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Across 1. measure of the ellipse's size, "a", 1/2 the length of the long or major axis 4. total amount of matter an object contains 7. geocentric, the circle, deferent & epicycle with center around Earth 9. 3 basic laws of motion, inertia, force & mass, force 1 acts on force 2 10. Earth-centered, early thoughts, actual motions lead to downfall of entire theory 11. tug-of-war between Sun's gravity and planet's inertia, results in a stable orbit 13. Renaissance period, accounts for retrograde motion & brightness, heliocentric 18. time needed to complete one orbit around the Sun, "P", sidereal orbit 19. most accurated naked-eye measurements of stars and planets, observational astronomer 20. worked with Brahe, heliocentric and mathmatical theorist

Down 2. distance between Sun and Earth, semi-major axis of Earth's orbit around the Sun 3. Sun-centered, newer at end of 16th century, religiously sensational 5. rate of change of the velocity of an object, speed up, slow down, change direction 6. radical approach because performed experiments to test his ideas, telescopes 8. object having mass exerts a force on another object with mass 12. path of the sun, celestial orientation,relative to the stars, over one year 14. how planets orbit, a flattened circle 15. push or pull 16. body will keep moving at a constant velocity and same direction 17. 2 points within an ellipse, planets orbit with the Sun at one