# The Venus Transit 2004

## ... Brief InfoSheet G3

# Definition of the Astronomical Unit and other units of distance in Astronomy

While the official unit of length is the meter, defined as the length traversed in vacuum by light during a 1/299 792 458th of a second, the basic unit used in astronomy to measure the distances is the **"Astronomical Unit (AU)" **. This corresponds to - roughly - the mean distance of the Earth to the Sun. All the distances in the solar system may be deduced from only one of them thanks to the famous laws of Kepler.

To express the Astronomical Unit in terms of meters, one needed in previous time to calculate the parallaxes of planets or asteroids. Nowadays, we can rely on the use of radar or even laser measurements.

The most recent estimates give **149 597 870 691 meters ** for one Astronomical Unit. That is, we have now reached a precision of one meter on a distance of several millions of kilometres !

From these, one can define auxiliary units of length, more useful when dealing with galactic or extragalactic distances. The first, the light-year, corresponds to the distance travelled by light during one year. It is equal to 63 241,077 10 a.u. The second is the parsec, corresponding to the distance to which an Astronomical Unit is seen under an angle of one second of degree. This is equivalent to 206 264,806 248 a.u. or 3,26 light-years.

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