We now all grow up knowing that the Sun is the center of the Solar System, so it can be hard to understand what a momentous effect it had on scientific thought in the 17 century. The most interesting thing about changing from an Earth centered to a Sun centered model was that the data didn’t really change. The Ptolemaic system did a perfectly good job of predicting the positions of planets. Copernicus Sun-centered model doesn’t predict the positions of the planets any better, it just seems simpler. In fact, the thing that we actually see, the motion of the Sun and planets compared to Zodiac constellations looks very similar in both models.
In this lab we will explore the Ptolemaic Sun-centered model and the Earth-centered model of Copernicus. In particular we will look at how the same motion as seen from the Earth can be caused by very different motions.
Laboratory Tools
In this lab we will be making use of two simulators developed by the astronomy department at the University of Nebraska-Lincoln. The simulators are:
- The Ptolemaic System Simulator, which shows the motion of a planet in the Ptolemaic Earth centered system. The simulator shows an Orbit View from above the plane of motion and Zodiac Strip that shows how the planet and Sun move relative to the Zodiac constellations. The Planetary Parameters box allows you to change the planet or change the parameters to a planet that doesn’t exist. The Control and Settings box lets you start the animation and control what is shown.
- The Planetary Configuration Simulator, which shows the motion of a planet in the Copernican Sun centered system. This simulator shows a top down view of two planets orbiting the Sun in the Diagram box and again how the planet and Sun move relative to the Zodiac constellations in the Zodiac Strip. You can choose which planets are used in the Orbit Sizes box and the animation in the Animations Control box. The Timeline box gives the names of certain astronomical events, you can ignore this box.
Assignment
First open the Ptolemaic System Simulator. Start the animation and look at the Zodiac Strip. Describe how the planet moves relative to the Zodiac constellations. Try all four preset planets and describe what is similar in their motion and any differences. Now look at the Orbit View. What in the Orbit View causes the motions you have noted in the Zodiac Strip view? You can adjust the planets parameters using the sliders. Which parameter is most important in changing the motion of the planet?
Now let’s open the Planetary Configuration Simulator. Set the observer’s planet as Earth and the target planet as which every planet you were using last with the previous simulator. Looking at the Zodiac Strip is the motion the same or different? Now look at the Diagram box. Using this view explain the what causes the motions you see in the Zodiac Strip. Is it easier to explain these motions from this view or using the Ptolemaic model? The Planetary Configuration Simulator also allows one to change the planet you are observing from. Change the observer’s planet to Jupiter and let the target planet be the Earth. Describe the motion in the Zodiac Strip under these conditions. How about if the target is Mercury, Venus or Mars?
Questions
- Would aliens living on Jupiter be more or less likely to have come up with a Sun centered model for the Solar System?
- For what purpose is the Ptolemaic model actually a “better” model of the Solar System?
- For what purpose is it absolutely necessary that one uses the Sun-centered model of the Solar System?
