The distance between the Earth and the Sun could have a value of 1 (0.000016, or ). To communicate, planet researchers must first say which UNITS they use!
The International System (S.I. or MKSA) defines a consistent system of units, but it is not always suitable for Astrophysics, because astronomers work with either very small or very immense scales.
For distances, the unit of the International System (SI) is the meter.
- In the Solar System, astronomers use the astronomical unit, which is the mean distance between the Earth and the Sun.
1 astronomical unit (AU) = 150 000 000 km = km (more exactly meters)
- Concerning the stars, astronomers use the parsec, which is a shortcut for "per second": a parsec is the distance at which one AU underlies an arc of 1 second.
1 parsec (pc) = 3.26 l.y. = km (m) = 200 000 AU
- The distances between stars or galaxies are given in light-years, the distance covered by light in one year.
The speed of light is c = 300 000 km/second.
One light-year (km) = 365(days) * 24(hours) * 3600 (seconds) * 300000(km/second)
1 l.y. = m = 60 000 AU
!! Warning!!, on these large scales, time plays a significant role : the radiation coming from a star located 2000 light-years away is left over from a star 2000 years ago. It is impossible to know what happened to the star over the past 2000 years!!