1. History of Astronomy: Don’t memorize dates  Who were the scientists?  What did they contribute to what we know today?  When (approximate) did they make this contribution?  What is the significance of that time frame?

2. Ancient History-Greeks (400B.C.-A.D. 150)  Aristotle (384-322 B.C.)  Determined that the earth is round. He observed shadows cast on the Moon during lunar eclipses.  Before Christopher Columbus!

3. Greeks-Two models  Heliocentric Model - Aristarchus(312-230 B.C.) = first Greek to believe this model  Figured out the size of and distances to the moon and sun.  Used that to determine the sun as the center of the universe  Earth and other planets revolve around the sun.  Not accepted by most Greeks

4. Ancient History - Greeks  Eratosthenes (276-195 B.C.)  Determined circumference of the earth using geometry and trigonometry.  Hipparchus (190-120 B.C.)  Determined the location of 850 “fixed” stars and the length of the year.

5. Greeks-Two models  Geocentric Model  Earth is the center of the universe.  Celestial sphere = transparent bubble that contained all the stars, planets, etc.  Fixed stars = all the heavenly bodies that remain stationary on the celestial sphere  Wanderers = 7 bodies that move within the fixed (background) stars.  Included the sun, moon, Mercury, Venus, Mars, Jupiter, and Saturn

6. Greeks- Accepted Model  Claudius Ptolemy- Proposed the Ptolemaic System (A.D. ~127-145)  Circular Orbits  Geocentric model  Unchallenged for 13 centuries because it predicted the motion of the planets.  Explained Retrograde motion-planets move eastward across the night sky, but sometimes will stop and go backward before going eastward again.

7. Ptolemaic System

8. Recent History  Nicolaus Copernicus (1473-1543)  Proposed Earth as a planet  Supported heliocentric model  Explained most motions by a rotating Earth.  Religious/Social implications = goes against the Roman Catholic Church  Think Inquisition of 1600’s! Acceptance means to be fined or ostracized.

9. Tycho Brahe (1546-1601)  Famous for detailed and precise measurements of the heavens  Extensive amounts of very precise data  Conclusions? None published, the Catholic Church reigns supreme, also a believer of Ptolemaic System. (Trying to prove it with his research.)

10. Galileo Galilei (1564-1642)  1609 constructed a telescope(first to use for astronomy, not the inventor.)  Discovered 4 satellites, moons, of Jupiter  Data that supported Copernicus  Planets are round  Venus has phases  Moon’s surface has mountain, craters, etc.  Sun has sunspots, and rotates.

11. Johannes Kepler (1571-1630)  Assistant to Brahe  Kept/Stole/Inherited data?  Using Brahe’s data, discovered the three laws of planetary motion.

12. Kepler’s Laws of Planetary Motion  Law Number 1  The orbit of a planet around the sun is an ellipse with the sun's center of mass at one focus.

13. Law #1-Summary  All orbits are ovals!  We draw circles with one focus. Ellipses have two foci instead of one.  A.k.a. a mathematically perfect oval.  Circles are a special kind of ellipse.  So, all circles are ellipses,  But all ellipses are not circles.  In orbits, the sun is at one focus.

14. Sun (focus) Path of planet (ellipse)

16. Kepler’s Laws of Planetary Motion  Law Number 2  A line joining a planet/comet and the Sun sweeps out equal areas in equal intervals of time.  Another way of saying this is that the planet moves FASTER in its orbit the closer it is to the sun.  Perihelion = Point in orbit where planet is closest to the sun, therefore, traveling faster  Aphelion = Point in orbit where planet is farthest from the sun, therefore, traveling slower

20. Great! So?  Why would the planet move faster as it got closer?  Many factors to consider...  Can partially explain it by the Universal Law of Gravitation.

21. Sir Isaac Newton(1642-1727) Universal Law of Gravitation  All things have a force of gravity.  Dependant upon mass and distance.  The greater the mass the more gravity.  The closer two objects, the more gravity.  INVERSE RELATIONSHIP!  Difference between mass and weight.  Mass is the same everywhere.  Weight is a measure of the force of gravity.  Why don’t the Astronauts drift away from the moon?  2nd law Tie in: Force of gravity is greatest when the planet is closest to the sun.

22. Gravity in a nutshell  Mass matters!  We fall to the earth, not our neighbor, because Earth is so much more massive.  Astronauts feel lighter on the moon because it is less massive and has less gravity.  Distance matters!  We fall to the earth, not the sun, because Earth is much closer.  Sun is still massive enough to keep Earth, and other planets in orbit.  How does it relate to Kepler’s Laws?  When the planet is nearing perihelion, it will feel a greater force of gravity from the sun. It will go faster!

23. Gravity according to Einstein (1879-1955)  Einstein has given us a frame of reference for understanding Newton’s law of gravity.  Bent space model - Anything with mass “bends” space.  More massive objects will stay put and bend the space around them.  Less massive objects will “sink” in toward the more massive ones.  Will investigate this in lab later this week.

24. Einstein’s Gravity

25. Kepler’s Laws of Planetary Motion  Law Number 3  The period of a planet increases rapidly with the radius of its orbit. (T 2 =d 3 )  Period=Time it takes for the planet to make one revolution around the sun.

26. Law #3-Summary  Don’t memorize the formula! (We’ll save that for your physics class.)  Logically-it takes longer to travel a larger distance.  Think about the outside lane on a race track vs. the inside lane. Planet Distance from sun Period Mercury0.38 AU88 days Neptune30 AU60,266 days (165 years)