| A | B |
|---|
| firmament | structure similar to an inverted bowl that formed the boundary between the Earth's sky and the heavens |
| retrograde | observed motion "backwards" or opposite the normal direction in the sky |
| epicycle | smaller spheres or discs that the planets moved on to produce retrograde motion according to Ptolemy |
| heliocentric | Sun centered model of solar system |
| geocentric | Earth centered model of solar system |
| Ellipse | true shape of planetary orbits around the Sun, Kepler's First Law |
| crystal spheres | heavenly structures that were believed to support the planets as they orbited the Earth |
| planets | heaven bodies that "wandered" among the constellations |
| eccentricity | how far "off-centered" an ellipse is from a circle |
| magnetism | force Kepler believed to be driving the planets |
| gravity | force Newton identified as driving the planets |
| foci | two points on major axis of ellipse from which all points on the ellipse are equidistant |
| major axis | the longest distance across an ellipse through its center point |
| minor axis | the shortest distance across an ellipse through its center point |
