1. Kepler's _____ law says that the line joining the Sun and a planet sweeps equal areas in equal times.
6. Newton's _____ law: F = ma.
7. Newton's _____ law says that things continue moving in a straight line at a constant speed unless acted on by an outside force.
8. In the early 17th century, this man helped to prove the heliocentric theory by using a telescope to discover four large moons around Jupiter and the phases of Venus.
10. Newton's _____ law says that for every action there is an equal and opposite reaction.
12. This 2nd century astronomer used an Earth-centered model with circles-on-circles (epicycles) to explain the motion of the planets.
14. Calculated the correct size of the Earth in about 250 B.C.
15. 19th century French physicist who used a pendulum to prove that the Earth moves.
16. Giant 25 ton stones in England that are aligned with astronomical objects.
2. In the 17th century this great scientist discovered the "Laws" that describe gravity.
3. Kepler's _____ law says that the square of the period of revolution is proportional to the cube of the planet's average distance from the Sun.
4. Used Tycho's observations of the planets to develop his own "Laws of Planetary Motion" in the early 17th century.
5. Greek philosopher (around 350 B.C.) thought that the Earth was at the center of the universe.
9. 16th century Polish astronomer famous for his theory that the Sun was at the center of the universe and that the Earth moved..
10. 16th century Danish observer of the planets (lost part of his nose in a sword fight).
11. Newton's first law is also known as the law of _____.
13. Kepler's _____ law says that the planets orbit the Sun in ellipses, with the Sun at one focus.