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Compare Object
Distances

Analyze data to compare distances between objects in our solar system: from the inner rocky planets to the far reaches of the outer giants.

Earth & Space Science

What Is an Astronomical Unit?

When distances in space are too large for kilometers or miles to be practical, scientists use a special measuring stick called the Astronomical Unit (AU).

Think of it this way: if you placed a ruler from the Sun to Earth, that ruler is exactly 1 AU long. Every other distance in the solar system is measured as a multiple of that ruler.

Why AU Makes Comparison Easy

Raw km
Mars is 227,900,000 km from the Sun. Hard to picture and compare.
Using AU
Mars is \( 1.52 \) AU from the Sun: about 1.5 times the Earth-Sun distance. Instant comparison!

The Inner Solar System

Sun Mercury 0.39 AU Venus 0.72 AU Earth 1.00 AU Mars 1.52 AU Inner (Rocky) Planets: 0.39 to 1.52 AU

The four inner planets are all within about \( 1.5 \) AU of the Sun. They are relatively close together and are made of rock and metal.

The Outer Solar System

Sun Inner Jupiter 5.20 AU Saturn 9.58 AU Uranus 19.2 AU Neptune 30.07 AU Outer (Gas/Ice Giant) Planets: 5.2 to 30.1 AU

Core Rule: Distance Scales Rapidly

Jupiter
\( 5.2 \) AU
Saturn
\( 9.6 \) AU ≈ \( 1.8 \times \) Jupiter
Uranus
\( 19.2 \) AU ≈ \( 2 \times \) Saturn
Neptune
\( 30.1 \) AU ≈ \( 1.6 \times \) Uranus

Visualizing AU on a Number Line

0 5 10 15 20 25 30 AU Me Ve Ea Ma Jupiter Saturn Uranus Neptune Inner 4: all < 2 AU Outer 4: spread across 5 to 30 AU

On a true scale number line, the inner planets cluster near the Sun while the outer planets are vastly spread apart.

Inner vs Outer: Key Distinction

Inner Planets Outer Planets
Distance Range \( 0.39 \) to \( 1.52 \) AU \( 5.2 \) to \( 30.1 \) AU
Total Spread ≈ \( 1.1 \) AU ≈ \( 24.9 \) AU
Gap Between Neighbors ≈ \( 0.3 \) to \( 0.5 \) AU ≈ \( 4 \) to \( 11 \) AU
Composition Rocky / Terrestrial Gas / Ice Giants

Converting Between AU and km

1
AU to km: Multiply the AU value by \( 1.496 \times 10^{8} \)
2
km to AU: Divide the km value by \( 1.496 \times 10^{8} \)

How to Compare Distances

1
Identify the two objects and find each distance from the Sun (in AU or km).
2
Use the same unit. Convert so both distances are in AU (or both in km).
3
Compute the ratio: Divide the larger distance by the smaller: \( \text{ratio} = \frac{d_{\text{far}}}{d_{\text{near}}} \).
4
Interpret: "Object A is \( n \) times farther from the Sun than Object B."
5
For distance between two objects: subtract the smaller from the larger: \( \Delta d = d_{\text{far}} - d_{\text{near}} \).

Example 1: Earth vs Mars

Sun Earth Mars 1.00 AU 0.52 AU Mars: 1.52 AU total from Sun

Example 2: Earth vs Jupiter

Sun Earth 1.00 AU Jupiter 5.20 AU 4.20 AU gap

This gap alone is about 4 times the entire Earth-Sun distance. A spacecraft traveling at \( 60{,}000 \) km/h would need over 1 year to cross it.

Example 3: Jupiter vs Neptune

Jupiter
\( 5.20 \) AU from the Sun
\( 7.78 \times 10^{8} \) km
Neptune
\( 30.07 \) AU from the Sun
\( 4.50 \times 10^{9} \) km

Planet-to-Planet Distance Table

Planet Pair Gap (AU) Gap (km) Times Earth-Sun
Mercury to Venus 0.33 \( 4.9 \times 10^7 \) \( 0.33 \times \)
Earth to Mars 0.52 \( 7.8 \times 10^7 \) \( 0.52 \times \)
Mars to Jupiter 3.68 \( 5.5 \times 10^8 \) \( 3.68 \times \)
Saturn to Uranus 9.62 \( 1.4 \times 10^9 \) \( 9.62 \times \)
Uranus to Neptune 10.87 \( 1.6 \times 10^9 \) \( 10.87 \times \)

Common Mistakes

✕ Mixing Units
Comparing one planet's distance in AU with another's in km. Always convert to the same unit before comparing.
✕ Trusting Textbook Diagrams
Most diagrams are not to scale. They make inner and outer planets look evenly spaced. Always check the actual AU values.
✕ Confusing Ratio with Difference
Saying "Jupiter is 4.2 AU farther" (difference) is not the same as "Jupiter is 5.2 times farther" (ratio). Both are valid but answer different questions.

Question 1 of 5

One Astronomical Unit (AU) is defined as the average distance from:

Question 2 of 5

Saturn is approximately \( 9.58 \) AU from the Sun and Mars is \( 1.52 \) AU. Approximately how many times farther from the Sun is Saturn compared to Mars?

Question 3 of 5

The distance from the Sun to Jupiter is \( 5.20 \) AU. What is this distance in kilometers?
(\( 1 \text{ AU} = 1.496 \times 10^{8} \text{ km} \))

Question 4 of 5

Which statement best describes the spacing of planets in our solar system?

Question 5 of 5

A student says: "Neptune is about \( 30 \) AU from the Sun, so it must be \( 30 \) times farther from the Sun than Mars." Is this reasoning correct?