The Idea of Earth's Rotation

Earth and Stars

The simulation below illustrates the motion of the stars across the sky for an observer on Earth, as well as two models (Aristotelian and Copernican) to explain those motions. The Sky View on the right shows the sky as seen by an observer, much like a "star map". Cardinal directions are shown, with south at the top. (Note that directions may seem reversed but star maps are designed to display what you see when you look UP, not down.) White dots show visible stars and cyan markers indicate the locations of the North and South Celestial Poles.

The view on the left shows a model that explains the observed motions of the stars. The model shows the stars located on a Celestial Sphere centered on the Earth. The celestial equator is shown as a transparent blue plane. The Earth is shown (not to scale!) at the center, with a green dot indicating the location of the observer and a transparent green plane showing that observer's horizon plane. Relative to the Celestial Sphere the Earth should really be shrunk down to a point (which can be simulated by unchecking the Show Earth checkbox), and only stars above the observer's horizon plane can be seen. The green hemisphere accounts for this effect by covering the stars below the horizon. Cyan arrows show the cardinal directions along the horizon plane, while a cyan line indicates the rotation axis of the Celestial Sphere or Earth.

Explore!

You can use the simulation to explore the Aristotelian and Copernican models yourself. Controls allow the user to play/pause the simulation, advance the simulation by one time step, initialize the simulation using current settings, or reset the simulation to the default settings. Drop-down menus let the user select the observer's location on Earth (with latitudes shown in parentheses) or choose a specific star to highlight in red. Checkboxes let the user decide whether or not to show the Earth, as well as whether to show a rotating Earth (Copernican model) or a rotating Celestial Sphere (Aristotelian model). The user can also click and drag to rotate the 3D view on the left. Use the instructions below to guide your exploration, or just play around!

Aristotle's Model

• Play the simulation with the location set to Berry College (35 N) and the Earth NOT rotating. Watch the motion of the stars in both views. In the Sky View the stars should move from east to west, generally. (Hint: it may be helpful to highlight the star Arcturus so you can focus on the motion of a single star.) How does the rotation of the Celestial Sphere in the 3D view produce this visual effect? Is the North Celestial Pole (NCP) visible? If so, in what direction and how far above the horizon?
• Now change the location to Bangkok. Notice how the orientation of the Celestial Sphere changes. Why does it change this way? (Hint: because Earth's surface is curved, when you change location you also change the orientation of the observer's horizon plane relative to the Celestial Sphere.) Play the simulation. How does the observer's view of the stars change? Is the NCP still visible? Where does it appear? Try to understand why changing location on Earth produces this change in the observed stars.
• Now change the location to Rio. How does the observer's view of the stars change? Is the NCP still visible? What about the South Celestial Pole? Where does it appear? Are there stars visible from Rio that are not visible from Berry College? (Hint: highlight Rigil Kentaurus and see if it is visible from both locations.)
• Now examine the simulation with the location set to the North Pole, and also with the location set to the South Pole. How do the stars move as seen from the poles? What is the difference in the motion of the stars betweeen the North and South Poles? Can observers at the North Pole see any of the same stars as observers at the South Pole?

Copernicus' Rotating Earth

• Now set the simulation to display a Rotating Earth. Repeat some of the investigations from above with this new setting. Do any of your answers change? Which model does a better job of matching the actual motion of the stars (or are they both equally good)?
• How does the direction of Earth's rotation in Copernicus' model compare to the direction of the Celestial Sphere's rotation in Aristotle's model? (Hint: you can check and uncheck the Rotating Earth checkbox to go back and forth between the two models.)
• Keep working with the simulation until you understand how the combination of a spherical Earth and a Celestial Sphere of stars (with ONE of these spheres rotating) can account for the apparent motions of the stars in our night sky.