| Parsecs (pc) | Decimeters (dm) |
|---|---|
| 1 Parsec | 3.08567758149 × 1017 dm |
| 2 Parsecs | 6.17135516298 × 1017 dm |
| 3 Parsecs | 9.25703274447 × 1017 dm |
| 4 Parsecs | 1.2342710326 × 1018 dm |
| 5 Parsecs | 1.54283879075 × 1018 dm |
| 10 Parsecs | 3.08567758149 × 1018 dm |
| 20 Parsecs | 6.17135516298 × 1018 dm |
| 25 Parsecs | 7.71419395373 × 1018 dm |
| 50 Parsecs | 1.54283879075 × 1019 dm |
| 100 Parsecs | 3.08567758149 × 1019 dm |
| Reference | Parsecs (pc) | Decimeters (dm) |
|---|---|---|
| A sheet of A4 paper (long side) | 9.62511 × 10-18 pc | 2.97 dm |
| Average adult human height | 5.50932 × 10-17 pc | 17 dm |
| A football pitch (length) | 3.40282 × 10-15 pc | 1050 dm |
| A marathon | 1.36745 × 10-12 pc | 421950 dm |
| Height of Mount Everest | 2.86777 × 10-13 pc | 88490 dm |
The parsec is a unit of distance equal to about 3.0857 × 1016 metres, or 3.26 light years. The symbol is pc. It is the standard distance unit in professional astronomy, used in preference to the light year in research literature.
The name is a contraction of parallax second, and the definition follows directly from how stellar distances are actually measured. As the Earth orbits the Sun, nearby stars appear to shift slightly against the more distant background. Half of that annual shift is the star's parallax angle. A star with a parallax of one arcsecond lies at a distance of one parsec. The relationship is a simple reciprocal: distance in parsecs equals one divided by parallax in arcseconds, which is why the unit is convenient for observers. No conversion is needed between the measurement and the result.
Formally the parsec is defined as 648,000 divided by pi astronomical units, a value the International Astronomical Union fixed exactly in 2015. Herbert Hall Turner proposed the name in 1913.
No star is close enough to have a parallax of a full arcsecond. Proxima Centauri, the nearest, has a parallax of about 0.77 arcseconds and lies 1.3 parsecs away. The angles involved are minute, which is why parallax was not successfully measured until 1838 despite being sought since antiquity. The Gaia spacecraft has since measured parallaxes for over a billion stars with microarcsecond precision.
Multiples handle larger scales. The Milky Way is about 30 kiloparsecs across, and distances between galaxy clusters run to megaparsecs. The Hubble constant is conventionally quoted in kilometres per second per megaparsec.
The unit is widely known from a line in Star Wars in which the Millennium Falcon completes a route in under twelve parsecs, a distance where a time would be expected.
Cosmology works in megaparsecs and gives the unit a role in one of its central quarrels. The Hubble constant, the rate at which the universe expands, is quoted in kilometres per second per megaparsec: a galaxy one megaparsec away recedes at about seventy kilometres a second, one two megaparsecs away at twice that. Measurements from the cosmic microwave background give a value near 67, while those from supernovae and variable stars give about 73, and the gap between them has resisted a decade of effort to close it. Whatever resolves the disagreement, it will be argued over in this unit.
One parsec equals 3.26156 light years, 206,265 astronomical units, or 30.857 trillion kilometres.
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.