1 light year = 9.5 million million kilometres, or 6.0 million million miles
Within the solar system it is common to talk of distance measured in Astronomical Units defined as the distance between the Earth and the Sun.
1 AU = 150 x 10 9 metres, equivalent to 93 x 10 6 miles, i.e. 93 million miles.
Another local measure is the Lunar Unit, i.e. the distance from the Earth to the Moon. This is particularly useful in expressing the threat to the Earth of sizeable known “Near Earth Objects” describing their closest approach as multiples of LUs (hopefully!).
1 Lunar Unit =404x 10 3 km, equivalent to 250 x 10 3 miles, i.e. 250,000 miles
For relatively close objects, like stars in our local galaxy, the Milky Way; a method is usedwhich applies basic trigonometry to the Solar System. It uses as a baseline the observation of a star’s apparent position and measures the angle to the star from either side of a line running through the Sun to the Earth’s position diametrically opposite. This is known as the parallax method and gives rise to the unit of the “Parsec” from parallax arc second. In formal Astronomical papers it is normal to express distances in Parsecs, Mega Parsecs, Giga Parsecs, etc. The further away the star is the smaller the apparent movement is and thus other methods had to be developed.
1 Parsec = 3.26 Light Years, equivalent to 31 million, million kilometres,
and therefore using the conversion above:
1 AU = 4.85 x 10 -6 Parsecs
The cosmic distance ladder
is the name given to a succession of techniques which allow
progressively larger distances to be measured using a variety of
sophisticated techniques not usually available to the amateur. Several
of these methods have evolved from simple concepts used in light and
time measurement for the last 400 hundred years. The idea of a Standard Candle
which had a measurable luminosity and burnt for a consistent time has
been around for a long time. Astronomers were able to use this idea to
look for objects which had a known brightness and could be fitted on the
Main sequence of star life. They then compared the luminosity of the
object with its observed brightness, applied the inverse square law and
computed its distance. Gradually a series of Standard Candles was
developed, and is still being refined as measuring technology improves.
These techniques have allowed astronomers to make measurements to the
edge of the observable universe of 13.7 billion light years.