Distances in astronomy

The unit of distance used in astronomy is the parallax-second, or parsec. It is defined to be the distance at which the mean radius of the Earth's orbit about the Sun subtends an angle of one second of arc. In metres, the parsec, abbreviated to pc, is 3.0856 × 1016 m. For many purposes, it is sufficiently accurate to adopt 1 pc = 3 × 1016 m. The parsec is a recognised SI unit and it is often convenient to work in kiloparsecs (1 kpc = 1000 pc = 3 × 1019 m), megaparsecs (1 Mpc = 106 pc = 3 × 1022 m) or even gigaparsecs (where 1 Gpc = 109 pc = 3 × 1025 m).

Sometimes, it is convenient to measure distances in light-years, which is the distance light travels in one year: 1 light-year = 9.4605 × 1015 m. Thus, 1 pc = 3.26 light-years.

Another commonly used distance unit in astronomy is the astronomical unit, abbreviated to AU, which is the mean radius of the Earth's orbit about the Sun: 1 AU = 1.49578 × 1011 m. The very nearest stars to the Earth are at a distance of about 1 pc, and so they are about 2 × 105 times as far away as the Earth is from the Sun.

Accurate distances are among the most difficult measurements to make in astronomy, and there must, therefore, exist corresponding uncertainties in all the derived physical properties of astronomical objects.