1. The Copernican Revolution
Chapter 2
The Copernican Revolution
Chapter Opener. Caption: Astronomy came into its own as a viable and important academic subject during the 20th century. In this collage, clockwise from upper left, are four famous astronomers: Harlow Shapley (1885–1972) discovered our place in the “suburbs” of the Milky Way. Annie Cannon (1863–1941) carefully analyzed photographic plates to classify nearly a million stars over the course of fifty years. Karl Jansky (1905–1950), working at Bell Labs in New Jersey, discovered
radiation at radio wavelengths coming from the Milky Way. Edwin Hubble (1889–1953) discovered the expansion of the universe. (HARVARD OBSERVATORY; NRAO; CALTECH)

2. Units of Chapter 2 2.1 Ancient Astronomy 2.2 The Geocentric Universe
2.3 The Heliocentric Model of the Solar System
The Foundations of the Copernican Revolution
2.4 The Birth of Modern Astronomy

3. Units of Chapter 2, continued
2.5 The Laws of Planetary Motion
Some Properties of Planetary Orbits
2.6 The Dimensions of the Solar System
2.7 Newton’s Laws
The Moon is Falling!
2.8 Newtonian Mechanics
Weighing the Sun

4. 2.1 Ancient Astronomy Ancient civilizations observed the skies
Many built structures to mark astronomical events
Summer solstice sunrise at Stonehenge:
Figure 2-1. Caption: Stonehenge. This remarkable site in the south of England was probably constructed as a primitive calendar or almanac. The inset shows sunrise at Stonehenge at the summer solstice. As seen from the center of the stone circle, the Sun rose directly over the “heel stone” on the longest day of the year. (English Heritage)

5. 2.1 Ancient Astronomy
Spokes of the Big Horn Medicine Wheel are aligned with rising and setting of Sun and other stars
Figure 2-2a. Caption: Observatories in the Americas. (a) The Big Horn Medicine Wheel in Wyoming, built by the Plains Indians, has spokes and other features that roughly align with risings and settings of the Sun and other stars.

6. 2.1 Ancient Astronomy
This temple at Caracol, in Mexico, has many windows that are aligned with astronomical events
Figure 2-2b. Caption: Observatories in the Americas. (b) The Caracol temple in Mexico, built by the Mayan civilization, has some windows that seem to align with astronomical events, suggesting that at least part of Caracol’s function may have kept track of the seasons and the heavens.

7. 2.2 The Geocentric Universe
Ancient astronomers observed:
Sun
Moon
Stars
Five planets: Mercury, Venus, Mars, Jupiter, Saturn
Figure 2-3. Caption: Turkish Astronomers at Work. During the Dark Ages, much scientific information was preserved and new discoveries were made by astronomers in the Islamic world, as depicted in this illustration from a 16th-century manuscript. (The Granger Collection)

8. 2.2 The Geocentric Universe
Sun, Moon, and stars all have simple movements in the sky
Planets:
Move with respect to fixed stars
Change in brightness
Change speed
Undergo retrograde motion
Figure 2-4. Caption: Planetary Motion. Most of the time, planets move from west to east relative to the background stars. Occasionally—roughly once per year—however, they change direction and temporarily undergo retrograde motion (east to west) before looping back. The main illustration shows an actual retrograde loop in the motion of the planet Mars. The inset depicts the movements of several planets over the course of several years, as reproduced on the inside dome of a planetarium. The motion of the planets relative to the stars (represented as unmoving points) produces continuous streaks on the planetarium “sky.” (Boston Museum of Science)

9. 2.2 The Geocentric Universe
Inferior planets: Mercury, Venus
Superior planets: Mars, Jupiter, Saturn
Now know:
Inferior planets have orbits closer to Sun than Earth’s
Superior planets’ orbits are farther away
Figure 2-5. Caption: Inferior and Superior Orbits. Diagram of Earth’s orbit and two other possible planetary orbits. An “inferior” orbit lies between Earth’s orbit and the Sun. Mercury and Venus move in such orbits. A “superior” orbit (such as the orbit of Mars, Jupiter, or Saturn) lies outside that of Earth. The points noted on the orbits indicate times when a planet appears to come close to the Sun (conjunction) or is diametrically opposite the Sun on the celestial sphere (opposition); they are discussed further in the text. The inset is a schematic representation of the orbit of an inferior planet relative to the Sun, as seen from Earth.

