European Southern Observatory (ESO)European Association for Astronomy Education (EAAE) Observatoire de ParisInstitut de Mécanique Céleste et de Calcul des Éphémérides (IMCCE)Astronomical Institute of the Academy of Sciences of the Czech Republic The Venus Transit 2004 European Science & Technology Week 2004Visit the websites of the VT-2004 organisers

The Venus Transit 2004

... Brief InfoSheet G5

Distances in the Universe

The kilometer, while very practical to measure distances on the Earth, is much too small a unit to be used in astronomy. Astronomers use two different distance units (rulers) to measure the universe.

The first one, useful within the solar system, is the Astronomical Unit (AU) : this is the mean distance of the Earth from the Sun (original definition)

1 AU = 149 597 870 691 m (149.6 million km) = 499.005 light-seconds

One light-second is the distance traveled by the light in 1 second, or 299 792 km.

Another unit, necessary for much larger stellar and extragalactic distances is the light-year (ly) . This is the distance traveled by the light during one year:

1 light-year (ly) = 63 240 AU = 9.450 10 12 km

Astronomers also use a third distance unit, the parsec (pc) equal to 3.26 light-years.

How do we measure the distances?

Historically: After the Kepler's Third Law was discovered, astronomers could determine the relative distances in the solar system. However, to learn the trus size of the solar system, the distance of one planet had to be measured in absolute terms, e.g., in kilometres. They early realised that one possibility to do this was to determine the value of 1 AU by means of observaitons of a Venus Transit.

Today: radar and laser measurements provide distances to major and minor planets with an accuracy of a few metres.

Assume that we send out a beam of light from Earth. How long will it then take for this light - moving with the highest possible speed according to the special theory of relativity (299 792 km/s) - to reach different objects in the Universe? Look at the table below for the answers.

The Moon 1.2 seconds
The Sun 8 min 20 sec
Planet Pluto 5.3 hours
Proxima Centauri - the nearest star 4.2 years
Sirius - the brightest star in the sky 8.6 years
The Polar Star (Polaris) 432 years
Center of the Milky Way galaxy 30 000 years
Andromeda Galaxy 2 million years
Virgo Cluster of Galaxies 60 million years
3C273 - a quasar 2 500 million years

Want to know more?

This topic is discussed in a more detailed way in the associated Extended InfoSheet.

Back to the List of Brief InfoSheets.