Units for measuring distances

The distance between the Earth and the Sun can be 1, or 0.000016, or even 1.5x10¹¹. In order to communicate, planet hunters must first of all say which UNITS they are using!
The International System (SI or MKSA) defines a coherent system of units, but it is not always suitable for Astrophysics, which involves scales that are both immense and extremely small.

For distances, the International System (SI) unit is the metre.

• In the Solar System, astronomers use the astronomical unit ^? , the mean distance between the Earth and the Sun. 1 astronomical unit ^? (AU ^? ) = 150,000,000 km = 1.5x10⁸ km (or, more exactly, 1.496x10¹¹ metres)
• For stars, astronomers use the parsec ^? , which is an abbreviation of “parallax second”: a parsec ^? is the distance at which an AU ^? subtends an angle of one arc-second. 1 parsec ^? (pc) = 3.26 l.y. = 3x10¹³ km (3x10¹⁶ m) = 200,000 AU ^? (or more exactly 206264 astronomical units ^? ).
• Greater distances, between stars or galaxies ^? , are given in light years ^? , the distance travelled by light in one year. The speed of light is c = 300,000 km per second. One light year ^? (km) = 365 (days) x 24 (hours) x 3,600 (seconds) x 300,000 (km per second)
  One light year ^? (l.y.) = 9.46x10¹⁵ m = 60,000 AU ^? .
• Caution: at these great distances, time begins to play a major role: the radiation which reaches us from a star located 2,000 light years ^? away left the star 2,000 years ago. It is impossible to know what has happened to the star during this 2,000 year period!