1. Early Views of the Solar System (With thanks to Mark Ritter & John Bloom)

2. The Roots of Astronomy
Already in the stone and bronze ages, human cultures realized the cyclic nature of motions in the sky.
Monuments dating back to ~ 3000 B.C. show alignments with astronomical significance.
Those monuments were probably used as calendars or even to predict eclipses.

3. Stonehenge
Constructed: 3000 – 1800 B.C.
Summer solstice
Heelstone
Alignments with locations of sunset, sunrise, moonset and moonrise at summer and winter solstices
Probably used as calendar.

4. Big Horn Medicine Wheel (Wyoming)
Examples All Over the World
Big Horn Medicine Wheel (Wyoming)

5. Examples All Over the World
Caracol (Maya culture, approx. A.D. 1000)

6. Ancient Astronomers
No written documents about the significance of stone and bronze age monuments.
First preserved written documents about ancient astronomy are from ancient Greek philosophy.
Greeks tried to understand the motions of the sky and describe them in terms of mathematical models.

7. Ancient Greek Astronomers
Models were generally wrong because they were based on wrong “first principles”, believed to be “obvious” and not questioned:
1. Geocentric Universe:
Earth at the Center of the Universe.
2. “Perfect Heavens”:
Motions of all celestial, ethereal bodies described by motions involving objects of “perfect” shape, i.e., spheres or circles.

8. General Greek Principles of Science
Nature can be understood
(Not just randomly occurring phenomena)
The diverse behavior observed in nature is held together in patterns
Tendency to
“Save the Phenomenon”

9. Euclid ~ 600 BC What he got right: What he got wrong:
Predicted solar eclipses
Moon shines due to reflected sunlight
Universe explainable by ordinary knowledge and reason
What he got wrong:
Thought Earth was a flat, rotating disk
Thought water was a fundamental element from which all other matter derives

10. Pythagoras ~ 530 BC First to suggest that Earth is a sphere
                                                                                                                        
First to suggest that Earth is a sphere
One of the earliest advocates of a geocentric solar system:
Earth at center, surrounded by system of concentric, rotating, transparent spheres
Bodies attached to spheres in this order:
Moon, Mercury, Venus, Sun, Jupiter, Saturn, stars

11. Philolaus ~ 410 BC (student of Pythagoras)
Suggested Earth moves around a central fire
(NOT the Sun)
What were his new ideas???

12. Ancient Greek Astronomers
Eudoxus (409 – 356 B.C.):
- Model of 27 nested spheres
Aristotle (384 – 322 B.C.), major authority of philosophy until the late middle ages:
- Universe can be divided in 2 parts:
Imperfect, changeable Earth
Perfect Heavens
(described by spheres)
Aristotle expanded Eudoxus’ Model to use 55 spheres.

13. the Aristotelian universe ~ 340 BC
In ancient times philosophers argued from first principles, things that were accepted as “obviously” true
Several important ideas ruled the ancient world…

14. Aristotle ~ 340 BC first: the earth was round
gave 4 reasons for earth’s roundness:
Symmetry: the sphere is a perfect shape
Elements have their natural places: Earth’s pieces fall naturally to Earth’s center, pressing it into a spherical shape (circular reasoning!)
Shadow: Lunar eclipse shadow (Earth’s shadow on Moon) is always circular (what if Earth were a circular disk?)
North Star: Polaris gets higher in the sky the further north one goes

15. second: the earth was located in the center of the cosmos!
third: the heavens were perfect, the earth imperfect
all the cosmos traveled in perfect circles around us
Plato pushed this, his student, Aristotle adopted it
he gave us the Aristotelian way of looking at the cosmos…

16. geocentric universe with all the heavenly objects moving in uniform circular motion
Aristotelian concepts dominated ancient ideas about the universe for thousands of years…
Two major themes here:
1) our place in the cosmos
2) the character of planetary motion

17. Aristarchus ~ 240 BC
Rotation of Earth on its own axis accounts for daily motion of stars
Earth revolves around Sun in a yearly orbit
He had it right before 200 BC!
But his ideas failed to catch on…

18. Eratosthenes ~ 235 BC
Calculated Earth’s radius to within about 5% accuracy!
Basic method:
Measured shadow lengths at two different cities directly North-South of each other (Syene and Alexandria)
Calculated Earth’s circumference and radius using geometry
Central angle = 7.5 degrees, 500 miles from A to S, times 48 = 24,000 miles;
Actual angle = 7.2 degrees, 500 mi x 50 = 25,000 miles.

