# 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

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