Parsec
A parsec is the distance at which one astronomical unit subtends an angle of one arcsecond. It is about 3.26 light-years.
Stellar parallax: The apparent shift in a nearby star’s position against distant stars as Earth moves around the Sun. A star’s parallax angle determines its distance in parsecs.
Stellar distance: The separation between a star and an observer, usually expressed in astronomical distance units. Parsecs are a standard unit for distances to stars.
Friedrich Wilhelm Bessel: A German astronomer who made the first reliable measurement of stellar parallax, for 61 Cygni, in 1838. His measurement demonstrated the geometric method later expressed through parsecs.
Light-year: The distance light travels in vacuum in one Julian year, about 9.46 trillion kilometres. It measures the same kind of distance but is about 3.26 times shorter than a parsec.
Astronomical unit: The mean distance between Earth and the Sun, defined as exactly 149,597,870,700 metres. This baseline forms the right triangle in the parsec’s geometric definition.
Kiloparsec: A unit equal to 1,000 parsecs, commonly used for distances within galaxies. It scales the parsec to the size of galactic structures.
61 Cygni: A nearby binary star system whose parallax Bessel measured in 1838. Its measured parallax provided an early concrete stellar distance for the unit’s underlying method.
Astronomical distance unit: A unit used to express distances between astronomical objects or within the universe. The parsec is one member of this broader family of distance units.
Arcsecond: An angular unit equal to one sixtieth of an arcminute, or one 3,600th of a degree. One arcsecond is the angle specified in the parsec’s definition.
Megaparsec: A unit equal to one million parsecs, commonly used for distances between galaxies. It extends parsec-based measurements to the extragalactic universe.