| Hectometers (hm) | Parsecs (pc) |
|---|---|
| 1 Hectometer | 3.24077928944 × 10-15 pc |
| 2 Hectometers | 6.48155857889 × 10-15 pc |
| 3 Hectometers | 9.72233786833 × 10-15 pc |
| 4 Hectometers | 1.29631171578 × 10-14 pc |
| 5 Hectometers | 1.62038964472 × 10-14 pc |
| 10 Hectometers | 3.24077928944 × 10-14 pc |
| 20 Hectometers | 6.48155857889 × 10-14 pc |
| 25 Hectometers | 8.10194822361 × 10-14 pc |
| 50 Hectometers | 1.62038964472 × 10-13 pc |
| 100 Hectometers | 3.24077928944 × 10-13 pc |
| Reference | Hectometers (hm) | Parsecs (pc) |
|---|---|---|
| A sheet of A4 paper (long side) | 0.00297 hm | 9.62511 × 10-18 pc |
| Average adult human height | 0.017 hm | 5.50932 × 10-17 pc |
| A football pitch (length) | 1.05 hm | 3.40282 × 10-15 pc |
| A marathon | 421.95 hm | 1.36745 × 10-12 pc |
| Height of Mount Everest | 88.49 hm | 2.86777 × 10-13 pc |
The hectometre is one hundred metres, written hm. It uses the SI prefix hecto, from the Greek hekaton meaning hundred. Like the decametre and the decimetre it is a legitimate unit that ordinary usage has largely bypassed in favour of metres and kilometres.
Its clearest application is in land measurement, where it appears indirectly. A square hectometre is 10,000 square metres, which is exactly one hectare. The hectare is the standard unit of land area in agriculture, forestry and property registration across most of the world, so the hectometre is embedded in daily practice even though the length itself is seldom named. A square field measuring one hectometre on each side covers one hectare.
Rail and road infrastructure provides the other common use. Several European railway networks mark distances with hectometre posts, placing a marker every hundred metres along the track. These give maintenance crews and signallers a precise location reference, and incident reports commonly cite a kilometre and hectometre figure. Dutch and Belgian motorways use hectometre markers in the same way, and drivers are directed to quote them when reporting a breakdown.
Athletics offers a familiar length without using the name. The straight of a standard outdoor track is one hectometre, and the 100 metres is the shortest standard sprint distance. Nobody calls it the one-hectometre sprint.
Meteorology uses the unit for cloud base height in some aviation reporting formats, where reporting in hundreds of metres or hundreds of feet keeps the figures compact.
Nautical charts and aviation both work in comparable steps without adopting the name. Runway visual range is reported in hundreds of metres in many countries, and visibility in aviation weather reports is given in metres up to five thousand, effectively in hectometre increments. Where the unit is used explicitly, as on Dutch motorways, its advantage is precision: a hectometre marker locates an incident to within fifty metres.
Radio engineering names a whole band after it. The international classification calls waves between 100 and 1000 metres hectometric, corresponding to frequencies from 300 kilohertz to 3 megahertz, and that band is the medium wave used for broadcasting since the 1920s. A transmitter working at 1000 kilohertz radiates a wave three hectometres long, which is why medium-wave aerials are tall masts rather than the short whips that serve higher frequencies. The naming continues in both directions: the band below is kilometric and the one above decametric, so the ladder of metric prefixes maps directly onto the divisions of the radio spectrum.
One hectometre equals 100 metres, 10 decametres, or 0.1 kilometres. It is approximately 328.084 feet, or 109.361 yards.
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.