| Decimeters (dm) | Astronomical units (au) |
|---|---|
| 1 Decimeter | 6.68458712227 × 10-13 au |
| 2 Decimeters | 1.33691742445 × 10-12 au |
| 3 Decimeters | 2.00537613668 × 10-12 au |
| 4 Decimeters | 2.67383484891 × 10-12 au |
| 5 Decimeters | 3.34229356113 × 10-12 au |
| 10 Decimeters | 6.68458712227 × 10-12 au |
| 20 Decimeters | 1.33691742445 × 10-11 au |
| 25 Decimeters | 1.67114678057 × 10-11 au |
| 50 Decimeters | 3.34229356113 × 10-11 au |
| 100 Decimeters | 6.68458712227 × 10-11 au |
| Reference | Decimeters (dm) | Astronomical units (au) |
|---|---|---|
| A sheet of A4 paper (long side) | 2.97 dm | 1.98532 × 10-12 au |
| Average adult human height | 17 dm | 1.13638 × 10-11 au |
| A football pitch (length) | 1050 dm | 7.01882 × 10-10 au |
| A marathon | 421950 dm | 0.000000282056 au |
| Height of Mount Everest | 88490 dm | 0.0000000591519 au |
The decimetre is one tenth of a metre, written dm. It is a legitimate SI-derived unit formed with the prefix deci, but it is among the least used members of the metric ladder. Most people move directly from centimetres to metres and skip the intermediate step entirely.
The reason is practical rather than technical. A length of 3 dm is more naturally expressed as 30 cm or 0.3 m, and neither alternative requires the reader to pause. Units survive when they answer a question no neighbouring unit answers as well, and the decimetre rarely does.
One derived form is a striking exception. The cubic decimetre, dm3, is exactly one litre, because a cube measuring 10 cm on each side holds precisely that volume. This relationship is the foundation of the metric volume system and was deliberate: the litre was defined in 1795 as the volume of a cubic decimetre. The unit therefore appears constantly in chemistry, where concentration is expressed in moles per cubic decimetre, written mol/dm3. School and university chemistry courses use this notation routinely even though the same quantity could be written as moles per litre.
The square decimetre sees occasional use in specifying small areas, particularly in materials testing and in some European technical standards for coatings and surface treatment.
Outside these niches the decimetre appears mainly in teaching, where the full sequence of prefixes is demonstrated, and in tables of unit conversions. Some countries have used it historically for shoe sizing and for textile measurement.
Aquarium and tank capacities show the same relationship at work. A tank measuring five by three by four decimetres holds sixty cubic decimetres, which is sixty litres, and the arithmetic can be done without conversion factors. This is the practical advantage the metric system was designed to deliver, and the decimetre is the length at which volume in litres and length in whole units line up most neatly.
Scandinavia is the exception to its general neglect. Swedish, Norwegian and Danish speakers use the decimetre in ordinary conversation, giving the size of a fish, a shelf or a snowfall in decimetres where a French or German speaker would say tens of centimetres. Schools there teach it alongside the centimetre and the metre rather than skipping it, and rulers are marked accordingly. The habit is a reminder that which prefixes feel natural is a matter of custom rather than logic: the SI offers the whole ladder, and each language community has quietly settled on the rungs it finds comfortable.
One decimetre equals 10 centimetres, 100 millimetres, or 0.1 metres. It is approximately 3.937 inches, a little under four inches.
The astronomical unit is a length equal to exactly 149,597,870,700 metres, or very nearly 150 million kilometres. The symbol is au. It approximates the mean distance between the Earth and the Sun and is the standard measure for distances within the solar system.
The unit was originally defined by that orbital relationship rather than by a fixed number. Earlier definitions tied it to the properties of a hypothetical body orbiting the Sun, which meant its value depended on the gravitational constant and was subject to revision as measurements improved. The International Astronomical Union ended that dependence in 2012 by fixing the astronomical unit as an exact number of metres. The change simplified calculations and removed an inconvenience: a unit whose length shifted whenever a physical constant was refined.
Determining its value was one of the great problems of observational astronomy. Transits of Venus across the Sun's disc, observed from widely separated points on Earth, allowed the distance to be triangulated. Expeditions were mounted for the transits of 1761, 1769, 1874 and 1882, and James Cook's first Pacific voyage was organised around the 1769 event. Radar ranging to Venus in the 1960s eventually settled the figure far more precisely than any optical method.
The unit makes solar system distances legible. Mercury orbits at 0.39 au, Mars at 1.52 au, Jupiter at 5.2 au and Neptune at 30.1 au. The Kuiper Belt extends to roughly 50 au. Voyager 1, the most distant human-made object, has passed 165 au. Light takes about 499 seconds to cover one astronomical unit, so the Sun is a little over eight light minutes away.
Beyond the solar system the unit becomes unwieldy, and astronomers switch to light years and parsecs. One parsec is 206,265 au.
Spacecraft navigation depends on the unit being exact rather than approximate. Trajectories to the outer planets are computed over distances of tens of astronomical units, and an error in the length of the unit itself would propagate into every position calculation. Fixing the value in 2012 removed that source of drift from the ephemerides used for mission planning.
One astronomical unit equals 149,597,870.7 kilometres, or about 92.956 million miles.