10. 2.2 The Geocentric Universe
Early observations:
Inferior planets never too far from Sun
Superior planets not tied to Sun; exhibit retrograde motion
Superior planets brightest at opposition
Inferior planets brightest near inferior conjunction

11. 2.2 The Geocentric Universe
Earliest models had Earth at center of solar system
Needed lots of complications to accurately track planetary motions
Figure 2-7. Caption: Ptolemaic Model. The basic features, drawn roughly to scale, of Ptolemy’s geocentric model of the inner solar system, a model that enjoyed widespread popularity prior to the Renaissance. Only the five planets visible to the naked eye and hence known to the ancients—Mercury, Venus, Mars, Jupiter, and Saturn—are shown. The planets’ deferents were considered to move on spheres lying within the celestial sphere that held the stars. The celestial sphere carried all interior spheres around with it, but the planetary (and solar) spheres had additional motions of their own, causing the Sun and planets to move relative to the stars. To avoid confusion, partial paths (dashed) of only two planets—Venus and Jupiter—are drawn here.

12. The Geocentric Theory
The earth is located at the center of the universe and all the planets revolve around the earth.

13. The Geocentric Theory
The Geocentric theory was believed by the Catholic church especially because the church taught that God put earth as the center of the universe which made earth special and powerful.
The idea of the Earth actually moving was widely felt as a foolish suggestion because, as they saw it, if the Earth was moving they would be able to feel it.

14. Aristotle ( B.C.)
Developed an early model based on the concept of uniform circular motion. He placed the earth at the center of the universe and all of the planets, sun and stars around it.
When Aristotle lived, if a person could “reason” out why something happened, then you didn’t need to do any experiments to see what would happen.

15. In the realm of change, the natural motion of earthy materials was to seek the center of the universe.
This is why Aristotle placed the earth at the center of the cosmos. This is also his explanation for why objects fall when dropped. A dropped object is just following its natural tendency to seek the center of the universe.

16. Aristotle reasoned that if the earth rotated about its axis, we should fly off into space. Since we don't, the earth must be stationary.
It would be almost 1900 years before Galileo introduced the concepts of gravity and inertia that explain why these effects are not observed even though the earth does move.

17. Greek Contributions
The Ancient Greek philosophers refined astronomy, changing it from merely observational science into a full-blown theoretical science.

18. Greek Contributions
The ancient astronomers used astronomy to track time and cycles, for agricultural purposes, as well as adding astrology to their sophisticated observations.
Rather than generating astronomical theories, they were more concerned with what was happening and when, but without addressing the why, other than vague attributions to Gods and astrology.

19. Ptolemy (140 A.D.)
Ptolemy advanced the geocentric theory in a form that prevailed for 1400 years.
He added mathematics to support the theory

20. Ptolemy’s Problem
Many supporters of the geocentric theory had one piece of evidence they couldn’t explain – the movement of MARS.
Let’s take a break and investigate this movement.

21. Mars’ Motion

22. How did Ptolemy Explain this Problem?
Ptolemy used geometric models to predict the positions of the sun, moon, and planets, using combinations of circular motion known as epicycles.
An epicycle is an orbit within an orbit
Having set up this model, Ptolemy then went on to describe the mathematics which he needed in the rest of the work.

23. Ptolemy’s Model – Epicycles Included

24. 2.3 The Heliocentric Model of the Solar System
Sun is at center of solar system. Only Moon orbits around Earth; planets orbit around Sun.
This figure shows retrograde motion of Mars.
Figure 2-9. Caption: Retrograde Motion. The Copernican model of the solar system explains both the varying brightnesses of the planets and the phenomenon of retrograde motion. Here, for example, when Earth and Mars are relatively close to one another in their respective orbits (as at position 6), Mars seems brighter. When they are farther apart (as at position 1), Mars seems dimmer. Also, because the (light blue) line of sight from Earth to Mars changes as the two planets orbit the Sun, Mars appears to loop back and forth in retrograde motion. Follow the lines in numerical order, and note how the line of sight moves backward relative to the stars between locations 5 and 7. The line of sight changes because Earth, on the inside track, moves faster in its orbit than does Mars. The actual planetary orbits are shown as white curves. The apparent motion of Mars, as seen from Earth, is indicated by the red curve.