19. Ptolemy ~ 120 AD great ancient astronomer of the 2nd century
wanted to put it all this orbital motion into a mathematical model
final effort to “Save the Phenomenon”
no significant changes for 1300 years!
Ptolemaic view = geocentric view

20. why did they think we were in center anyway?
popular worldview
the Sun appears to rise, transit the sky, and set
it doesn’t feel like we are moving!
and there was the problem of…

21. parallax, the apparent motion of an object b/c of the motion of the observer
if we were going around the sun, the background stars should change position!
(they do, but not a lot since they are so far away)
Ancients didn’t have telescopes
Had no idea how far away the stars were

22. planetary motion was difficult for the ancients to explain
planets (wanderers) would suddenly backtrack!
called retrograde motion

23. Ptolemy tried to explain it with epicycles (little circles - blue) on deferents (big circles - white)
all in an attempt to keep the Aristotelian (geocentric) view of the cosmos

24. all Ptolemy’s attempts to mathematically explain Aristotle's universe were put together in a book preserved by the Arabs, Al Magisti, then translated into Latin, The Almagest
but unknown to them it was all wrong! enter…

25. Copernicus ~ 1500 AD
a Polish monk, Mikolai Kopernik (Lt. Nicolaus Copernicus) rediscovered the heliocentric model of the universe
but being for heliocentrism was being against Aristotle, and  against the Church!
After Aristarchus!

26. De Revolutionibus
finished in 1530, Copernicus wrote De Revolutionibus Orbium Coelestrium (“on the revolution of the celestial orbs”)
he didn’t give permission for its publication until he was dying

27. most important idea?
he put the sun (Sol) in the center!

28. with the sun at the center the whole universe was simplified and elegant, and could explain things like retrograde motion
we, moving faster on the inside, see other planets “seem” to move backward
but!…

29. Copernicus insisted on circular orbits!
so he had to come up with his own epicycles so it worked better…

30. his model was incorrect
but his hypothesis with the sun in center was right
but without a telescope it couldn’t be substantiated with evidence
AND his circle obsession meant it couldn’t predict things very well
AND it upset people because their whole universe changed!
so it wasn’t accepted for a long time

31. Tycho Brahe ~ 1500s
the greatest pre-telescopic observational astronomer
(also known for his fake nose and bladder problem)

32. in 1572, he saw a supernova which he called a new star
it showed no parallax, so he deduced it to be a part of the starry sphere
but the star sphere was “perfect and unchanging”!!!

33. so the new star challenged Ptolemy and Aristotle
Brahe wrote a book on it called De Stella Nova
which got the notice of lots of people and made him famous…

34. Danish king Frederick II gave him island of Hveen to build an observatory (Uraniburg)

35. here Tycho shows off the latest mural quadrant
with tools like these (notice no scopes) he observed the heavens and took over 20 years of data!

36. Tycho had his own version of what was going on
neither Ptolemaic nor Copernican
Earth was at center, but all the other planets went around the sun
Tychonic view

37. Tycho ended up in Prague and hired some new assistants to help prove his system was correct
one of the assistants was Johannes Kepler

38. Johannes Kepler (1571 – 1630) Kepler had a not-so-fun life
born in 1571 in Germany, he was poor, had an absent father, was sickly, had a nasty mother, etc., etc.

39. a Copernican, Kepler even wrote a book Mysterium Cosmographicum which tried to prove that all the planets were built with regular solids as spacers!

40. joining Tycho
eventually, after being persecuted for being a Protestant, he headed for Prague to work for Tycho
then Tycho died…
so, Kepler took all of Tycho’s work to finish the Rudolphine Tables himself

41. with all of Tycho’s data, Kepler tried to find out how the planets really moved
he discovered that Mars moves in an ellipse, not a circle!
and it traveled at different speeds!

