In this activity students use a computer model to study the planets in our Solar System.

How do the planets’ orbits change with their distance from the Sun?

This activity will look at the inner six planets (including Earth), which were the ones that were known by astronomers before the invention of telescopes. Each one has a different size, color, composition, and distance from the Sun. List these six planets in order, starting with the one closest to the Sun.

This activity addresses NSES standards for earth and space science and inquiry at grades 5-8 for earth in the solar system
(http://books.nap.edu/readingroom/books/nses/6d.html#es).

1. Each planet takes a different amount of time to go once around the Sun, called its period. To your list, add your guess of the period for each planet, in terms of Earth days and years. Don’t do any research on this question right now. Just make an “educated guess.”

1. Do you think the closer planets go around faster, or more slowly? Explain your reasoning.

1. Below is a NetLogo model of our Solar System. It is set up with the six planets at their proper relative distances from the Sun.

2. Measure the time for each orbit compared to one Earth year. For the close-in planets, use the ZOOM slider to get a better look at them. Slow down the model (using the slider above graphics window) and let them do one revolution and then stop the model. Then estimate what portion of a year the Earth has moved. For the farther-out planets, run the model full speed and make them do one revolution and stop the model. Count the number of earth years there were during that time. Fill out the following table, which you will need to re-create in the answer area or on paper.

3. Compare your measurements to the actual orbit times. How good were your measurements?
Note: An Astronomical Unit (AU) is defined as the distance from the Earth to the Sun. It’s a convenient unit for measuring planetary distances in the Solar System.

1. Compare your measured orbit time to the astronomically measured one. How good is the model? Did it give the correct orbit time?
2. Johannes Kepler proposed the following law, based on his observations of the planets:
(period in years) squared = (radius in AU) cubed

This is true for the Earth, since its period is 1 year and its radius (distance to the Sun) is 1 AU. See if it is true for the other planets. Fill out the following table, which you will need to re-create in the answer area or on paper.

How well does Kepler’s law fit the data?

Johannes Kepler (1571 – 1630), a German astronomer and mathematician, proposed three laws based on his and others’ observations of planetary motion.
1. The planets travel in ellipses, with the Sun at one focus (see the Ellipses activity).
2. The planets don’t move at constant speeds along these ellipses (see *Ellipses*).
3. The cube of each planet’s radius is proportional to the square of its period. You have observed this relationship in this activity.

Isaac Newton was able to demonstrate that a gravitational force between the Sun and the planets would lead to these three laws. This was very strong evidence that the Sun, rather than the Earth was the center of the Solar System. Up to that time, most observers and philosophers believed that the Sun and all the planets went around the Earth and moved in perfect circles.

Here’s a real challenge. If you watch the graph in the model, you will notice that the Earth-Sun distance changes with two different rhythms – a short cycle and a longer, more gradual cycle. See if you can figure out what each of these rhythms corresponds to and what causes it. Here are some hints:

• Compare the rhythm to the period of the earth’s year.
• Remember that other planets pull on the Earth just as the Sun does. So another planet could cause the Earth to be pulled closer to and farther from the Sun. Can you figure out which planet might do this? Does it match with the rhythm you observe in the model?