   This activity is designed to help middle school students understand that planets travel in nearly circular orbits around the sun, and that planetary motion obeys laws defined by Kepler and Newton. Students will explore interactive web sites demonstrating orbital motion and complete a series of modeling activities.  In this activity the learner will:

• define a planetary orbit and know that its shape is elliptical
• identify a circle, ellipse, parabola and hyperbola as conic sections, and distinguish them from other shapes
• define eccentricity as a measure of the elongation of an ellipse
• identify Kepler's First Law: a planet orbits the sun following a path of an ellipse with the sun at one focus.
• simulate Kepler's Second Law (The line joining a planet to the sun sweeps out equal areas in equal times) to understand the speed of a planet in its orbit.
• examine Kepler's Third Law (The closer a planet is to the sun, the greater its speed).
National Science Education Standards Exploration

Activity 1: Have students draw an orbit of a planet,  discuss the orbit's properties, and create a KWL Chart

Extensions: List observations of various images and diagrams of the solar system taken from the Web.

Concept Introduction

Activity 2: Construct an ellipse.

Activity 3: Learn about Kepler and Newton.

Concept Application

Activity 4: Kepler's Third Law. Building Background: Observing, Communicating, Comparing, Relating

Activity 1: Have students draw a simple picture of an orbit on paper.   Use these illustrations and their understanding of orbits to begin a unit-long KWL Chart and start a guided discussion.  Some sample questions might be:

 KNOW   What is an orbit ?  What are some things that orbit? What is the shape of an orbit? Do all orbits have the same shape? Are orbits natural or human made? WANT TO KNOW  What do we want to learn about planetary orbits? Why don't planets travel in straight lines? What's in the middle of the orbit?  Create a KWL Chart
Exploration: Observing, Communicating, Organizing, Relating, Inferring?

Extensions: Go to NASA's Windows to the Universe Solar System Formation web site.  Click on the small image of our solar system to get one that fills the screen.   Then ask students,  "What story does this picture tell?"  Direct students to list observations that can be made about the solar system by looking at the diagram and images.

Repeat the activity with the following sites:

1) The Solar System Live: Students can examine a different view of the solar system.  Point out the orbits of each of the planets shown on the diagram.  Ask the students the following questions:

1. If we were far out in space, looking at our solar system, would we see these lines
2. What do these lines represent?
3. Is there a pattern or patterns?
4. Can you describe the pattern(s) you see?
2) The NSSDC Photo Gallery: Solar System Family Portrait:
Students can look at a partial view of the solar system that includes actual photographic images taken by the Voyager probe.

Orbits are paths that describe dynamic phenomena.  It is hard to deduce the shapes of planetary orbits from our moving Earth, but in some cases we can directly observe objects that are orbiting the planets.  The following extension activities may be helpful.

There are commercial solar system simulators available as shareware.  One such (for PC's) is PlanetWatch.  Try downloading this simulator and watching as the planets (inner and outer) move around their orbits.  The sizes of the orbits and the orbital periods are correctly proportioned in this package.

Ask students if they have observed satellites moving overhead and, if so, to describe to the class what they have seen.

Have your students examine the pair of photos showing the moons of Uranus.  The two images were taken 90 minutes apart by NASA's Hubble Space Telescope.  Ask students to list their observations.  Why don't we have similar images of, for example, the orbiting moons of Jupiter?

The rings of Saturn consist of particles orbiting the planet. Students should view the movie (from Voyager) of the rings. What can they observe here?

Concept Introduction

Observing, ( Recording ), Communicating , Comparing, Organizing, Relating, Inferring

Activity 2: It is easy for students to construct an ellipse using simple tools.

Have the class complete the playground ellipse activity.  Direct students to write what they did and why they believe they got the shape they did.

Activity 3: Introduce students to Kepler and Newton with a short discussion of their lives, using information found in the Scientific Background Information.

Newton suggested a model for planetary orbits based on conic sections. To help the students visualize this concept, complete the conic section classroom activity Activity 4:  1. Kepler's third law (the Harmonic Law), relates the planets' orbital periods to their mean distances from the Sun. Ask students to complete the Harmonic Law activity

2. The ellipse is the shape of a planetary orbit. We describe how "squashed" an ellipse is by giving a number, between 0 and 1, called its eccentricity. An eccentricity of 0 corresponds to a circle; however, if the eccentricity is close to 1, the ellipse is very flattened.

(a) NASA has an online orbit simulator.  Ask students to explore the simulator at this site to learn about Kepler's laws of planetary motion, and have student groups discuss their findings.
(b) Ask students to complete the orbital eccentricity activity. Have students complete the KWL chart from the beginning of the lesson by filling in the "What We Learned" section (sample below).

Encourage students to reflect and report on how their knowledge changed.

What were the understandings that students had when they completed the "What We Know" section of the KWL chart?

How did their perspectives change? Do they think their views may change again? Why or why not?

WHAT DID WE LEARN? (Sample)

• What is an orbit ?
• What is its shape?
• Do all orbits have the same shape? If not, what shapes are possible?
• Why don't planets travel in a straight line?
• Are orbits natural or man made?
• Can you give some examples of orbits?
• What are foci?  Are they natural or man made?
• Were there any particular men or women associated with orbits?  Who?  What did they do?
• What do we want to learn further about planetary orbits? 