| Decameters (dam) | Light years (ly) |
|---|---|
| 1 Decameter | 1.05700083402 × 10-15 ly |
| 2 Decameters | 2.11400166805 × 10-15 ly |
| 3 Decameters | 3.17100250207 × 10-15 ly |
| 4 Decameters | 4.2280033361 × 10-15 ly |
| 5 Decameters | 5.28500417012 × 10-15 ly |
| 10 Decameters | 1.05700083402 × 10-14 ly |
| 20 Decameters | 2.11400166805 × 10-14 ly |
| 25 Decameters | 2.64250208506 × 10-14 ly |
| 50 Decameters | 5.28500417012 × 10-14 ly |
| 100 Decameters | 1.05700083402 × 10-13 ly |
| Reference | Decameters (dam) | Light years (ly) |
|---|---|---|
| A sheet of A4 paper (long side) | 0.0297 dam | 3.13929 × 10-17 ly |
| Average adult human height | 0.17 dam | 1.7969 × 10-16 ly |
| A football pitch (length) | 10.5 dam | 1.10985 × 10-14 ly |
| A marathon | 4219.5 dam | 4.46002 × 10-12 ly |
| Height of Mount Everest | 884.9 dam | 9.3534 × 10-13 ly |
The decametre is ten metres, written dam. It is a valid SI unit formed with the prefix deca, but like the decimetre it is rarely used. The prefix deca is the only SI multiplier whose symbol is two letters, which makes it slightly awkward to write and may have contributed to its neglect.
Spelling varies. Decametre is the international form; dekameter appears in some American texts, and the symbol dkm is occasionally seen in older material, though dam is the form recognised by the BIPM. The similarity between dam and the abbreviation for decimetre has caused confusion, and some standards bodies discourage the unit for that reason alone.
Where it does appear, it is usually in surveying, forestry and agronomy. Field dimensions, plot spacing and tree heights sit conveniently in the range of a few decametres. Some European cadastral records use the unit. It also occurs in specifications for cable and hose supplied in ten-metre lengths.
The derived area unit is more familiar than the length unit. A square decametre is 100 square metres, which is exactly one are. The are is itself little used, but its hundredfold multiple, the hectare, is the standard measure of land area across most of the world. A hectare is 100 ares, or 10,000 square metres, and can equally be described as a square hectometre.
In practice most people express ten metres simply as ten metres, or as 0.01 kilometres when working at larger scales. The decametre is best understood as a formally correct unit that everyday usage has passed over.
The unit does appear in one everyday context without being named. Swimming pool lane lengths, running track segments and the spacing of street lighting are frequently set in multiples of ten metres. Planning documents that lay out plot frontages or building setbacks often work to the same grid. The quantity is useful; it is only the word that has fallen out of use.
Its square has fared better than the length itself. The are, a hundred square metres, is the square decametre under another name, and although the are is now rare its hundredfold the hectare governs land measurement across most of the world. The same pattern shows in the decare of a thousand square metres, which is the standard unit of farmland in Turkey, Greece, Norway and parts of the Balkans and is simply ten ares. So the decametre survives chiefly by inheritance: nobody quotes a field boundary in decametres, but a great deal of the world's land is bought and sold in units built from it.
One decametre equals 10 metres, 1000 centimetres, or 0.01 kilometres. It is about 32.8084 feet.
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.