| A | B |
|---|
| geocentric | Earth in the center |
| heliocentric | Sun in the center |
| epicycles | Smaller circular spheres mounted on larger orbital spheres |
| Ptolemy | A.D. First century astronomer who devised epicycles to explain retrograde motion |
| retrograde | a motion that appears reverse from the normal direction of motion |
| Tycho Brahe | last of the great naked eye astronomers |
| Galileo Galili | Italian scientist who was first to use telescope to make many astronomical discoveries |
| Aristotle | ancient Greek philosopher who added a "fifth element" for the heavens |
| firmament | a barrier shaped like an overturned bowl to separate waters above the heavens from waters below the heavens |
| major axis | The longest length across an ellipse. It passes through both foci. |
| minor axis | The shortest length across an ellipse. |
| Copernicus | Polish cleric who proposed a heliocentric model of the solar system. |
| Kepler | the last scientific astrologer and the first astrophysicist |
| ancient view of planets | "wanderers" in the sky, heavenly bodies that may be gods or goddesses |
| Kepler's First Law | Planets move in ellipses. |
| Kepler's Second Law | Planets move fastest at perihelion and slowest at aphelion |
| Kepler's Third Law | The greater the planet's semi-major axis, the slower the planet's orbital speed |
