Sizes and distances in space

Planet diameters are measured in metres and kilometres.

Distances between the planets and the Sun are measured in AU (Astronomical Unit).

1 AU is the average distance between the Earth and the Sun and is approximately equal to 150,000,000 km (or 1.5 × 1011 m).

Look at this diagram. If you switch the torch on, the light from it will travel 9,460,730,472,580,800 metres (nine thousand four hundred and sixty trillion, seven hundred and thirty billion, four hundred and seventy two million, five hundred and eighty thousand, eight hundred) in one year. This distance is called a light year.

A torch projects a beam of light. The end of the beam is labelled 'After one year'. The distance the beam has travelled after one year is 9,460,730,472,580,800 metres, or one light year.
Distances between the Earth and other stars and galaxies are measured in light years (ly). Remember a light year is a unit of distance, not a unit of time.
  • 1 light year is the distance travelled by light in 1 year.
  • Light and all other electromagnetic waves have a speed of 300,000,000 m/s (or 3 × 108 m/s) in space. This is the speed of light.

To calculate the distance in metres for 1 light year, multiply

365 days (1 year) ×

24 hours (1 day) ×

60 minutes (1 hour) ×

60 seconds (1 minute) ×

300,000,000 m/s (speed of light in space).

All this works out as 9.46 × 1015 m.

Earth to -Distance
The Moon1.3 light seconds
The Sun8.3 light minutes
Alpha Centauri4.4 light years
Hercules Globular (M 13)25,000 light years
Andromeda galaxy2.5 million light years

Alpha Centauri is 4.4 light years away from us. Can you work out this distance in metres?

4.4 ly × 9.46 × 1015 m / ly = 4.16 × 1016 m


The Andromeda galaxy is 2.5 million light years away from Earth. How long does it take light from this galaxy to reach Earth?

2.5 million years.


It takes 1.3 light seconds for laser light fired from Earth to hit the Moon. How far away is it?

\text{distance =}{\text{~speed}}\times{\text{time}}

= 3 × 108 m/s × 1.3 seconds

= 3.9 × 108 m