The furthest your eye can see

Source: Adam Evans CC-BY 2.0

This is the Andromeda galaxy. The nearest galaxy.

The Earth

Source: NASA Public Domain

The radius is the distance from the centre to surface

The Moon

Source: Gregory H. Revera CC-BY SA 3.0

The Moon is the nearest body to the Earth

The Sun

Source: NASA Public Domain

The Earth orbits the Sun; the nearest star

Proxima Centauri

Source: 2MASS Public Domain

The nearest star other than the Sun

Let's use the metre (m)

• The Earth is 6,400,000 m in radius
• The Moon is 384,000,000 m away
• The Sun is 149,000,000,000 m away
• Proxima Centauri is... a lot of metres away
• The Andromeda galaxy is... a ridiculous amount of metres away

Let's use the kilometre (km)

1 km = 1000 m

• The Earth is 6,400 km in radius
• The Moon is 384,000 km away
• The Sun is 149,000,000 km away
• Proxima Centauri is... a lot of km away
• The Andromeda galaxy is... a ridiculous amount of km away

Let's use the Astronomical Unit (AU)

The Astronomical Unit is the distance between the Earth and the Sun.

1 AU = 149,000,000 km

• The Earth is 0.000043 AU in radius
• The Moon is 0.00258 AU away
• The Sun is 1 AU away - by definition
• Proxima Centauri is 269,000 AU away
• The Andromeda galaxy is 159,000,000,000 AU away

Light units

One light year is the distance travelled by light in one year.

• The speed of light is 300,000 km per second
• so 1 light second = 300,000 km
• and 1 light minute = 60 × 300,000 km = 18,000,000 km
• and 1 light hour = 60 × 18,000,000 km = 1,080,000,000,000 km
• and 1 light day = 24 × 1,080,000,000,000 km = 25,900,000,000,000 km
• and 1 light year = 365 × 25,900,000,000,000 km = 9,460,000,000,000,000 km

Remember: a light year is a distance, not a time.

Let's use the light units

• The Earth is 0.02 light seconds in radius
• The Moon is 1.28 light seconds away
• The Sun is 8.28 light minutes away
• Proxima Centauri 4.24 light years away
• The Andromeda galaxy is 2,500,000 light years away