The center of the Solar System: Heliocentric Model vs. Geocentric Model

1. The center of the Solar System: Heliocentric Model vs. Geocentric Model 7th grade Earth Science Mrs. Andres

2. Aristotle (384-322 B.C.) • Established the Geocentric view • States that the Earth is the center of all things and the Sun, moon and planets revolve around it • Works for the Sun and moon, • Doesn’t work for the planets • Can’t explain: • Their changing brightness and • Retrograde motion: • Appear to stop and move backwards in the sky during the year

3. A Geocentric Solar System

5. Retrograde Motion of the Planets as seen from Earth

6. Aristarchus of Samos (310-230 B.C.) • Greek, born 12 years after Aristotle died • First to propose the Heliocentric model, • Never accepted in his lifetime… century or millennium!! • Why: Aristotle was more influential, and famous with more followers and writings.

7. Questions Aristarchus couldn’t answer • Why doesn’t the earth feel like it’s moving? Shouldn’t there be some sort of wind or force as we move at high speed around the Sun, like a car speeding around a track? • Ans.: The earth isn’t moving through anything in space, so there is no wind resistance acting on the planet as it moves, • Secondly, Earth’s gravityoverpowers any ‘centrifugal force’ (inertia) we would feel from our revolving motion

8. Questions Aristarchus couldn’t answer • If Sun is center of the solar system’s gravity, why don’t things fall UP, towards the Sun, instead of down, towards the center of Earth? • Ans.: The effects of gravity become exponentially weaker the farther away you are from an object, so since we are so close to the Earth and so far from the Sun, we don’t feel the Sun’s gravity, although it does contribute to the tides in the oceans

9. Questions Aristarchus couldn’t answer • If the vantage point of earth changes, why don’t we see stellar parallax? • Ans.: Even the nearest stars are so far away that the stellar parallax we see is very small • Similar to looking down a long straight road at a far away mountain • Close your right eye, and note its position, then create parallax by opening the your right eye and closing your left. If you do this with your finger out in front of your face, your finger moves compared to the background, but the mountain is too far away and appears to stay still

10. Claudius Ptolemaeus (Ptolemy) – around 140 A.D. • Greek, 400 yrs. after Aristotle • Made the best Geocentric Model • “The Ptolemaic Model” • Problem: very complex • Had 80 circles to explain the sun, moon and 5 planets known then!

11. Ptolemy made “epicycles” to explain retrograde motion of planets

15. Occams’ razor • Named after English philosopher: William of Ockam • If two theories explain the facts and make the same predictions, the simpler one is better. Keep it simple! • Good theories have no more complexity than is absolutely necessary.

16. Nicholas Copernicus (1473-1543) • 2,000 years after Aristotle and Aristarchus • Copernican Revolution –The change from a belief in a Geocentric model to a Heliocentric model • Copernicus liked Heliocentric model because it’s simpler: • “…it is more pleasing to the mind”, • It also explained some small inconsistencies in the Geocentric theory.

17. A Drawing of the Heliocentric model made by Copernicus

18. Nicholas Copernicus • The only major flaw in his version of the heliocentric model was that he used circular orbits when we now know the planets travel in elliptical orbits • But that might have been because the orbit of Mars is very close to a perfect circle around the Sun. • His ideas were never accepted in his lifetime either, • But with the observations of some later astronomers, the world eventually did “revolve around” to the correct understanding. 

19. Galileo Galilei (1564-1642) • Italian born 21 years after Copernicus died • A key to his success: • He embraced cutting edge technology and techniques of his day like • Experimentation over logical reasoning and • The telescope • Invented in Holland around 1600, Galileo was already around 36 years old • He heard about it (never had seen one!) and built one for himself.

20. Galileo’s discoveries of Jupiter’s moons with his telescope showed that Earth was not the center of all orbits strongly supported a heliocentric model

21. Galileo also observed ‘blemishes’ on the Sun (sunspots): This contradicted ‘perfect spheres’ idea of Aristotle.

22. Galileo’s discoveries of Venus’ phases with his telescope showed that Venus must orbit the Sun strongly supported a heliocentric model

24. Galileo’s discoveries of Venus’ phases with his telescope showed that Venus must orbit the Sun strongly supported a heliocentric model Venus can only go through phases if it orbits the Sun.

