| Parsecs (pc) | Centimeters (cm) |
|---|---|
| 1 Parsec | 3.08567758149 × 1018 cm |
| 2 Parsecs | 6.17135516298 × 1018 cm |
| 3 Parsecs | 9.25703274447 × 1018 cm |
| 4 Parsecs | 1.2342710326 × 1019 cm |
| 5 Parsecs | 1.54283879075 × 1019 cm |
| 10 Parsecs | 3.08567758149 × 1019 cm |
| 20 Parsecs | 6.17135516298 × 1019 cm |
| 25 Parsecs | 7.71419395373 × 1019 cm |
| 50 Parsecs | 1.54283879075 × 1020 cm |
| 100 Parsecs | 3.08567758149 × 1020 cm |
| Reference | Parsecs (pc) | Centimeters (cm) |
|---|---|---|
| A sheet of A4 paper (long side) | 9.62511 × 10-18 pc | 29.7 cm |
| Average adult human height | 5.50932 × 10-17 pc | 170 cm |
| A football pitch (length) | 3.40282 × 10-15 pc | 10500 cm |
| A marathon | 1.36745 × 10-12 pc | 4219500 cm |
| Height of Mount Everest | 2.86777 × 10-13 pc | 884900 cm |
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 centimetre is one hundredth of a metre, written cm. Among the metric subdivisions it is the one people reach for most readily in daily life, occupying the range where objects are small enough to hold but too large to measure comfortably in millimetres.
Body measurement is its most common application. Height, waist, chest and inside leg are recorded in centimetres across most of the world, and clothing is sized accordingly. Paediatric growth charts plot height and head circumference in centimetres against age. Rainfall over long periods, snow depth and the dimensions of furniture and luggage are all reported the same way.
The unit had a formal role in science for nearly a century. The centimetre-gram-second system, adopted by the British Association for the Advancement of Science in 1874, took the centimetre as its base unit of length. CGS units such as the erg, the dyne and the gauss were standard in physics until the metre-kilogram-second system displaced them, and SI formally superseded CGS in 1960. Some CGS units persist in astronomy and in parts of electromagnetism.
Volume follows naturally. A cubic centimetre, written cm3 or cc, equals exactly one millilitre. Engine displacement is often quoted in cubic centimetres, particularly for motorcycles, and medical syringes are marked the same way.
Despite its usefulness, the centimetre sits awkwardly in engineering practice. Technical drawings prefer millimetres precisely to avoid mixing units that differ by a factor of ten, since a misplaced decimal point between the two is a plausible and expensive error.
Map scales often make the unit explicit. A 1:25,000 map means one centimetre on the paper represents 25,000 centimetres on the ground, or 250 metres, so four centimetres cover a kilometre. Walkers and orienteers use this relationship constantly, and it is one of the clearer illustrations of why a decimal system is convenient: converting between the two scales requires only moving a decimal point.
Two well-known wavelengths fall in this range and are named by it. Neutral hydrogen radiates at 21 centimetres, a line predicted in 1944 and detected seven years later, and because hydrogen fills the galaxy that single wavelength has mapped the spiral arms of the Milky Way and the rotation curves that first indicated dark matter. Domestic microwave ovens work at 12.2 centimetres, chosen from a band set aside for industrial and medical use rather than for any special resonance with water. In both cases the centimetre is the natural unit because the wave is the size of a hand.
One centimetre equals 10 millimetres or 0.01 metres, and is very close to 0.3937 inches.