Retrograde Motion

It took almost 2000 years before the Greek dream of knowing the solar system's true geometry was realized. In the 1400s, the brilliant Polish astronomer Nicholas Copernicus devised a heliocentric (sun-centered) model for the solar system. In his model, all the planets now orbited the Sun, with inner planets completing their years faster than the outer planets. The retrograde loops fell naturally out of Copernicus' heliocentric system, as rapidly moving inner planets overtook the slower moving outer worlds

INTERACTIVE Applet Direction:

This interactive applet allows you to play with Copernicus' idea (which we now know to be true) and experiment for yourself with the origin of retrograde motion. The interactive gives you a simplified version of a solar system with just 3 planets (Venus, Earth and Mars). With the powers provided by the slider bar, you can change the orbits of both Mars and Venus. Planets closer to the Sun always travel faster than those farther out, so by changing the orbit size you are also changing orbital period (the length of its 'year'). If you click on "Use Direction Coloring" the color of the planet will change from green to red when it undergoes retrograde motion.

Now you can choose which planet you want to live on and which planet you want to watch in the night sky for retrograde loops. For instance you can watch Mars from the Terrestrial sky or look at the movement of Venus in the Martian sky. As your home world moves around the Sun, the position of the planet you are watching is plotted against the fixed background of stars (as is the Sun). Play with the orbits and with the choice of planets you are watching (or watching from) until you get a good physical feeling for why retrograde motion occurs. This is what the Greeks so desperately wanted. Notice that the night sky is displayed as ring which wraps around the solar system. This may take some getting used to for a moment but it allows you to continuously view the motion of the Sun and the planets against the fixed background of stars

Visit this website for the interactive applet:

  1. Does Venus undergo retrograde motion when viewed from Mars or the Earth? Explain.
  1. Venus is called the morning or evening star. Explore the applet to find out why. Explain.
  1. Do you expect a planet to brighten (or appear larger through a telescope) during a retrograde loop? Try to answer the question by thinking about it before you go to the Applet to experiment. Explain.
  1. The heliocentric model was extremely controversial when it was proposed and was strongly resisted by many people. Can you identify two reasons either scientific or otherwise, which would have led to this resistance?

Planetary SciencePage 1