In the simulation below, there are two planets. One is Earth and one is Mars. Earth is more massive than Mars. In the configuration below, Earth is closer to the sun than Mars:

1. Which of these two planets has the fastest orbital period about the sun? The applet is set to pause every quarter orbit to help you discern the relative orbits between earth and mars. Click on the start button until 4 quarter orbits have passed and then hit the stop button. Has Mars completed an orbit yet? Now hit the reset button.

2. Now grab the earth and move it to an orbit that is outside the orbit of Mars. Move the earth until it reads 2.50 in the Earth AU box. Do you think the orbital period of the earth will still be faster around the sun than Mars because the earth is a more massive planet? Hit the start button and wait until 4 quarters of the earth orbit has elapsed. How many times has Mars gone around the sun, greater or less times than once? (this is given in the Mars Orbits box).

3. Hit the reset button and move the earth to a distance of 0.5 AU. What is its orbital period? Now move the earth to a distance of 4 AU - what is its orbital period? If you moved earth to 2 AU can your predict its orbital period?

4. Now hit the reset button and place the earth at exactly the same distance as Mars so the images overlap. Hit the start button and observe the system. Do the images of earth and mars ever separate? Does the orbital period of a planet depend on the mass of the planet?