1. The History of Astronomy

2. When did mankind first become interested in the science of astronomy?
With the advent of modern computer technology (mid-20th century)
With the development of the theory of relativity (early 20th century)
With the invention of the telescope (~ A.D. 1600)
During the times of the ancient greeks (~ 400 – 300 B.C.)
In the stone and bronze ages (several thousand years B.C.)

3. 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.

4. Egyptians Saw patterns in the Sun, moon, and Venus
Called the sun “Ra” (Sun god) – rode in his boat across the sky daily
Developed 365 day year solar calendar

5. Ra (on right)

6. Mathematical theory rather than just observations
Babylonians (3000 BC)
Mathematical theory rather than just observations
Motions of the sun and moon
Provided the first evidence that the earth is round
Developed basic constellations and astrology
Introduced the 24 hour day
Developed a lunar calendar

7. Greece Eudoxus, born 400 B.C. Ptolemy, AD 140
saw objects that moved in sky
called planets, sun, and moon “wanderers”
Earth motionless and at the center of universe
Ptolemy, AD 140
Leading astronomer
“geocentric” theory – earth is center of universe. Lasted for 1400 years.

8. Aristotle (384 BC – 322 BC) Geocentric Theory
Proved that the earth is spherical
Believed that the earth is the center of the solar system and that everything revolves around it
Believed that all stars are fixed points which rotate on a single celestial sphere

9. 9

10. Aristotle believed there were only a few basic Substances
1. Earthly Realm – Air, Earth, Fire, and Water
2. Heavenly Realm – Quintessence (Not found on Earth)
Aristotle agreed that the Earth is spherical BUT didn’t believe that the earth moved.
1. No Rotation- NO sense of motion...no strong winds…..No displacement of thrown object.
2. No Revolution – No measureable stellar parallax, BUT the stars were too far away to measure stellar parallax back then. They needed better equipment.

11. The soul passing into quintessence

12. Stellar Parallax – Aristotle said that if the earth revolved, the closer stars should shift among the background of further stars. They actually do shift but not enough for ancient astronomers to detect. 12

13. Aristarchus ( B.C.)
1st to place the sun at the center of the universe, but his ideas were to radical for anyone to accept.
1750 years BEFORE Copernicus!
Eratosthenes ( B.C.)
1st person able to measure the circumference of the Earth
Hipparchus ( B.C.)
Compiled first star catalog
Developed a scale for star brightness
Accurately calculated distance from Earth to the moon!

14. Stonehenge

15. Stonehenge
Constructed 3000 – 1800 B.C. in Great Britain
Alignments with locations of sunset, sunrise, moonset and moonrise at summer and winter solstices
Probably used as calendar.

16. Other Examples around the World
Big Horn Medicine Wheel (Wyoming)

17. Other Examples around the World
Caracol (Mexico); Maya culture, approx. A.D. 1000

18. Cahokia – Native American Site Outside of St. Louis

19. Ancient people of central Africa (6500 B. C
Ancient people of central Africa (6500 B.C.) could predict seasons from the orientation of the crescent moon.
Here’s an example of the practical application of observations: Africans could determine where they were in the rainy season or dry season from observations of the crescent moon.

20. Egyptian obelisk: Shadows tell time of day.

21. England: Stonehenge (completed around 1550 B.C.)

22. Mexico: model of the Templo Mayor

23. New Mexico: Anasazi kiva aligned north–south

24. SW United States: “Sun Dagger” marks summer solstice

25. Scotland: 4,000-year-old stone circle; Moon rises as shown here every 18.6 years.

26. Peru: lines and patterns, some aligned with stars
Note: fun to discuss the claims that these had to have been made by “ancient astronauts”…
Peru: lines and patterns, some aligned with stars

27. Macchu Pichu, Peru: structures aligned with solstices

28. South Pacific: Polynesians were very skilled in the art of celestial navigation.

29. France: Cave paintings from 18,000 B. C
France: Cave paintings from 18,000 B.C. may suggest knowledge of lunar phases (29 dots).

30. Bone or tortoiseshell inscription from the 14th century B.C.
"On the Jisi day, the 7th day of the month, a big new star appeared in the company of the Ho star."
"On the Xinwei day the new star dwindled."
Stopped here 4/3/2018 3A; stopped here 5/4/2018 3B
Bone or tortoiseshell inscription from the 14th century B.C.
China: earliest known records of supernova explosions (1400 B.C.)

31. Ancient Greek Astronomers
Models were based on unproven “first principles”, believed to be “obvious” and were not questioned:
1. Geocentric “Universe”: The Earth is at the Center of the “Universe”.
2. “Perfect Heavens”: The motions of all celestial bodies can be described by motions involving objects of “perfect” shape, i.e., spheres or circles.

32. Arabs Caliph Harun-al-Rashid, ruler
Scholars translated Greek texts into Arabic and preserved in “House of Wisdom” library – in Baghdad
Measured positions of stars and planets with fine instruments, astrolabe, can perform calculations
Named the red giant Betelgeuse, in Orion
325 light years away

33. Betelgeuse – “shoulder”
Orion constellation

34. Ptolemy: Geocentric model, including epicycles
Central guiding principles:
1. Imperfect, changeable Earth,
2. Perfect Heavens (described by spheres)

35. What were the epicycles in Ptolemy’s model supposed to explain?
The fact that planets are moving against the background of the stars.
The fact that the sun is moving against the background of the stars.
The fact that planets are moving eastward for a short amount of time, while they are usually moving westward.
The fact that planets are moving westward for a short amount of time, while they are usually moving eastward.
The fact that planets seem to remain stationary for substantial amounts of time.