25. The Heliocentric Theory
The Sun is the center of our solar system

26. Copernicus ( )
Polish astronomer who advanced the theory that the Earth and other planets revolve around the Sun. This was highly controversial at the time.
The Ptolemaic model had been widely accepted in Europe for 1000 years when Copernicus proposed his model.

27. Nicolaus Copernicus
In 1543,Copernicus published On the Revolutions of the Heavenly Spheres which argued that the sun, rather than the Earth, stood at the center of the universe and that the planets revolved around the sun

28. Nicolaus Copernicus
Copernicus’s idea harmonized much better with observed data than did Ptolemy’s, but it was not warmly received
If Copernicus was right, then the Earth was just another planet and human beings were not the center of the universe
At the time, this had serious religious ramifications

29. The Church became so angry – the Geocentric theory made human beings seem closer to God and since earth was in the center that meant humans were more special. The heliocentric theory changed that perspective completely, making humans lose that position in the universe.

30. 2.4 The Birth of Modern Astronomy
Telescope invented around 1600
Galileo built his own, made observations:
Moon has mountains and valleys
Sun has sunspots, and rotates
Jupiter has moons (shown):
Venus has phases
Figure Caption: Galilean Moons. The four Galilean moons of Jupiter, as sketched by Galileo in his notebook. The sketches show what Galileo saw on seven nights between January 7 and 15, The orbits of the moons (sketched here as asterisks, and now called Io, Europa, Ganymede, and Callisto) around the planet (open circle) can clearly be seen. More of Galileo’s remarkable sketches of Saturn, star clusters, and the Orion constellation can be seen in the opener to Part 1 on page 1. (From Sidereus Nuncius)

31. Discovery 2-1: The Foundations of the Copernican Revolution
Earth is not at the center of everything.
Center of earth is the center of moon’s orbit.
All planets revolve around the Sun.
The stars are very much farther away than the Sun.
The apparent movement of the stars around the Earth is due to the Earth’s rotation.
The apparent movement of the Sun around the Earth is due to the Earth’s rotation.
Retrograde motion of planets is due to Earth’s motion around the Sun.

32. 2.4 The Birth of Modern Astronomy
Phases of Venus cannot be explained by geocentric model
Figure Caption: Venus Phases. Both the Ptolemaic and the Copernican models of the solar system predict that Venus should show phases as it moves in its orbit. (a) In the Copernican picture, when Venus is directly between Earth and the Sun, its unlit side faces us and the planet is invisible to us. As Venus moves in its orbit (at a faster speed than Earth moves in its orbit), progressively more of its illuminated face is visible from Earth. Note the connection between the orbital phase and the apparent size of the planet: Venus seems much larger in its crescent phase than when it is full because it is much closer to us during its crescent phase. This is the behavior actually observed. (The insets at bottom left and right are actual photographs of Venus taken at two of its crescent phases.) (b) The Ptolemaic model (see also Figure 2.7) is unable to account for these observations. In particular, the full phase of the planet cannot be explained. Seen from Earth, Venus reaches only a “fat crescent” phase, then begins to wane as it nears the Sun. (Both these views are from a sideways perspective; from overhead, both orbits are very nearly circular, as shown in Figure 2.19.) (Images from New Mexico State University)

33. 2.5 The Laws of Planetary Motion
Kepler’s laws were derived using observations made by Tycho Brahe
Figure Caption: Tycho Brahe. The astronomer in his observatory Uraniborg, on the island of Hveen in Denmark. Brahe’s observations of the positions of stars and planets on the sky were the most accurate and complete set of naked-eye measurements ever made. (Royal Ontario Museum)

34. Tycho Brahe

35. Tycho Brahe
Tycho Brahe created extensive and detailed observations of the positions of the planets, all with his naked eye!
At Uraniborg - part observatory, part palace - Brahe's quadrant that allowed him to measure the positions of planets and stars was 2 metres high and let him measure to an accuracy of 1/360th of a degree - the size of a baseball seen from a mile away.

