| Leagues (lea) | Light years (ly) |
|---|---|
| 1 League | 5.1032338507 × 10-13 ly |
| 2 Leagues | 1.02064677014 × 10-12 ly |
| 3 Leagues | 1.53097015521 × 10-12 ly |
| 4 Leagues | 2.04129354028 × 10-12 ly |
| 5 Leagues | 2.55161692535 × 10-12 ly |
| 10 Leagues | 5.1032338507 × 10-12 ly |
| 20 Leagues | 1.02064677014 × 10-11 ly |
| 25 Leagues | 1.27580846267 × 10-11 ly |
| 50 Leagues | 2.55161692535 × 10-11 ly |
| 100 Leagues | 5.1032338507 × 10-11 ly |
| Reference | Leagues (lea) | Light years (ly) |
|---|---|---|
| A sheet of A4 paper (long side) | 0.0000615157 lea | 3.13929 × 10-17 ly |
| Average adult human height | 0.00035211 lea | 1.7969 × 10-16 ly |
| A football pitch (length) | 0.021748 lea | 1.10985 × 10-14 ly |
| A marathon | 8.73959 lea | 4.46002 × 10-12 ly |
| Height of Mount Everest | 1.83284 lea | 9.3534 × 10-13 ly |
The league is a unit of length equal to three miles, 24 furlongs, or exactly 4828.032 metres. It is obsolete in every practical sense, surviving mainly in literature and in historical documents.
The unit originated as a measure of how far a person could walk in an hour, which explains both its rough magnitude and its variability. Ancient and medieval leagues differed widely between regions. The Roman leuga was about 2.2 kilometres. French, Spanish and Portuguese leagues each had their own values, and the Spanish legua used in colonial land grants in the Americas differed again, complicating the interpretation of old property records in Texas and the American southwest.
English usage settled on three miles, a figure that was never especially precise in practice because it depended on the mile in force at the time. Since the mile itself was fixed at 5280 feet in 1593, the league inherited that definition, giving the modern value of 4828.032 metres.
Its cultural persistence far exceeds its practical use. Jules Verne's Twenty Thousand Leagues Under the Seas measures a journey rather than a depth, a distinction frequently lost in casual reference: twenty thousand leagues is roughly 96,000 kilometres, more than twice the circumference of the Earth and vastly greater than any ocean depth. Seven-league boots appear in French fairy tales as a device for covering ground in single strides. Tennyson's Charge of the Light Brigade opens with half a league onward.
The unit was formally abolished for trade in the United Kingdom long ago and has no legal standing. It appears occasionally in nautical contexts, where the marine league of three nautical miles is a separate and slightly longer unit.
Because the unit is so variable across languages and centuries, translations of older works can mislead badly. A distance given in leagues in a Spanish colonial document, a French medieval chronicle and an English statute may differ by a factor of two, and historians working with such sources generally convert to modern units and state which league they have assumed.
One league equals 3 miles, 4828.032 metres, or 4.828032 kilometres. A brisk hour's walk covers rather less than one.
The light year is the distance light travels in vacuum during one Julian year, equal to exactly 9,460,730,472,580,800 metres, or about 9.46 trillion kilometres. The symbol is ly.
It is a unit of distance, not of time, a point the name obscures and which causes persistent confusion. Its exactness follows from two defined quantities: the speed of light is fixed at 299,792,458 metres per second, and a Julian year is defined as exactly 365.25 days of 86,400 seconds. Multiplying the two gives a figure with no measurement uncertainty at all.
The unit's appeal is that it makes the finite speed of light explicit. Looking at an object twelve light years away means seeing it as it was twelve years ago, and this delay is not a curiosity but the basis of observational cosmology. The most distant galaxies observed lie billions of light years away, so telescopes are effectively instruments for looking into the past.
Nearby distances are modest by the standard of the unit. Proxima Centauri, the closest star to the Sun, is 4.25 light years away. Sirius is 8.6. The centre of the Milky Way lies about 26,000 light years from Earth, and the galaxy spans roughly 100,000. The Andromeda Galaxy is some 2.5 million light years distant and is visible to the unaided eye from a dark site, which makes it the most remote object most people will ever see directly.
Professional astronomers generally prefer the parsec, which arises naturally from parallax measurement, and journals report distances in parsecs, kiloparsecs and megaparsecs. The light year dominates popular writing because it needs no explanation beyond the speed of light.
Related units follow the same principle at smaller scales. The light second, about 300,000 kilometres, is close to the Earth-Moon distance, and radio engineers use the light nanosecond, roughly 30 centimetres, when reasoning about signal propagation along a cable. The light minute and light hour occasionally appear in descriptions of the outer solar system.
One light year equals 63,241 astronomical units, about 0.3066 parsecs, or 9.4607 trillion kilometres.