1. Models of the Solar System

2. The Greeks
Greeks inherited knowledge from both Babylonians and Egyptians.
Egyptians and Babylonians had kept records for several thousand years
However, Egyptians and Babylonians used their astronomical knowledge for practical and political means
The Greeks, however tried to answer theoretical questions and understand how the universe actually worked.

3. The Greeks
More was discovered from 300 BC to 150 AD, than in the 2000 years before, or 1500 years after.
This was due to the Greek’s motivation to understand all things, coupled with highly advanced mathematics (trigonometry, for example)

4. Aristarchus of Samos Aristarchus lived in 300 BC Founded the
Alexandria School of Astronomy
Figured out the size ratio of the moon compared to the earth using earth’s shadow.

5. Question
Suppose it takes an hour for the moon to enter the earth’s shadow.
Suppose it takes four hours for the moon to cross the earth’s shadow.
How many times larger is the moon that the earth?

6. Aristarchus of Samos Estimated the distances of the sun, the
moon, and the earth from each other.
He was off by a factor of 10, however

7. Aristarchus of Samos Finally, Aristarchus proposed that the
sun was in the center of the solar system.

8. Eratosthenes Lived around 200 BC
Worked at the Alexandria School of Astronomy
Determined the earth’s circumference within 100 miles of the correct answer.

9. Eratosthenes The angle of the shadow was 7°
He heard that in Syene, on a specific day of the year, when the Sun was overhead, there was no shadow cast.
However, in Alexandria, on that same day, the sun cast a small shadow.
The angle of the shadow was 7°
The distance from Alexandria to Syene was 575 miles

10. Question
If the angle is 7°, and the distance was 575 miles, what would have been Eratosthenes answer?

11. Hipparchus Lived around 150 BC Built one of the first observatories
Invented or refined many astronomical tools
Created the magnitude system of star brightness
Discovered the earth’s precession
Figured out the orbit of the moon around the earth, and the earth around the sun.

12. Geocentric vs. heliocentric
The reason why the greeks decided that the earth was the center of the solar system is understandable, given what they saw.
They expected that the stars should shift their angle in the sky in the six months between June and December
They did not see this shift
So they concluded that the earth is the middle, not the sun.

13. Question
We now know that the Sun is the center of the solar system, and not the earth.
So why did the Greeks not observe the predicted shift of the stars?

14. Hypatia of Alexandria Lived in the 400s AD
Was the first woman instructor of
Astronomy at the Alexandria School
Of Astronomy.
Created the Astrolabe
Was killed for being a female scientist by a mob.
The belief at that time was that women had no business being educated, or being teachers, and instead should stay at home and raise children.

15. Ptolemy Ptolemy lived in 150 AD
Also worked at the Alexandria School of Astronomy
Created the final form of the geocentric model of the solar system

16. Ptolemaic Model
Because the circle was held to be the perfect shape, and planets were also held to be perfect:
Ptolemy put the planets in perfect circular orbits.
Moon closest, then Mercury, Venus, Sun, Mars, Jupiter, Saturn, and then the shell of stars.

17. Ptolemaic Model To explain things like the retrograde motion of mars:
Ptolmey put circles within the circular orbits.
These were called epicycles.

18. Ptolemaic Model Example of the epicycles.
Epicycles made the orbits of the planets look like this:

19. Ptolemaic Model
In addition, both Mercury and Venus’ epicycles were fixed in an orbit that always kept them in a directly line with the earth and the sun.

20. Ptolemaic Model
This is the overall effect of the epicycles and orbits:

21. Astronomy after the Greeks
The Romans did almost nothing with astronomy.
Rome was full of engineers, not scientists.
European Middle Ages also had no progress in astronomy.
Europeans assumed that the Greeks had already discovered all knowledge.

22. Astronomy after the Greeks
Arabic Middle Ages was different.
Islam held that the heavens was Allah’s creation, and worthy of intensive study
Much work was done on navigation based astronomy and algebra based astronomy.
Our number system (1,2,3,4) was created at this time.
Arabs also preserved the works of Greek astronomers like Hipparchus and Ptolemy.

23. Astronomy after the Greeks
Why did the European Middle Ages see no progress when the Arabic Middle Ages did?
The Catholic Church ruled that the geocentric model was ordained by God, because it gave a clear location for Satan (inside Earth) and a clear location for God (in the stellar circle)

24. Astronomy after the Greeks
Not all was stagnant
Giodano Bruno was a monk in Italy during the late 1500s.
He proposed that the stars had their own solar systems, and that intelligent life could live on the planets there.
He was burned at the stake for this “heresy” in 1600.

