronunciation is contested. The stress pattern KIL-o-metre follows the pattern of other SI prefixed units such as kilogram and kilowatt, where the prefix is unstressed. The alternative, ki-LOM-eter, is nonetheless dominant in North America and common in Australia and New Zealand, and both are recorded as standard by major dictionaries.
The kilometre also anchors two other units. A square kilometre is one million square metres, or 100 hectares, and is the usual measure for the area of towns, lakes and protected land. Speed is expressed as kilometres per hour, written km/h.
Astronomy uses the kilometre for objects within the solar system, where larger units would be unwieldy. The Moon averages 384,400 km from Earth and the Sun about 149.6 million km. Beyond that distance astronomers switch to astronomical units, light years and parsecs, because the kilometre figures become too long to read. Orbital velocities are given in kilometres per second: the International Space Station travels at roughly 7.66 km/s.
One kilometre equals 1000 metres or 100,000 centimetres. It is approximately 0.621371 miles, so a five-kilometre race is a little over three miles.
The parsec is a unit of distance equal to about 3.0857 × 1016 metres, or 3.26 light years. The symbol is pc. It is the standard distance unit in professional astronomy, used in preference to the light year in research literature.
The name is a contraction of parallax second, and the definition follows directly from how stellar distances are actually measured. As the Earth orbits the Sun, nearby stars appear to shift slightly against the more distant background. Half of that annual shift is the star's parallax angle. A star with a parallax of one arcsecond lies at a distance of one parsec. The relationship is a simple reciprocal: distance in parsecs equals one divided by parallax in arcseconds, which is why the unit is convenient for observers. No conversion is needed between the measurement and the result.
Formally the parsec is defined as 648,000 divided by pi astronomical units, a value the International Astronomical Union fixed exactly in 2015. Herbert Hall Turner proposed the name in 1913.
No star is close enough to have a parallax of a full arcsecond. Proxima Centauri, the nearest, has a parallax of about 0.77 arcseconds and lies 1.3 parsecs away. The angles involved are minute, which is why parallax was not successfully measured until 1838 despite being sought since antiquity. The Gaia spacecraft has since measured parallaxes for over a billion stars with microarcsecond precision.
Multiples handle larger scales. The Milky Way is about 30 kiloparsecs across, and distances between galaxy clusters run to megaparsecs. The Hubble constant is conventionally quoted in kilometres per second per megaparsec.
The unit is widely known from a line in Star Wars in which the Millennium Falcon completes a route in under twelve parsecs, a distance where a time would be expected.
Cosmology works in megaparsecs and gives the unit a role in one of its central quarrels. The Hubble constant, the rate at which the universe expands, is quoted in kilometres per second per megaparsec: a galaxy one megaparsec away recedes at about seventy kilometres a second, one two megaparsecs away at twice that. Measurements from the cosmic microwave background give a value near 67, while those from supernovae and variable stars give about 73, and the gap between them has resisted a decade of effort to close it. Whatever resolves the disagreement, it will be argued over in this unit.
One parsec equals 3.26156 light years, 206,265 astronomical units, or 30.857 trillion kilometres.