1. How has the model of the solar system changed over time?

2. The Basic Problem: If observed from one night to the next, a planet appears to move from East to West against the background stars most of the time. Occasionally, however, the planet's motion will appear to reverse direction, and the planet will, for a short time, move from West to East against the background constellations.

3. How can we explain this? Astronomers have been trying to design models of our solar system to explain this backward motion of planets for over 2,000 years.

4. Plato’s Explanation The ancient Greeks believed the circle to be the perfect form, and since they "knew" the heavens were perfect, then the Earth must stand still and everything move around it in perfect circle orbits! This is called the Geocentric Model. Cutaway of the Geocentric Model

5. Ptolemy’s Explanation This model assumed that the motions of the Sun, Moon, and stars were based on perfect circles. To account for the retrograde motion of the planets, a system of epicycles were “invented” where the planets moved around small circular paths (Epicycles) that in turn moved around larger circular orbits around the Earth (GEOCENTRIC.)

6. Copernican Explanation Copernicus replaced the geocentric universe of Ptolemy with one that was centered on the Sun (heliocentric), with only the Moon orbiting the Earth. His explanation for retrograde motion in this system arises from the fact that the planets further from the sun are moving more slowly in their orbits than those closer to the sun.

7. Kepler’s Explanation Kepler became convinced the ellipse was the shape of planet orbits, not the circle. This idea went against the 2,000 years of belief! Kepler had a hard time convincing other scientists of his time that planet orbits are not circles. Even the great scientist, Galileo, disagreed with Kepler.

8. The Current Model The current model of the Solar System has identified nine planets orbiting the Sun (HELIOCENTRIC MODEL). The orbit of Pluto is different from the rest of the planets. Some scientists are even beginning to think Pluto doesn’t meet the definition of “planet.”

10. And we now know there are solar systems other than our own! Recently, other stars with orbiting planets have been discovered. These planets (called exoplanets) have never been seen by us, but they have been detected by the very slight "wobble" they induce in their star.

11. First Exoplanet Image Astronomers confirm the first Image of a planet outside of our Solar System on April 30, 2005

12. March 2005 Announcement

13. The End