25. Copernicus In the late 1400s and early 1500s, the
Renaissance was in full swing in Italy.
Nicolas Copernicus was a Polish seminary student attending school in Italy.
There he studied astronomy, and may have read Aristarchus’ heliocentric model

26. Copernicus He rejected the Ptolemaic model, with
its complicated epicycles
Instead, Copernicus put the
Sun in the middle of the solar
system, and then the
planets in order orbiting around it.

27. Copernicus There were two major problems with the Copernican model:
Copernicus made the orbits perfectly circular, which made the model difficult to explain or predict some movements of the planets correctly
Copernicus could not explain why the Earth didn’t leave the Moon behind as the Earth orbited the sun.
Gravity didn’t exist yet, you see!

28. Tycho Brahe Lived in the late 1500s Danish nobleman
Built the largest, most expensive
observatory the world had seen to date.
Made the most accurate and precise measurements of stars and planets ever done without a telescope.
In fact telescopes couldn’t equal his work until the early 1700s!
Made observations over decades, amassing a huge amount of data.

29. Johannes Kepler
Tycho Brahe was an excellent observer, but a terrible mathematician.
Brahe acquired a young assistant named Johannes Kepler to aid him in this.
Kepler was a member of the low-class
However, Kepler’s mathematical genius was so great that Brahe overlooked his “low status” and brought him to work at the great observatory.

30. Johannes Kepler Kepler lived in the early 1600s.
He took Brahe’s observations of the motion of the planets, and spent 8 years calculating an answer.
After those 8 years, he created Kepler’s Laws of Planetary Motion

31. Kepler’s Laws of Planetary Motion
The orbit of every planet is an ellipse with the Sun at one of the two foci
A line joining the planet and the sun sweeps out equal areas during equal intervals of time
The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit.

32. Kepler’s Laws of Planetary Motion
Kepler’s first law:
Kepler’s second law:

33. Kepler’s Laws of Planetary Motion
Kepler’s third law:

34. Galileo Galileo Galilei was a contemporary of Kepler’s.
He adopted the heliocentric theory very quickly
He believed that nature should prove theories, not human reasoning, a revolutionary concept at the time!
Proved that gravity works on all objects equally

35. Galileo Built one of the first telescopes First to use the telescope
in astronomy
Discovered the following:
The Milky Way is made up of stars
Many of the mountains on the moon
Planets had visible disks, while stars remained as points
The four large moons of Jupiter, and named them
Sunspots on the surface of the sun
Planets underwent phases like the moon does

36. Galileo Galilei
Planetary phases were used to disprove the geocentric theory.

37. The Phases of Venus

38. Galileo In the late 1630s, under threat of
Torture, Galileo was forced to claim:
That the Heliocentric model of the
universe was a lie
That Kepler’s Laws were mathematical tricks
That there was no proof that the Earth moves

39. Newton
Created the Laws of Motion, which are so fundamental that they are still used today, essentially unchanged.
A body at rest will stay at rest (etc. etc.)
For every action there is an equal and opposite reaction
The total force on a body is equal to its mass times its acceleration.

40. Newton
Newton used the third law of motion to define the law of gravity:
The Force between two objects is equal to the mass of each object multiplied, divided by the distance squared, and multiplied by the universal constant of gravity.

41. Law of Gravitation
Newton applied his new Law of Gravitation to not just to objects on the Earth, but to the planets themselves.
When he did this, he discovered WHY Kepler’s laws worked

42. Kepler + Newton = AWESOME
The reason why Kepler’s second law works (for example) is because as a planet gets closer to the sun, it travels faster.
This is because, according to the Law of Gravity, the attractive force is growing stronger BECAUSE the planet is closer.
Angular momentum =
Mass*speed*distance from sun
Since mass can’t change, as the distance
becomes smaller, the speed has to become
larger.

43. Final Proof of the Heliocentric Model
So, let’s go back to the Greeks for a minute.
The Greeks expected the angle of the stars to change, but they didn’t.

44. Final Proof of the Heliocentric Model
It turns out that the stars actually DO change their angle though, through the year.
The amount the angle changes is 20 arcseconds.
20 arc seconds is about degrees, which is only possible to detect with a telescope.

45. Final Proof of the Heliocentric Model
This angular change was discovered in 1729 by James Bradley in England.
He was trying to explain parallax.

46. Parallax
Parallax is the apparent motion of a fixed object in reference to the background caused by the movement of the observer.

47. Parallax
Parallax is the apparent motion of a fixed object in reference to the background caused by the movement of the observer.

48. Parallax
Parallax of the stars is only possible if the Earth travels around the sun.
And those stars are at varying distances away from each other.