arcsecond
Angleexact by definition
The arcsecond is exactly 1/3600 of a degree, or 1/60 of an arcminute. In radians it is about 4.848 × 10⁻⁶, an exact definition with an irrational decimal value. It is the working unit of astrometry, telescope pointing and precision surveying.
| 1 ″ | 0.00027777778 ° |
The name comes from pars minuta secunda, the "second small part" of a degree, completing the Babylonian sexagesimal chain. Its astronomical importance comes from stellar parallax: a star whose annual parallax is one arcsecond is one parsec away, which is how the parsec got both its value and its name.
About the arcsecond
An arcsecond is small enough to be hard to imagine, so here is a handle: a coin about 24 mm across, viewed from 5 km away, subtends roughly one arcsecond. The full Moon is about 1,800 arcseconds wide. Human vision resolves at best around 60 arcseconds, so everything measured in this unit is beyond the unaided eye by design.
Turbulence in the atmosphere blurs starlight into a disc typically 1 to 2 arcseconds across even at good observatory sites, a figure astronomers call the "seeing". That limit, not the size of the mirror, is what caps the resolution of most ground telescopes, and it is the reason space telescopes and adaptive optics exist: Hubble resolves to about 0.05 arcsecond. Precision here has changed enormously in two centuries. Bessel measured the first stellar parallax in 1838 at about 0.3 arcsecond, a heroic result at the time; the Gaia spacecraft now measures parallaxes to tens of microarcseconds, meaning it can detect a shift ten thousand times smaller than Bessel's and map the distances of over a billion stars.