| Nautical leagues (nlea) | Meters (m) |
|---|---|
| 1 Nautical league | 5556 m |
| 2 Nautical leagues | 11112 m |
| 3 Nautical leagues | 16668 m |
| 4 Nautical leagues | 22224 m |
| 5 Nautical leagues | 27780 m |
| 10 Nautical leagues | 55560 m |
| 20 Nautical leagues | 111120 m |
| 25 Nautical leagues | 138900 m |
| 50 Nautical leagues | 277800 m |
| 100 Nautical leagues | 555600 m |
| Reference | Nautical leagues (nlea) | Meters (m) |
|---|---|---|
| A sheet of A4 paper (long side) | 0.0000534557 nlea | 0.297 m |
| Average adult human height | 0.000305976 nlea | 1.7 m |
| A football pitch (length) | 0.0188985 nlea | 105 m |
| A marathon | 7.59449 nlea | 42195 m |
| Height of Mount Everest | 1.59269 nlea | 8849 m |
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.
The metre is the base unit of length in the International System of Units, written m. Every other SI length unit is defined as a multiple or fraction of it, and units of area, volume, speed and many others are built from it in turn.
Its definition has been revised four times, each revision replacing a physical object with something more reproducible. The French Academy of Sciences proposed in 1791 that the metre should be one ten-millionth of the distance from the North Pole to the equator along the meridian through Paris. Surveying that arc took six years and produced a small error, since the Earth is not a perfect sphere. In 1889 the definition moved to a platinum-iridium bar held at Sèvres, near Paris, with copies distributed to signatory nations. In 1960 it was redefined as 1,650,763.73 wavelengths of orange-red light emitted by krypton-86.
The current definition dates from 1983 and is unusual in that it fixes a different quantity. The speed of light in vacuum is defined as exactly 299,792,458 metres per second, and the metre follows as the distance light travels in 1/299,792,458 of a second. Light speed is therefore no longer measured; it is a defined constant, and improvements in measurement now refine the metre rather than the speed.
The unit is used worldwide for building dimensions, athletics tracks, swimming pools, water depth, fabric and rope. Only a small number of countries retain imperial units for general purposes, and even there the metre dominates science and medicine.
Everyday reference points help fix the scale. A standard interior door is roughly two metres tall. A single stride for an adult is close to three quarters of a metre, which is why pacing is a workable rough measure of distance. The width of a single traffic lane is between three and three and a half metres in most countries.
One metre equals 100 centimetres, 1000 millimetres, or 0.001 kilometres. It is approximately 3.28084 feet, or 39.3701 inches.