42. he wrote a lot after this and in 1619 published The Harmony of the Worlds in which he said that the radii of the planets’ orbits are related to their periods
all this led to his three fundamental laws of planetary motion…

43. Kepler’s three laws of planetary motion
first law relies on the ellipse – an oval drawn around 2 points
points called foci (sing: focus)
semimajor axis (a) = half the longest diameter
eccentricity (e) is half the distance between foci divided by the semimajor axis
a circle is an ellipse with e = 0
greater e  more elliptical
Greater e = more elliptical, less symmetric/circular

44. Kepler’s First Law (Law of Ellipses) states that all planets travel in ellipses with the sun at one focus

45. Second Law (Equal Areas-Equal Times) planets sweep out equal areas in equal times
translation: they go faster when closer to the Sun

46. Third Law
T2 = R3
states that the closer a planet is to the Sun the shorter its “year”
T = period in Earth years
R = distance in AUs
given one, you should be able to find the other
Kepler never knew why his laws worked, just that they did

47. the Rudolphine Tables
finally, in 1627, Kepler finished the R Tables, a precise model of planetary motion
(notice Tycho as part of the pillars of astronomy)
a contemporary of Kepler was…

48. Galileo Galilei (1594 – 1642)
helped push science to a new level…

49. telescopic observations
GG had an academic upbringing and was quietly Copernican
it was the telescope that pushed him over the top
he used this recent invention to examine the sky and gather evidence for the heliocentric view

50. he immediately published his findings in Siderius Nuncius (The Sidereal Messenger)
here he reported several big discoveries…

51. 1: The Moon was not perfect!!!
remember Aristotle said everything in the heavens was perfect?
but the Moon looked sort of like Earth!!!

52. Major Discoveries of Galileo
Surface structures on the moon; first estimates of the height of mountains on the moon

53. 2: There were lots of stars up there (many more than were previously supposed)
the Milky Way (and the rest of the heavens) is substantial, like the earth, not ethereal

54. 3: He saw four little “stars” near Jupiter
these points of light were going around Jupiter!
but they weren’t supposed to!!!! they should have been left behind!!!
So, not everything orbits the Earth…
and, if Jupiter can move and keep its “moons” so could Earth

55. Major Discoveries of Galileo
Moons of Jupiter
(4 Galilean moons)
(What he really saw)
Rings of Saturn
(What he really saw)

56. Major Discoveries of Galileo
4: He saw that the sun had spots (proving that the sun is not perfect!), and that it rotated

57. 5: He observed that Venus went through phases (including “full Venus”)
So what? it can only have all phases if it goes around the sun, not the Earth!

58. but even with all this evidence people were still upset
some didn’t even look through the telescope!
The Catholic Church, still in love with Aristotle, told Galileo to be quiet
it was in 1629 that he wrote his great defense of the Copernican way…

59. Galileo’s Dialogue it was an imaginary debate between three friends:
Salviati (pro-Copernican) Sagredo (indifferent) Simplicio (pro-Ptolemy)
problem: Simplicio presented the Pope’s view of things … bad move!

60. Galileo was brought before the Inquisition
charged with not shutting up
condemned to life imprisonment
note: what Copernicus and Kepler and Galileo taught and discovered didn’t contradict the Bible…
it contradicted people’s interpretations, traditions, and worldviews

61. The Birth of Modern Science
in trying to understand the skies in the 16/17 century, modern science was born
nothing more than a logical way of observing, studying, and thinking about nature
scientific method (Roger Bacon, 13th century)
cause and effect (Francis Bacon, 17th century)
et al…

63. Isaac Newton and orbital motion
by now we have a new outlook on the universe and new ways to study it all
perfect time for this guy to show up…

64. Isaac Newton
Isaac was very smart, raised in England, educated at Trinity, but…
during the Black Plague of 1665 went home to Woolsthorpe
while there he thought through his laws of motion…

65. which he wrote about in his Principia
from the work of all the great guys before him he gave us three laws of motion which we still use
he realized the same force that brings an apple to the Earth keeps the Moon in orbit
and those other things must be pulling on us, too

66. he figured if something has mass it has gravity
mass is how much stuff/matter something has
weight is the force that something exerts because gravity pulls on it

67. He also realized that the distance plays a huge role
and is an inverse square relation
double the distance and gravity decreases by 4x.

68. know the relationships…
the force of gravity attracting two objects is proportional to their masses, but inversely proportional to the squares of their distances

69. orbital motion here’s why satellites orbit celestial bodies
throw something hard enough and the Earth will curve out of the way
it will “fall” forever
that is being in orbit
orbital motion

70. this animation shows how

71. circular velocity is the speed you have to achieve to orbit in a circle
less, you crash; more, you might even leave!
the Shuttle needs to get to about 8 km/s to do it.
less over smaller bodies