| Light years (ly) | Chains (ch) |
|---|---|
| 1 Light year | 470290029854689 ch |
| 2 Light years | 940580059709377 ch |
| 3 Light years | 1.41087008956 × 1015 ch |
| 4 Light years | 1.88116011942 × 1015 ch |
| 5 Light years | 2.35145014927 × 1015 ch |
| 10 Light years | 4.70290029855 × 1015 ch |
| 20 Light years | 9.40580059709 × 1015 ch |
| 25 Light years | 1.17572507464 × 1016 ch |
| 50 Light years | 2.35145014927 × 1016 ch |
| 100 Light years | 4.70290029855 × 1016 ch |
| Reference | Light years (ly) | Chains (ch) |
|---|---|---|
| A sheet of A4 paper (long side) | 3.13929 × 10-17 ly | 0.0147638 ch |
| Average adult human height | 1.7969 × 10-16 ly | 0.0845065 ch |
| A football pitch (length) | 1.10985 × 10-14 ly | 5.21952 ch |
| A marathon | 4.46002 × 10-12 ly | 2097.5 ch |
| Height of Mount Everest | 9.3534 × 10-13 ly | 439.881 ch |
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 chain is a unit of length equal to 66 feet, 22 yards, or exactly 20.1168 metres. It occupies an unusual position in the imperial system as the unit that connected everyday measurement to land surveying.
It is named after a physical object. Edmund Gunter, an English mathematician, introduced a surveying chain of 100 iron links in 1620. The design solved a real problem: land area was recorded in acres, a unit inherited from medieval agriculture, while distances were measured in feet and yards. Gunter chose a length that reconciled the two systems. Ten square chains make exactly one acre, and 80 chains make one mile, so a surveyor could measure in chains and compute both area in acres and distance in miles without awkward conversion factors.
The chain shaped the landscape of several countries. The United States Public Land Survey System, which divided much of the American west into townships and sections from 1785, was laid out in chains. A section is one square mile, or 80 chains on a side, and the resulting grid remains visible in road patterns and field boundaries across the Midwest. Similar surveys in Canada, Australia and New Zealand used the same unit, and older property deeds in those countries still cite chain measurements.
Cricket preserves it in sporting form. The pitch measures 22 yards between the wickets, which is exactly one chain, a length fixed long before the game's laws were formally written.
British railways continue to use the chain for locating features along a line. Positions are recorded as a mileage and a chain figure, so 12 miles 40 chains identifies a point precisely. The convention survives in signalling diagrams and engineering records despite general metrication.
The chain subdivides into 100 links of 7.92 inches each, giving surveyors a decimal subdivision within an otherwise non-decimal system.
One chain equals 22 yards, 66 feet, or 20.1168 metres.