Distances in the solar system are so big that kilometers become clumsy. Astronomers use the astronomical unit (AU), the average distance from Earth to the Sun: about 150 million km. Then you can compare distances with ratios.
| Planet | Distance from the Sun (AU) | Width compared with Earth |
|---|---|---|
| Mercury | 0.39 | 0.38 |
| Venus | 0.72 | 0.95 |
| Earth | 1 | 1 |
| Mars | 1.52 | 0.53 |
| Jupiter | 5.2 | 11.2 |
| Saturn | 9.6 | 9.4 |
| Uranus | 19.2 | 4.0 |
| Neptune | 30.1 | 3.9 |
Key idea
1 AU ≈ 150 million km. In a true scale model, the planets are tiny dots, very far from the Sun and from each other.
Why it works
A scale model shrinks every length by the same factor, so all the ratios stay true. Shrink the Sun to a ball 1 m wide. Earth, 109 times smaller, becomes about 9 mm across, the size of a pea, and it sits about 107 m away, a whole soccer field. Neptune would be over 3 km away. Pictures in books squeeze the planets close together to fit them on a page.
Worked example: From AU to kilometers
Jupiter is about 5.2 AU from the Sun. About how far is that in kilometers?
- 1 AU is about 150 million km.
- $5.2 \times 150 = 780$, so about 780 million km.
Answer: About 780 million km
Worked example: A model on a field
In a school model, 1 AU is 10 m. How far from the model Sun should Saturn (9.6 AU) go? And Neptune (30.1 AU)?
- Each AU is 10 m, so multiply by 10.
- Saturn: $9.6 \times 10 = 96$ m. Neptune: $30.1 \times 10 = 301$ m.
Answer: Saturn at 96 m, Neptune at 301 m
Common mistake
Trusting book pictures that squeeze the planets together and draw them nearly as big as the Sun. On a true scale, the space between planets is enormous: Earth is about 107 Sun-widths from the Sun. Check: divide distance by size; if a picture fits on a page with visible planets, it can't be to scale.
Light from the Sun takes about 8.3 minutes to reach Earth, 1 AU away. It takes about 4 hours to reach Neptune.