| Parsecs (pc) | Nautical leagues (nlea) |
|---|---|
| 1 Parsec | 5553775344657 nlea |
| 2 Parsecs | 11107550689314 nlea |
| 3 Parsecs | 16661326033971 nlea |
| 4 Parsecs | 22215101378628 nlea |
| 5 Parsecs | 27768876723284 nlea |
| 10 Parsecs | 55537753446569 nlea |
| 20 Parsecs | 111075506893138 nlea |
| 25 Parsecs | 138844383616422 nlea |
| 50 Parsecs | 277688767232844 nlea |
| 100 Parsecs | 555377534465688 nlea |
| Reference | Parsecs (pc) | Nautical leagues (nlea) |
|---|---|---|
| A sheet of A4 paper (long side) | 9.62511 × 10-18 pc | 0.0000534557 nlea |
| Average adult human height | 5.50932 × 10-17 pc | 0.000305976 nlea |
| A football pitch (length) | 3.40282 × 10-15 pc | 0.0188985 nlea |
| A marathon | 1.36745 × 10-12 pc | 7.59449 nlea |
| Height of Mount Everest | 2.86777 × 10-13 pc | 1.59269 nlea |
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.
The nautical league is a unit of length equal to three nautical miles, or exactly 5556 metres. It is longer than the land league by about 728 metres, since it is built from the nautical mile of 1852 metres rather than the statute mile.
The unit belonged to an era when it was the practical measure of coastal jurisdiction. The cannon shot rule, articulated in the seventeenth and eighteenth centuries, held that a state's authority extended as far as it could defend from shore, conventionally taken as three nautical miles. That distance became the standard territorial sea limit and was expressed as one marine league. Many older treaties and statutes define maritime boundaries in leagues for this reason.
The three-mile limit held for centuries before eroding through the twentieth. States progressively claimed wider waters, and the United Nations Convention on the Law of the Sea settled the matter in 1982 by establishing a twelve nautical mile territorial sea and a two hundred mile exclusive economic zone. Both figures are stated in nautical miles rather than leagues, and the older unit fell out of legal use.
Charts and pilot books from the age of sail used the league routinely for describing offshore distances and the visibility of landmarks. Reading those sources accurately requires knowing which league is meant, since the same word could refer to a land league, a marine league or a Spanish or Portuguese variant, and the differences are large enough to matter when reconstructing a historical position.
Today the unit has no navigational role. Charts are marked in nautical miles, and distances at sea are quoted the same way. The nautical league appears only in historical and legal texts.
The unit also underlies a familiar figure in visibility calculations. From a height of about three metres above sea level, the horizon lies roughly one nautical league away, which is why the three-mile limit corresponded loosely to what a lookout could see as well as to what a gun could reach.
One nautical league equals 3 nautical miles, 5556 metres, 5.556 kilometres, or about 3.452 statute miles.