36. Introduced to explain retrograde (westward) motion of planets
Epicycles
Introduced to explain retrograde (westward) motion of planets
Stopped here 5/3/2018 1A; Stopped here 5/4/2018 1B
The ptolemaic system was considered the “standard model” of the Universe until the Copernican Revolution.

37. The Copernican Revolution
Nicolaus Copernicus (1473 – 1543):
Heliocentric Universe (Sun in the Center)

38. New (and correct) explanation for retrograde motion of the planets:
Retrograde (westward) motion of a planet occurs when the Earth passes the planet.
This made Ptolemy’s epicycles unnecessary.
Described in Copernicus’ famous book “De Revolutionibus Orbium Coelestium” (“About the revolutions of celestial objects”)

39. In the Copernikan “Universe”, the orbits of planets and moons were …
Perfect Circles
Ellipses
Spirals
Epicycles
None of the above.

40. What do you think?
Why do you think it was difficult for people to accept a heliocentric model (sun centered) over the geocentric model (Earth centered)?
Geocentric model
Heliocentric model

41. Denmark
Tycho Brahe – 1576
Collected data of position of planets for 20 years
Able to make accurate predictions of positions without telescopes
Had own “Tychonic Universe” – combination of Ptolemy and Copernicus
Earth is stationary
Believed in circular orbits

42. Tychonic Universe

43. Why the Circles?
Why do you think these astronomers believed in circular orbits?

44. Found a consistent description by abandoning both
Johannes Kepler (1571 – 1630)
Used the precise observational tables of Tycho Brahe (1546 – 1601) to study planetary motion mathematically.
Found a consistent description by abandoning both
Circular motion and
Uniform motion.
Planets move around the sun on elliptical paths, with non-uniform velocities.

45. Kepler’s Laws of Planetary Motion
The orbits of the planets are ellipses with the sun at one focus. c
Eccentricity e = c/a

46. Eccentricities of Ellipses1) 2) 3)
e = 0.02
e = 0.1
e = 0.2 5) 4)
e = 0.4
e = 0.6

47. Eccentricities of planetary orbits
Orbits of planets are virtually indistinguishable from circles:
Most extreme example:
Pluto: e = 0.248
Earth: e =

48. A line from a planet to the sun sweeps over equal areas in equal intervals of time.
Fast
Slow
Animation

49. Are all four seasons equally long?
Yes.
No, summer is the longest; winter is the shortest.
No, fall is the longest; spring is the shortest.
No, winter is the longest; summer is the shortest.
No, spring is the longest; fall is the shortest.

50. Why is the summer longer than winter?
Because of the precession of the Earth’s axis of rotation.
Because of the moon’s 5o inclination with respect to the Ecliptic.
Because the Earth is rotating around its axis more slowly in the summer (→ longer days!).
Because the Earth is closest to the sun in January and most distant from the sun in July.
Because the Earth is closest to the sun in July and most distant from the sun in January.

51. Autumnal Equinox (beg. of fall)
Summer solstice (beg. of summer)
July
Winter solstice (beg. of winter)
Fall
Summer
Winter
Spring
January
Vernal equinox (beg. of spring)

52. Think critically about Kepler’s Laws: Would you categorize his achievements as physics or mathematics?
Mathematics
Physics

53. Isaac Newton (1643 - 1727) Major achievements:
Adding physics interpretations to the mathematical descriptions of astronomy by Copernicus, Galileo and Kepler
Major achievements:
Invented Calculus as a necessary tool to solve mathematical problems related to motion
Discovered the three laws of motion
Discovered the universal law of mutual gravitation

54. Newton’s Laws of Motion (I)
Newton’s Laws of Motion (I)
A body continues at rest or in uniform motion in a straight line unless acted upon by some net force.
An astronaut floating in space will float forever in a straight line unless some external force is accelerating him/her.

55. Velocity and Acceleration
Acceleration (a) is the change of a body’s velocity (v) with time (t): a
a = Dv/Dt
Velocity and acceleration are directed quantities (vectors)! v

56. Which of the following involve(s) a (non-zero) acceleration?
Increasing the speed of an object.
Braking.
Uniform motion on a circular path.
All of the above.
None of the above

57. Velocity and Acceleration
Acceleration (a) is the change of a body’s velocity (v) with time (t): a
a = Dv/Dt
Velocity and acceleration are directed quantities (vectors)! v
Different cases of acceleration:
Acceleration in the conventional sense (i.e. increasing speed)
Deceleration (i.e. decreasing speed)
Change of the direction of motion (e.g., in circular motion)

58. 5) Impossible to tell from the given information.
A ball attached to a string is in a circular motion as shown. Which path will the ball follow if the string breaks at the marked point? 2) 1) 3) 4)
5) Impossible to tell from the given information.

59. Newton’s Laws of Motion (II)
The acceleration a of a body is inversely proportional to its mass m, directly proportional to the net force F, and in the same direction as the net force.
a = F/m  F = m a

60. Newton’s Laws of Motion (III)
Newton’s Laws of Motion (III)
To every action, there is an equal and opposite reaction.
The same force that is accelerating the boy forward, is accelerating the skateboard backward.

61. The Universal Law of Gravity
Any two bodies are attracting each other through gravitation, with a force proportional to the product of their masses and inversely proportional to the square of their distance: Mm
F = - G r2
(G is the Universal constant of gravity.)