Key concepts
Solar system

From National Science Education Standards: Objects in the sky

Have you ever built a model of the solar system for school—or even just seen a picture of the solar system in a book? The planets are usually pretty close together—and close to the sun. In the real world (that is, in the real solar system) the planets are incredibly far apart from one another and from the sun (which is a good thing for us; if Earth were close to the sun, it would be too hot for us to live!).

The closest planet to the sun is Mercury, and it's about 36 million miles (57 million kilometers) from the sun. What about Earth? We're orbiting the sun at a safe—and comfortable—distance of 93.2 million miles (150 million kilometers).

That's a big distance to try to understand. But using some (very small) household objects and (a lot) of space on the floor or ground, we can get a better sense of just how far apart the sun and planets are, especially in comparison with their sizes.

To simplify the distances in this activity, you can use short-cut measurements, such as the length of an adult foot for each foot or the length of an adult walking pace for each yard or meter. We'll provide exact standard and metric measurements below.

•    Nine small, round objects about the size of a peppercorn (0.1 inch, or 2.5 millimeters, across)
•    Measuring tape
•    Nine small pieces of paper labeled for the sun and planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune)
•    A clear, 21.5-foot- (6.6-meter-) long path on the floor inside or ground outdoors
•    A grapefruit (optional)
•    Two grains of table salt (optional)
•    Two grains of sea salt (optional)
•    35 feet (10.7 meters) of space in at least one direction (optional)

•    How big is the solar system? Is it mostly crowded with planets or is it mostly empty space?
•    Try to clear a straight path that is 21.5 feet (8.6 meters) long. If you don't have that much room, clear at least four feet (1.2 meters) of space.

•    Place the first peppercorn at one end of the path, and label it as the sun.
•    Measure 3.3 inches (eight centimeters) and place the second peppercorn. Place the label for Mercury next to it—it's the closest planet to the sun. (If we were making a scale model of the solar system, it would not be the same size of the peppercorn sun, but it would be practically invisible to the naked eye: 0.0002 inch, or 0.005 millimeter, across!)
•    Return to the sun and measure out 6.2 inches (15.7 centimeters). Place the third peppercorn and label it Venus.
•    Do you know what the next peppercorn will represent? Can you guess how far it will be from the sun?
•    Measure 8.6 inches (21.8 centimeters) out from the sun, and place the next peppercorn—this one represents Planet Earth!
•    The next planet is Mars, and it should be placed 13.1 inches (33.3 centimeters) from the sun.
•    Return to the sun and now measure out three feet, 8.2 inches (1.1 meters) before placing the next peppercorn. Label this planet Jupiter. (This is the largest planet, but if the sun were the size of a peppercorn, it would still be smaller than grain of table salt.)
•    The next peppercorn will be Saturn, which would be six feet, 10 inches (2.1 meters) from the sun.
•    Now measure 13 feet, nine inches (4.2 meters) from the sun before placing the next peppercorn, which is Uranus.
•    The last planet in our solar system is Neptune. Measure out 21 feet, 6.6 inches (6.6 meters) from the sun and place your final peppercorn and planet label there.
•    Stand next to the Neptune peppercorn and look at the sun. If you lived on Neptune, would the sun look as big as it does from Earth? The sun throws out a tremendous amount of light and heat, but how much of it would Neptune get as compared with Earth?
•    Extra: Want to get a better sense of how small the planets are compared with the sun? Try using a grapefruit—about four inches (10 centimeters) in diameter—as the sun. If the planets were to be proportionally sized and spaced in this model, Mercury—the closest planet to the Sun—would be a grain of table salt 13 feet, 10.5 inches (4.2 meters) away. Venus would be about the size of a grain of sea salt 25 feet, 10.7 inches (7.9 meters) away from the sun. And Earth would also be about the size of a rough grain of sea salt—35 feet, 8.2 inches (10.9 meters) away from the sun. What about Neptune? It would be more than 1,416 feet (431.6 meters) away from a grapefruit-size sun.
•    Extra: Want to try building a solar system with other size objects? Try plugging in their sizes in the Exploratorium's Build a Solar System Model calculator.

Read on for observations, results and more resources.

Observations and results
Can you see the Neptune peppercorn when you stand next to the sun? Were you surprised how far apart the planets were? Of course, the space between the planets is not totally empty. There are floating rocks called asteroids, many of which orbit around the sun in the Asteroid Belt between Mars and Jupiter.

The solar system is just one tiny part of our galaxy (which is just a tiny part of the universe). The nearest star to our sun is about 25 trillion miles (4.24 light-years) away. What about in the peppercorn-scale sun universe? It would be the equivalent of 36.6 miles (58.9 kilometers) away from your peppercorn-size sun. That's a long trip—even in a microscopic space shuttle.

Share your solar system model observations and results! Leave a comment below or share your photos and feedback on Scientific American's Facebook page.

Be sure to pick up all of the peppercorns (or whatever objects you used to mark the planets and sun).

More to explore
"8 Wonders of the Solar System" from Scientific American
"Hubble Spies Evidence of Seasons on Neptune" from Scientific American
Solar System Exploration for kids from NASA
Solar System Scale Model from Phrenopolis
What's Out There? A Book about Space by Lynn Wilson, ages 4–8
Don't Know Much about the Solar System by Kenneth C. Davis, ages 9–12

Up next…
Get the Iron out—of Your Breakfast Cereal

What you'll need
•    Breakfast cereal that contains iron, such as fortified cornflakes (check the label to see how much iron each serving contains—the more the better!)
•    Bowl and spoon (or mortar and pestle)
•    Magnet (as strong as possible)
•    White piece of paper
•    Resealable zip-top bag (optional)
•    Water (optional)