25. Galileo Galilei • Some key observations he made in that trusty telescope of his: • Mountains, valleys and craters on the Moon, • The sun has imperfections - sunspots, which he watched rotating • 4 moons of Jupiter (if other planets have moons, earth is not the center of all things) • Venus goes through phases like the moon, which can only be explained by the heliocentric theory

26. Johannes Kepler (1571-1630) • German, 7 years younger than Galileo • Poor eyesight • So he based his laws on Tycho Brahe’s observations • Discovered 3 simple laws that accurately described the planets’ motion in the heliocentric theory

27. Johannes Kepler • Called: “The 3 Laws of Planetary Motion” • Kepler’s Laws pretty much sealed the lid on the Geocentric model’s coffin and allowed most scientists to accept the Heliocentric model. • Especially because they followed the Occam’s razor principle (simple) • They work for every planet in our solar system, even the ones that Kepler never knew about!

28. Kepler’s model for the Solar System: A Concentric Series of Crystalline Spheres

29. Kepler’s Three laws • 1st Law: All orbits are Elliptical with Sun at one focus (circular orbits are special case of elliptical with e = 0) • 2nd Law: Planets sweep out equal areas in equal time • 3rd Law: P2 = a3(P in yrs, a in AU)

30. The Copernican Revolution

31. 9/13/2005

32. Kepler’s Friend and Boss: Tycho Brahe

33. Tycho Brahe • (1546-1601) Dutch, 25 years older than Kepler and 18 years older than Galileo) • Arguably the best observational astronomer of all time • Didn’t have the telescope yet, that was invented a couple decades later, (around the time he died, actually) • So he had to essentially just use his eyes and kept careful records and passed them on to Kepler when he died.

35. Tycho Brahe • Met Kepler when he moved to Germany (He ticked off a bunch of the aristocracy in Denmark) • Kepler was a great mathematician, so Brahe (rich boy) paid him to work with his observations and try to make sense of them with him. • After Brahe died, Kepler spent most to the remaining 29 years of his life carrying on Brahe’s legacy and making sense of his observations (resulted in the 3 laws of planetary motion). • An amazing example of scientific collaboration and the fulfillment of a promise from one friend to another.

36. Isaac Newton • (1642-1727) British • Born the same year that Galileo died • Finally explained WHY the earth orbits the Sun. • Before him Galileo, Brahe and Kepler explained WHAT the solar system looked like, but couldn’t say WHY it looked that way. • Newton came up with… • The 3 laws of motion • The law of Universal Gravitation • and helped invent Calculus to explain WHY.

37. Isaac Newton • The law of gravity says that all masses attract each other and that the more mass an object has the greater the attraction it has. • Explains why the earth and planets are pulled towards the Sun, but why don’t they get pulled INTO the Sun??

38. Isaac Newton • The first law of motion is the law of inertia • An object tends to maintain its current state of motion • In both speed and direction • When the solar system formed, the Sun “caught” giant masses in space, which are now the planets. • Those masses moving through space also wanted to maintain their forward motion through space because of their inertia, so they ended up moving around the Sun instead of stopping and sitting still in space.

39. Isaac Newton • The planets continue to move because there is no friction in space to slow them down • If gravity was suddenly “turned off”, they would fly off in a straight line pointing out from their orbit because of their inertia. • ball on a string example

40. So to sum up: • Gravity keeps the planets pulled inward • towards the Sun • Inertia keeps them moving outward, • results in a circular path

41. Isaac Newton Quote • “If I have been able to see farther than others it is because I stood on the shoulders of giants.” --- Newton’s letter to Robert Hooke, • probably referring to Galileo and Kepler

42. Another Sweet Quote… • “ I know not what I appear to the world, but to myself I seem to have been only like a boy playing on the sea-shore, and diverting myself in now and then finding a smoother pebble or a prettier sea-shell, whilst the great ocean of truth lay all undiscovered before me.”