### Overview

Activity Notes
Students construct -- and where appropriate, calculate -- a scale model of the solar system using beads and string. Students will observe the relative distances of the planets, asteroid belt and dwarf planet Pluto from one another and from the sun; and gain a better understanding of the vast distances between planets in the outer solar system compared with those in the inner solar system.

### Management

• To speed up the activity for younger students, the string may be pre-cut and a set of solar system beads may be put into a plastic zip-lock bag for each student.  Also, for younger students, a measured marking grid can be put on a table top so the students can mark their measured distances and then tie off the beads.  If the pre-marking method is used, extra distance must be added to each planet distance to accommodate the string within each knot (approximately 4 centimeters for a double knot around the bead).

• Tape newspaper to the surface where the students will be marking their strings so they do not mark up the counter or floor.

• For older students, measurements are made each time from the sun to the planet and tied on after each measurement.

### Background

Our solar system is immense in size. We think of the planets as revolving around the sun, but rarely consider how far each planet is from the sun or from each other. Furthermore, we fail to appreciate the even greater distances to the other stars. Astronomers refer to the distance from the sun to the Earth as one "astronomical unit" or AU. This unit provides an easy way to calculate the distances of the other planets from the sun and build a scale model with the correct relative distances.

Astronomical Unit (1AU) = Approximately 150 million kilometers (93 million miles)
(149,597,871 kilometers or 92,955,807 miles to be exact!)

How big are the planets and how far away are they compared to each other? See how the sizes of planets and the distances between them compare in this video. Credit: NASA/JPL-Caltech | Watch on YouTube

Keep two important Solar System facts in mind:

1. The planets never align in a straight line. Occasionally, though, sky-watchers are treated to the sight of two bright planets apparently close together as viewed from our planet.

2. The string solar system is a radius of the orbits of the planets. To see how large the solar system is, hold the sun in one location and swing the planets in a circle around it. If you move counter-clockwise you will be moving the planets in the direction they move as viewed from above their plane.

### Procedures

Students will construct a distance model of the solar system to scale, using colored beads as planets. The chart below shows the planets and asteroid belt in order, along with their distance from the sun in astronomical units.

 Planet AU Scale Value (centimeters) Bead Color Sun 0.0 AU 0 cm Yellow Mercury 0.4 AU 4 cm Solid Red Venus 0.7 AU 7 cm Cream Earth 1.0 AU 10 cm Clear Blue Mars 1.5 AU 15 cm Clear Red Asteroid Belt 2.8 AU 28 cm Black Jupiter 5.2 AU 52 cm Orange Saturn 9.6 AU 96 cm Clear Gold Uranus 19.2 AU 192 cm Dark Blue Neptune 30.0 AU 300 cm Light Blue Pluto (closest) 29.7 AU 297 cm Brown Pluto (average) 39.5 AU 395 cm Brown Pluto (most distant) 49.3 AU 493 cm Brown

Student Procedure

1. Complete the distance chart by multiplying each AU distance by a scale-factor of 10 centimeters per astronomical unit.

2. Start your scale model by cutting a 4.5-meter piece of string (5.0 meters if you are doing the Pluto extension) and tying the largest "sun" bead to one end using a double knot.

3. Using the distances (in centimeters) that you calculated, measure the distance from the sun on the string to each planet.

4. Tie a colored bead in place for each planet using a double knot. Note: The bead colors are rough approximations of the colors of the planets and the sun.

5. When you are finished, wrap your string Solar System around the cardboard holder.

### Extensions

• Add dwarf planet Pluto’s nearest and most distant points to illustrate that Pluto’s orbit is much different than the eight major planets. This was one element of the decision to describe Pluto as a dwarf planet. When Pluto is closest to the sun, it is inside the orbit of Neptune. If a bead is to be added for Pluto at this point, it needs to be on the string before the Neptune bead is tied off. So the order will be: Uranus, Pluto (closest), Neptune, Pluto (average), Pluto (most distant).

• Consider that if you were traveling at the speed of light, it would take 8 minutes and 19 seconds (or about 8.3 minutes) to travel from the sun to the Earth (1 AU). Using this measure, students can calculate the light-time from the sun to each planet. It would take 4.3 years (traveling at the speed of light – 300,000 kilometers per second) to reach the next nearest star system, Alpha Centauri!

• The Voyager spacecraft are the most distant human-made objects. Launched in 1977, the spacecraft flew by all four of the gas giant planets -- Jupiter, Saturn, Uranus and Neptune (only Voyager 2 flew past Uranus and Neptune) -- and kept on going. In August 2012, Voyager 1 made history as the first man-made object to enter interstellar space.