Subscribe to BBC Focus magazine for fascinating new Q&As every month and. The nearest star, Proxima Centauri, is about 1.3 parsecs away, while the Milky Way is more than 31,000 parsecs across.

‘But cosmologists are so comfortable with millions and billions that none of them feel the urge to invent a unit. One parsec is defined as the distance at which one astronomical unit (the average distance between the Earth and Sun) subtends an angle of one arc-second. One parsec corresponds to the distance at which the mean radius of the earth's orbit subtends an angle of one second of arc. To clear things up, arcsec is a measure of an angle, while parsec is a measure of distance. A unit of distance used in astronomy, equal to about 3.26 light years (3.086 × 10 kilometres). These angles are most of the time so small that they can only be denoted using arcminutes or arcseconds.Īrcsec is also often taken to be related to parsec, another term in astronomy. This unit of measurement is extremely useful in astronomy where the apparent size of a celestial object is usually gauged by means of angular measurements. The term arcsecond is also abbreviated to arcsec, but this term is often confused with the arc secant, which is a trigonometric function that bears the same abbreviation. As an example, 1 arcsecond is written as 1”. Defining a unit is often more complex than first appears. The symbol used to mark an arcsecond is the double quote. While smaller than the parsec, it is still an incredibly large distance. Simply put, it is equal to 1/3600 degrees of an arc. An arcsecond, also called a second of an arc, is a unit of measurement that amounts to one sixtieth of an arcminute.

Parsec definition