In this activity students use a computer model to study the elliptical orbits of planets around a sun.

What shapes are planetary orbits?

Earth is not always the same distance from the sun. Do you think that explains the seasons? Explain your reasoning.

The PhET model Orbits is opened in a separate window.

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).

What shapes are planetary orbits?

1. Open the PhET model Orbits. Use START, STOP, and RESET to control the model. This picture shows how to adjust the model.
   
2. In the drop-down menu on the upper right, select the “Ellipses” model. Click START and observe the model. Three orbits are shown. What do you notice about the shapes? How are they similar and how are they different?
3. Is the speed of each planet constant as it travels around its orbit? You can measure differences roughly by using the clock. Start and stop the model and compare the time for the half-orbit closest to the sun with the time for the half-orbit farthest from the sun. What is the difference?

1. An ellipse is a special shape a bit like a distorted circle. It has two foci, which is the plural of focus. It is defined as all points for which the sum of the distances from the two foci is constant. Go here for a picture.
2. If the two foci are in the same place, what will the shape look like?
3. Explain why this statement is true: “a circle is always an ellipse but an ellipse is not always a circle.”
4. Since the Earth’s path is an ellipse, its distance from the Sun changes a bit during the year. You may be surprised to learn that the Earth is closest to the Sun in January and farthest in July! Do you think this elliptical shape can explain the seasons?

Johannes Kepler (1571 – 1630) discovered that the planets travel in ellipses, with the Sun at one focus. He also discovered that they move faster when closer to the Sun. Isaac Newton was able to demonstrate that a gravitational force coming from the Sun would have these two results. This was very strong evidence that gravity exists between the Sun and the planets, as well as planets and their moons. After this it was generally accepted that the planets revolve around the Sun rather than the Earth.

• An ellipse can be drawn with two pins, a loop of string, and a pencil. The pins are placed at the foci and the pins and pencil are enclosed inside the string. The pencil is placed on the paper inside the string, so the string is taut. The string will form a triangle. If the pencil is moved around so that the string stays taut, the sum of the distances from the pencil to the pins will remain constant, satisfying the definition of an ellipse.

• Try drawing ellipses using this method. You can stick pins into cardboard to make the foci. Sketch your drawing below.

For other PhET models, go to the Physics Education Technology website.