Until the Renaissance, it was generally accepted that the Earth was at the center of all things. Copernicus found that putting the Sun at the center solved some problems with the “geocentric model”, such as the apparent “retrograde motion” of the planets (something we’ll explore in this lab). However, his “heliocentric model” with circular orbits for the planets still did not adequately fit the data on the apparent motion of the planets as seen from Earth.   Since a key aspect of a scientific model is to make accurate predictions, its inability to do so indicated that it needed refining. It was Kepler who provided the necessary refinements to the Copernicus' theories and dramatically increased the accuracy of the heliocentric model. These changes had profound effects on navigation and ultimately all of astronomy.
Click Here to view the Historical Background of Johannes Kepler and Tycho Brahe. It is not required for this lab.

The purpose of this lab is to learn how astronomical objects move through space. Under the influence of gravity planets, stars and galaxies move according to well defined laws called Kepler's laws. You will learn about the three Kepler's laws in this lab.


  1. Read the background material by clicking on the topics listed on the menu on the left side of this page. For a given topic there may be multiple pages each of which can be accessed using the navigation buttons arranged across the top of each page.
  2. Click on the menu item "Problems". A window should open at the bottom of your screen with the questions.
  3. Read the questions carefully. Examine the text, diagrams and animations in order to answer the questions. Note that there are separate pages for each of Kepler's Laws and for retrograde motion.
  4. Answer the questions by filling in the form fields on the computer. Hit the button labeled "Submit" when you are done. A confirmation page will appear with all your answers imbedded. Be careful not to close the problems window until you are finished and want to submit or your answers may be lost and you will have to reenter your answers.
  5. Check over the confirmation page to make sure that you are happy with your answers. If you need to change your answers go back to step 4 and re-submit. We will only grade the last form you submit.

    Good Luck!