Topic 4.1: The Scale of the Solar System: AU and Light-Years
Miles and kilometers are fine for driving to Birmingham. For space they become silly: the distance from Earth to Neptune is about 4.5 billion kilometers. So astronomers use bigger rulers.
Ruler 1: The Astronomical Unit (AU)
One AU is the average distance from Earth to the Sun: about 149.6 million km (93 million miles). It's the natural ruler for our own solar system.
| Planet | Average distance from the Sun |
|---|---|
| Mercury | 0.39 AU |
| Venus | 0.72 AU |
| Earth | 1.00 AU |
| Mars | 1.52 AU |
| Jupiter | 5.20 AU |
| Saturn | 9.54 AU |
| Uranus | 19.2 AU |
| Neptune | 30.1 AU |
Ruler 2: The Light-Travel Time
Light travels at 299,792 km/s, about 300,000 km/s. Nothing goes faster. We can measure distance by how long light takes to cross it.
- Moon: 1.3 seconds away.
- Sun: 8 minutes 20 seconds away. So you always see the Sun as it was 8 minutes ago.
- Jupiter: about 43 minutes (5.2 x 8.3).
- Neptune: about 4 hours.
- Voyager 1, launched in 1977 and now over 160 AU away, is more than 22 light-hours from us. A radio message takes nearly a day to get there, and another day for the answer.
Ruler 3: The Light-Year
A light-year is the distance light travels in one year:
300,000 km/s x 31.5 million seconds = about 9.46 trillion km (5.88 trillion miles).
A light-year is a distance, not a time. It is about 63,000 AU. The nearest star after the Sun, Proxima Centauri, is 4.24 light-years away. At Voyager's speed (about 17 km/s) it would take about 75,000 years to get there. Our Milky Way galaxy is about 100,000 light-years wide.
A Scale Model You Can Walk
Make the Sun a ball 10 cm (4 inches) across, about the size of a grapefruit.
- Earth is a speck 1 mm wide, about 11 meters (36 feet) away.
- Jupiter is a 1 cm marble, about 56 meters away.
- Neptune is a 4 mm bead, about 320 meters away, three football fields.
- Proxima Centauri would be about 2,900 km away, farther than a flight from Montgomery to Denver.
Almost all of space is empty, and that is one reason we cannot see the planets from far off.
Worth a Pause
The Psalmist wrote, "Great is our Lord, and of great power: his understanding is infinite" (Psalm 147:5), right after he said God "telleth the number of the stars." Feeling small is a good first reaction to this lesson. The better second reaction is to notice that we, tiny as we are, can measure the whole distance with a stopwatch and a bit of arithmetic.
Try It: Scale Model Dash. Place the planets on a walkable scale model and learn how empty space really is. Play Scale Model Dash »
Practice Problems
- How many kilometers is one AU?
- How long does light from the Sun take to reach Earth?
- Jupiter is 5.2 AU from the Sun. About how long does sunlight take to reach it?
- Is a light-year a measure of time or distance? Explain.
- How many kilometers is a light-year, roughly?
- In the 10-cm-Sun scale model, about how far away is Earth, and how big is it?
- Why do astronomers use AU and light-years instead of kilometers?