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.
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 Moon||1.3 light seconds|
|The Sun||8.3 light minutes|
|Alpha Centauri||4.4 light years|
|Hercules Globular (M 13)||25,000 light years|
|Andromeda galaxy||2.5 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?
= 3 × 108 m/s × 1.3 seconds
= 3.9 × 108 m