| Yards (yd) | Light years (ly) |
|---|---|
| 1 Yard | 9.66521562632 × 10-17 ly |
| 2 Yards | 1.93304312526 × 10-16 ly |
| 3 Yards | 2.8995646879 × 10-16 ly |
| 4 Yards | 3.86608625053 × 10-16 ly |
| 5 Yards | 4.83260781316 × 10-16 ly |
| 10 Yards | 9.66521562632 × 10-16 ly |
| 20 Yards | 1.93304312526 × 10-15 ly |
| 25 Yards | 2.41630390658 × 10-15 ly |
| 50 Yards | 4.83260781316 × 10-15 ly |
| 100 Yards | 9.66521562632 × 10-15 ly |
| Reference | Yards (yd) | Light years (ly) |
|---|---|---|
| A sheet of A4 paper (long side) | 0.324803 yd | 3.13929 × 10-17 ly |
| Average adult human height | 1.85914 yd | 1.7969 × 10-16 ly |
| A football pitch (length) | 114.829 yd | 1.10985 × 10-14 ly |
| A marathon | 46145 yd | 4.46002 × 10-12 ly |
| Height of Mount Everest | 9677.38 yd | 9.3534 × 10-13 ly |
The yard is a unit of length equal to exactly 0.9144 metres, or three feet. The symbol is yd. It is the base unit of the imperial and United States customary length systems, with the foot and inch defined as fractions of it.
That relationship is the reverse of what most people assume. Since 1959 the yard has been defined against the metre, and the foot and inch follow from it, so every imperial length unit is ultimately derived from an SI definition. The International Yard and Pound Agreement fixed the value at 0.9144 metres exactly, reconciling small differences between the American and British yards that had persisted for decades.
The origin of the name is uncertain. Old English gierd meant a rod or measuring stick. The frequently repeated story that Henry I defined the yard as the distance from his nose to his outstretched thumb is not supported by contemporary evidence, though it has proved durable. A physical standard yard bar was maintained in London from 1760, and its destruction in the 1834 Palace of Westminster fire forced a lengthy reconstruction from surviving copies.
Sport keeps the unit prominent. An American football field is 100 yards between goal lines, and the game's vocabulary is built around yardage. Golf course distances are marked in yards in Britain and the United States. Cricket sets the distance between wickets at 22 yards, a figure inherited from the surveyor's chain.
Fabric and carpet are sold by the yard in the United States, and textile widths are quoted in inches alongside. Landscaping materials such as topsoil and gravel are sold by the cubic yard, which is 27 cubic feet or about 0.7646 cubic metres.
Its cube runs the construction trade in North America. Concrete is ordered, delivered and billed by the cubic yard, about 0.765 cubic metres, and a standard mixer truck carries eight or nine of them; excavation, gravel, topsoil and demolition waste are all quoted the same way. Golf keeps the linear unit just as firmly, with every hole on every scorecard measured in yards and every club sold on the distance it carries, so a player who reads a shot as a hundred and fifty yards is using a measurement the rest of the game's arithmetic depends on. Between them the two uses make the yard unlikely to disappear.
One yard equals 3 feet, 36 inches, or 0.9144 metres exactly. It is a little shorter than a metre, by about 8.4 centimetres.
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.