Units for Distance and Size in the Universe

Astronomers use many of the same units of measurement as other scientists. They often use meters for length, kilograms for mass, and seconds for time. However, the distances and sizes in the universe can be so big, that astronomers have invented more units to describe distance.

Astronomical Units:

Distances in the solar system are often measured in astronomical units (abbreviated AU). An astronomical unit is the average distance between the Earth and the Sun:

1 AU = 1.496 × 108 km = 93 million miles

Jupiter is about 5.2 AU from the Sun and Pluto is about 39.5 AU from the Sun. The distance from the Sun to the center of the Milky Way is approximately 1.7 × 109 AU.


To measure the distances between stars, astronomers often use light-years (abbreviated ly). A light-year is the distance that light travels in a vacuum in one year:

1 ly = 9.5 × 1012 km = 63,240 AU

Proxima Centauri is the nearest star to Earth (other than the Sun) and is 4.2 light-years away. This means light from Proxima Centauri takes 4.2 years to travel to Earth.


Many astronomers prefer to use parsecs (abbreviated pc) to measure distance to stars. This is because its definition is closely related to a method of measuring the distances between stars. A parsec is the distance at which 1 AU subtends and angle of 1 arcsec.

1 pc = 3.09 × 1013 km = 3.26 ly 


For even greater distances, astronomers use kiloparsecs and megaparsecs (abbreviated kpc and Mpc).

1 kiloparsec = 1 kpc = 1000 pc = 103 pc

1 megaparsec = 1 Mpc = 1,000,000 pc = 106 pc