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Compare Object
Sizes in the Solar System

Learn to analyze data and compare the sizes of planets, moons, and our Sun using real measurements, ratios, and scientific notation.

Earth & Space Science

What Does "Size" Mean in Space?

When scientists compare objects in the solar system, they usually talk about diameter: the distance straight through the center of a sphere. Because space objects are so vastly different, we measure diameter in kilometers (km).

Diameter
The straight-line distance through the center of a spherical body.
Radius
Half the diameter: \( r = \frac{d}{2} \). Often used in volume calculations.
Ratio
Comparing two sizes: \( \text{ratio} = \frac{d_A}{d_B} \). Makes huge numbers understandable.

The Incredible Scale of Our Solar System

Sun 1,392,700 km Jupiter 139,820 km Earth 12,742 km Moon 3,475 km Pluto 2,377 km Note: Sizes of smaller bodies are exaggerated

Why Compare Sizes?

Raw numbers like 139,820 km are hard to picture. By comparing sizes using ratios and scale models, we turn abstract data into something our brains can actually understand.

1
Classify objects: Distinguish gas giants from rocky planets by their relative sizes.
2
Understand gravity: Larger diameter often means greater mass and stronger gravitational pull.
3
Build scale models: Scientists and engineers use size ratios to create accurate representations.

The Size Ratio Formula

To compare any two solar system objects, divide their diameters:

A ratio of 11 means the larger object's diameter is 11 times the smaller one's. For example, Jupiter's diameter divided by Earth's:

Scientific Notation for Big Numbers

Solar system diameters range from thousands to millions of kilometers. Scientific notation keeps these numbers manageable:

Sun
\( 1{,}392{,}700 \text{ km} \) = \( 1.3927 \times 10^6 \) km
Earth
\( 12{,}742 \text{ km} \) = \( 1.2742 \times 10^4 \) km
Moon
\( 3{,}475 \text{ km} \) = \( 3.475 \times 10^3 \) km

Rocky vs. Gas Giant Planets

Gas Giants Jupiter 139,820 km Saturn 116,460 km Rocky Planets Earth 12,742 km Venus 12,104 km Mars 6,779 km Mercury 4,879 km Circles scaled proportionally

Diameter vs. Volume

Diameter is a linear measurement, but volume grows with the cube of the radius. A planet twice as wide holds eight times the volume:

Comparison Diameter Ratio Volume Ratio
Jupiter vs. Earth \(\approx 11\times\) \(\approx 1{,}321\times\)
Earth vs. Moon \(\approx 3.7\times\) \(\approx 49\times\)
Sun vs. Earth \(\approx 109\times\) \(\approx 1{,}300{,}000\times\)

How to Compare Step-by-Step

1
Identify the measurement: Check whether the data gives you diameter, radius, or circumference.
2
Convert to the same unit: Make sure both values use km (or the same unit) before dividing.
3
Divide larger by smaller: \( \text{ratio} = \frac{d_{\text{larger}}}{d_{\text{smaller}}} \). This tells you "how many times bigger."
4
Interpret the result: A ratio of 4 means the larger object is 4 times the width of the smaller.
5
Use scientific notation if needed: For very large or very small ratios, express your answer as \( a \times 10^n \).

Example 1: Earth vs. Mars

Earth 12,742 km Mars 6,779 km 1.88x

Example 2: Sun vs. Jupiter

Sun 1,392,700 km Jupiter 139,820 km 9.96x

Example 3: Volume of Saturn vs. Earth

Saturn's diameter: 116,460 km. Earth's diameter: 12,742 km.

1
Find the diameter ratio: \( \frac{116{,}460}{12{,}742} \approx 9.14 \)
2
Cube it for volume: \( 9.14^3 \approx 764 \)
3
Interpret: Saturn could hold roughly 764 Earths by volume.

Building a Scale Model

If we shrink the Sun to a basketball (24 cm diameter), how big would the planets be?

Sun = Basketball 24 cm Jupiter 2.4 cm (grape) Saturn 2.0 cm (cherry) Earth 2.2 mm (peppercorn) Moon 0.6 mm (grain of sand) Scale factor: 1 cm = 58,029 km

Ordering Objects by Size

From smallest to largest diameter in the solar system:

Pluto 2,377 km Moon 3,475 km Mercury 4,879 km Mars 6,779 km Venus 12,104 km Earth 12,742 km Neptune 49,528 km Saturn 116,460 km Jupiter 139,820 km

Common Mistakes to Avoid

✕ Confusing Diameter with Radius
Remember: \( d = 2r \). If you accidentally use radius when you need diameter, your ratio will still be correct: but if you mix them up between objects, you will be off by a factor of 2.
✕ Assuming Size Equals Mass
Saturn is wider than Neptune, but density matters. A larger diameter does not always mean more mass: Saturn is less dense than water!
✕ Forgetting the Cube for Volume
If a planet is \( 10\times \) wider, it is \( 10^3 = 1{,}000\times \) the volume: not \( 10\times \). Linear ratios do not apply to 3D comparisons.

Solar System Size Summary

Object Diameter (km) Relative to Earth Type
Sun 1,392,700 109.2x Star
Jupiter 139,820 10.97x Gas Giant
Saturn 116,460 9.14x Gas Giant
Neptune 49,528 3.89x Ice Giant
Earth 12,742 1.00x Rocky
Mars 6,779 0.53x Rocky
Moon 3,475 0.27x Natural Satellite

Question 1 of 5

Jupiter's diameter is 139,820 km and Earth's diameter is 12,742 km. Approximately how many times wider is Jupiter than Earth?

Question 2 of 5

Which correctly expresses the Sun's diameter in scientific notation?

Question 3 of 5

If Planet X is 4 times the diameter of Planet Y, how many times greater is Planet X's volume?

Question 4 of 5

Which list correctly orders these objects from smallest to largest diameter?

Question 5 of 5

In a scale model where Earth is a peppercorn (2.2 mm), about how large would Jupiter be?