| Light years (ly) | Miles (mi) |
|---|---|
| 1 Light year | 5878625373184 mi |
| 2 Light years | 11757250746367 mi |
| 3 Light years | 17635876119551 mi |
| 4 Light years | 23514501492734 mi |
| 5 Light years | 29393126865918 mi |
| 10 Light years | 58786253731836 mi |
| 20 Light years | 117572507463672 mi |
| 25 Light years | 146965634329590 mi |
| 50 Light years | 293931268659180 mi |
| 100 Light years | 587862537318361 mi |
| Reference | Light years (ly) | Miles (mi) |
|---|---|---|
| A sheet of A4 paper (long side) | 3.13929 × 10-17 ly | 0.000184547 mi |
| Average adult human height | 1.7969 × 10-16 ly | 0.00105633 mi |
| A football pitch (length) | 1.10985 × 10-14 ly | 0.065244 mi |
| A marathon | 4.46002 × 10-12 ly | 26.2188 mi |
| Height of Mount Everest | 9.3534 × 10-13 ly | 5.49851 mi |
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.
The mile is a unit of length equal to 5280 feet, 1760 yards, or exactly 1609.344 metres. The symbol is mi. It is the principal unit of road distance in the United States and the United Kingdom.
Its name comes from the Roman mille passus, a thousand paces, where a pace counted two steps. That unit measured about 1479 metres, appreciably shorter than the modern mile. The English mile grew to its present length because it was reconciled with the furlong, an older agricultural measure. Making the mile eight furlongs required 5280 feet, and Parliament fixed that figure by statute in 1593, which is why the unit is sometimes called the statute mile to distinguish it from other historical miles.
Those alternatives were numerous. The Scots mile, the Irish mile and various German and Scandinavian miles all differed, some substantially. The Swedish mil, still used colloquially, is ten kilometres.
Road signs, speed limits and vehicle odometers use the mile in the United States, the United Kingdom, Liberia and Myanmar, and speeds are given in miles per hour. The United Kingdom's position is inconsistent by design: distances and speeds are imperial while most other official measurement is metric, an arrangement that has survived repeated proposals for change.
Athletics keeps the mile as the one non-metric distance with continuing international standing. World records are ratified over the mile alongside metric distances, largely because of the historical weight of the four-minute barrier broken by Roger Bannister in 1954. The current record stands under 3 minutes 44 seconds.
Fuel economy in the United States is expressed as miles per gallon, and aviation and shipping use the separate nautical mile, which is longer at 1852 metres and unrelated in derivation.
The map of the United States is drawn on it. The Public Land Survey, begun in 1785, divided the western territories into townships six miles square, each containing thirty-six sections of one square mile, and each section of 640 acres subdividing neatly into quarters and quarter-quarters. Roads followed the section lines, which is why so much of the American Midwest is a grid of straight routes one mile apart, and why farms there are described in quarter sections rather than in acres. Anyone flying over Iowa or Kansas is looking at the mile drawn across an entire landscape, two centuries after the surveyors laid it out.
One mile equals 1760 yards, 5280 feet, 8 furlongs, or 1609.344 metres. A five-kilometre race is about 3.107 miles.