Astronomy Notes

Part 2: Solar System Layout

John P. Pratt

Early Greek Astronomy

The Greeks deduced several basic astronomical results.

Aristotle's scientific evidence that the earth is the center of the universe (300 B.C.).

Eratosthenes and the Size of the Earth (200 B.C.)

Hipparchus and Precession (100 B.C)

Ptolemy's Epicycles (A.D. 150)

Renaissance Discoveries

Copernicus proposes a sun-centered system

Tycho made careful planetary observations in 1500's using a huge quadrant (before the telescope).

Kepler uses Tycho's observations to proposed Kepler's Laws:

  1. Each planet moves in an ellipse with the sun at one focus.
  2. The line connecting the sun and the planet sweeps out equal areas in equal times.
  3. P2 = a3. The squares of the planetary periods in years equals the cubes of the semi-major axes in a.u. (1 a.u. = astronomical unit = distance from earth to sun).

Galileo began using the telescope in 1610 to make many key astronomical discoveries

Newton, the greatest physicist of all time, discovered Newton's Three Laws of Motion

  1. A body stays at rest or in motion in a straight line at the same speed unless acted on by a force.
  2. If a body accelerates, it is being pushed by a force such that Force = mass x acceleration (F = ma).
  3. For every force on one body, there is an equal and opposite force reacting on another body.

Newton's Law of Gravity

Why all things fall at the same rate (ignoring air resistance)

  1. F = ma from Newton's second law of motion.
  2. F = GMm/r2 is Newton's law of gravity.
  3. ma = GMm/r2 because both are equal to the force F.
  4. a = GM/r2 after dividing both sides my the mass m.
  5. That means that the acceleration is the same for any mass m.

Bode's Rule