The distance at which a star shows a parallax of one arcsecond when the Earth moves by one astronomical unit; its name comes from “parallax second”. One parsec is 3.26 light-years, or 206,265 astronomical units. Professional astronomers use it more than the light-year.
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Related terms
- Light-year
The distance light travels in a year, at 300,000 km/s: about 9.46 trillion kilometres. It is a unit of distance, not time. The nearest star after the Sun, Proxima Centauri, is 4.2 light-years away.
- Astronomical unit (au)
The mean distance between the Earth and the Sun, fixed at 149,597,870.7 km, or about 150 million kilometres. Light covers it in 8 min 20 s. Jupiter is about 5.2 au from the Sun, Neptune 30 au.
- Absolute magnitude
How bright an object would look if it were placed 10 parsecs (32.6 light-years) away. It measures its true luminosity, regardless of distance. The Sun, dazzling at −26.7, has an absolute magnitude of only +4.8; Rigel reaches about −7.
- Arcsecond
One sixtieth of an arcminute, or 1/3,600 of a degree, written ″: the apparent size of a coin about 2.5 cm wide seen from about 5 km away. Jupiter spans 30 to 50″, Mars 25″ at most. Seeing in western Europe often spreads stars over 2 to 3″.
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