The History of Astronomy

3. the eccentricity of a perfect circle
5. the point in a planet's orbit where it is travelling the slowest
7. this is what happens to the strength of the gravitational force between two objects when they move away from each other
10. loops made by a celestial object within its orbit around another celestial object
12. the Earth at the center of the universe
13. the relationship between a planet's distance and its orbital period describes this Kepler Law
16. his observations, which included the phases of Venus and Jupiter's moons were strong evidence to disprove the geocentric model
17. came up with a geocentric model of the universe with epicycles
19. his heliocentric model was very detailed and used perfect circles for the orbits of planets

1. the apparent backward movement of planets in the night sky
2. an ellipse has two of these
4. during this season in the northern hemisphere, the Earth travels the fastest in its orbit
6. this is what happens to the gravitational force between two objects when their mass is doubled
8. increases when the foci are farther apart
9. the type of relationship that exists between a planet's distance from the Sun and its orbital period
11. equal areas in equal time is a description of this Kepler Law
14. the Sun at the center of the universe
15. this early astronomer discovered that the orbits of the planets are ellipses
18. according to Kepler's 3rd Law, this does not affect a planet's orbit around the Sun