1. The Big Four: Copernicus Kepler Galileo Newton
Planets orbit Sun, not Earth
but assume circular orbits (Greek idea of perfection)
Kepler
Orbits elliptical, not circular
but described by simple laws
Galileo
Discovers moons of Jupiter
Orbital motion is Universal
Newton
Idea of forces theory of gravity

2. Newton’s Theory Idea of “Escape Velocity”
Gravity is a “force”
causes acceleration or deceleration (= negative acceleration)
g at Earth’s surface = 32 ft/sec/sec = 22 mph/sec = 9.8 meters/sec/sec
Example:
throw something up at 66 mph: it stops after 3 sec, then falls back in another 3 sec
How to calculate gravitational pull of a mass:
increases with amount of mass:
decreases with square of distance from center of mass:

5. Escape speed:
Competition: deceleration vs. weakening of gravity with distance

6. The Speed of Light
1675: Olaus Roemer (Danish astronomer) measures by timing “transits” of Jupiter’s moons
Kaufmann, Universe, 4th ed., p.80

7. The Idea of a Black Hole … until Einstein comes along
John Michell (1783) , Pierre-Simon Laplace (1796):
Ques: What happens if the escape speed from an object is greater than the speed of light?
Ans: If light consists of particles of matter, they would not be able to escape
The catch: early 19th cent - light is a wave (a disturbance), not a particle BH idea forgotten….
“dark star” ?
… until Einstein comes along