Illuminations: Mars Earth Orbit - Actual

Mars Earth Orbit - Actual

This applet uses the actual distances and speeds of the planets to simulate the relative positions of Earth and Mars.

Notice that both planets revolve around a different center point. The center of the Earth's orbit is approximately the Sun; the center of Mars' orbit is about 22 million kilometers from the Sun. Consequently, the orbits of the two planets are closer to one another on one side of the Sun than on the other side.

The orbital radius of the Earth is approximately 150 million kilometers, whereas the radius for Mars is, on average, 228 million kilometers.

Note that this approximation uses circular orbits for both planets, but the true paths are slightly eccentric ellipses.

Instructions

 The Orbit buttons allow for various explorations when the planets are in motion. You can also move the Earth and Mars by clicking and dragging them. The Show EM and Show E'M' buttons show the relative distance and orientation between Earth and Mars as they make their orbits about the Sun. When a portion of the path has been drawn, a red X will appear in the lower right corner; press this button to clear the graphic.

Exploration

 People used to believe that the Earth was the center of the Universe. Instead of thinking that the Earth was one of several planets that revolved around the Sun, they believed that the Sun and the other planets revolved around the Earth. Relative to the Earth, what would the path of Mars look like? Press the Put Earth in Orbit button. The graphic to the right will show the relative path of Mars if only the Earth were in orbit. Press the Show EM and Show E'M' buttons to see a segment connecting Earth and Mars; notice that the length and angle of these segments are identical, which explains how the path is being drawn. Press the Put Earth in Orbit button again to stop the orbit of the Earth. Press the Put Mars in Orbit button. The graphic will show the relative path of Mars if only Mars were in orbit. Press the Put Mars in Orbit button again to stop the orbit of Mars. Press the red X in the lower right corner to clear the graphic. Now, think about this: What would the path of Mars look like, relative to Earth, if both planets were in motion? To test your conjecture, press the Put Both in Orbit button. What does the path look like? Was your prediction correct? Reset the graphic and press the Put Both in Orbit, One Revolution by Mars button to see just one loop.