# Astronomical units

In the study of astronomy, many times the units of the International System (SI) are inefficient because the distances that must be expressed are very large.

For example: The distance from Earth to Mars is about 75 million kilometers, which in SI is expressed by 75,000,000,000 meters.

Due to the need for more efficient units are used: Astronomical Unit (UA), Light Years (AL) and Parsec (Pc).

## Astronomical Unit (UA)

It is the average distance between the earth and the sun. It is mainly used to describe orbits and distances within the solar system.

The average orbit size of the Solar System planets, ie their distance from the Sun is:

 Planet Sun Distance (UA) Mercury 0,39 Venus 0,72 Earth 1,00 Mars 1,52 Jupter 5,20 Saturn 9,53 Uranus 19,10 Neptune 30,00

## Light Year (al)

It is the distance traveled by light, in vacuum, in the time of 1 terrestrial year.

Being the speed of light c = 299 792,458 km / s, we have to:

1 al = 9,460,536 207,068 016 m = 63241,07710 UA

The nearest star to the sun is called Next Centauri, located in the constellation Centaur. Its distance to the sun is 4.22 al

## Parsec (Pc)

Is the distance at which 1 UA is represented by 1 "(1 arc second) in a parallax measurement.

This unit is used for very large distances, such as the distance between stars, galaxies or very distant objects such as quasars.