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parsec

Convert between parsecs and other major units of astronomical distance.

A unit of distance used in astronomy. Symbol, pc. One parsec = 3.26 light-years, or 3.0857 × 10¹³ kilometers. One parsec is the distance from Earth to a star that shows a parallax of one second of arc. The greater the parallax, the smaller the distance.

In 1922, the Commission des notations, des unités et de l'économie des publications of the International Astronomical Union accepted, and the First General Assembly adopted, the recommendation of an American committee¹ that distances to stars be described in parsecs.² In the early 1900s, the parsec was also known as the macron, astron, and astrometer.³

What is parallax?

The parsec is based on the phenomenon called parallax. Close one eye and hold up a finger at arm's length. Notice what is behind the finger. Now change eyes. The finger appears to jump against the background; something else appears behind it. Bring the finger closer to your face. Change eyes again. Notice that the nearer your finger is to your face, the bigger the jump appears to be.

This phenomenon can be used to measure the distance to stars that are not too far away. In place of your two eyes, astronomers use two positions of the earth, six months apart, so that the positions are at opposite sides of Earth's orbit. As your finger did, a nearby star will appear to jump against the background of very distant stars. The size of the jump can be expressed as the angle between the two positions the star had. This angle is very small; only 1″.5 for the nearest star.

The parallax of a star was first successfully measured in 1837, by Bessel.⁴

1. Proceedings of the National Academy of Sciences, volume 6, page 360 (1920).

2. A. Fowler, ed.
First General Assembly, held at Rome, May 2nd to May 10th, 1922.
Transactions of the International Astronomical Union. Volume 1.
London: Imperial College Bookstall, no date [1922 ?].

Pages 23, 138, 207.

3. Heber Curtis.
The unit of stellar distance.
Publications of the Astronomical Society of the Pacific, volume 25, page 213, (1913).

We thank Ken Croswell for calling this article to our attention.

4. F. W. Bessel.
II. A letter from Professor Bessel to Sir J. Herschel, Bart., dated Konigsberg, Oct. 23, 1838.
Monthly Notices of the Royal Astronomical Society, vol 4, no. 17 (9 Nov 1838).
doi:10.1093/mnras/4.17.152

The star was 61 Cygni.