36. Tycho Brahe
Later Brahe realised that the building wasn't stable enough, and he built a new part of his observatory underground, with an armillary sphere even larger than the quadrant - a huge structure of metal rings that let him measure the precise positions of objects in any part of the sky. (A quadrant would only allow measurements of things that were due south.)

37. Tycho Brahe
In 1572, Tycho Brahe observes a brilliant supernova (now called SN 1572). At the time, Brahe believed it to be a comet.
This supernova proves that it is traveling beyond Earth's atmosphere and therefore provides the first evidence that the heavens can change.
Brahe was the man!

38. How Kepler Got His Start…
Brahe's precise measurements laid the foundation for a new understanding of the motion of the planets.
German astronomer Johannes Kepler contacted Brahe at the end of the sixteenth century in an effort to obtain copies of the Danish astronomer's research. Brahe countered with a suggestion that Kepler could work as his assistant, helping him to compile his data.

39. How Kepler Got His Start…
However, Brahe proved more tightfisted than Kepler had anticipated and refused to share his measurements of the planets and their orbits.
Instead, he suggested Kepler work on solving the Mars dilemma that plagued astronomers.

40. How Kepler Got His Start…
Kepler, using Brahe's detailed observations, realized that the planets moved around the sun not in circles but in stretched out circles known as ellipses. However, the problem took him almost a decade to solve, and Kepler didn't publish it until well after Brahe's death.
Although Brahe's family intended to reap as much financial gain as possible from Brahe's observations, Kepler, by his own admission, less-than-ethically acquired them after Tycho died.

41. Johannes Kepler ( )
Johannes Kepler used mathematics to demonstrate that planetary orbits are elliptical, not circular as in the Ptolemaic theory

42. Kepler’s First Law
The orbits of the planets are ellipses, with the sun at one focus of the ellipse.

43. Kepler’s Second Law
The line joining the planet to the sun sweeps out equal areas in equal times as the planet travels around the ellipse.

44. Kepler’s Third Law
The ratio of the squares of the revolutionary periods for two planets is equal to the ratio of the cubes of their semimajor axes:

45. Kepler’s Third Law 61,500. 61,600. 248. 39.5 Pluto 27,200. 27,300.
165.
30.1
Neptune
7,060.
7,080.
84.0
19.2
Uranus
870.
868.
29.5
9.54
Saturn
142.
141.
11.9
5.20
Jupiter
3.53
3.51
1.88
1.52
Mars
1.00
Earth
0.378
0.615
0.723
Venus
0.058
0.241
0.387
Mercury
P**2
a**3
P (year)
a (AU)
Object

46. 2.5 The Laws of Planetary Motion
1. Planetary orbits are ellipses, Sun at one focus
Figure Caption: Ellipse. An ellipse can be drawn with the aid of a string, a pencil, and two thumbtacks. The wider the separation of the foci, the more elongated, or eccentric, is the ellipse. In the special case where the two foci are at the same place, the curve drawn is a circle.

47. 2.5 The Laws of Planetary Motion
2. Imaginary line connecting Sun and planet sweeps out equal areas in equal times
Figure Caption: Kepler’s Second Law. A line joining a planet to the Sun sweeps out equal areas in equal intervals of time. The three shaded areas A, B, and Care equal. Any object traveling along the elliptical path would take the same amount of time to cover the distance indicated by the three red arrows. Therefore, planets move faster when closer to the Sun.

48. 2.5 The Laws of Planetary Motion
3. Square of period of planet’s orbital motion is proportional to cube of semimajor axis

49. More Precisely 2-1: Some Properties of Planetary Orbits
Semimajor axis and eccentricity of orbit completely describe it
Perihelion: closest approach to Sun
Aphelion: farthest distance from Sun

50. 2.6 The Dimensions of the Solar System
Astronomical unit: mean distance from Earth to Sun
First measured during transits of Mercury and Venus, using triangulation
Figure Caption: Solar Transit. The transit of Mercury across the face of the Sun. Such transits happen only about once per decade because Mercury’s orbit does not quite coincide with the plane of the ecliptic. Transits of Venus are even rarer, occurring only about twice per century. The most recent took place in June (AURA)

51. 2.6 The Dimensions of the Solar System
Now measured using radar:
Ratio of mean radius of Venus’s orbit to that of Earth very well known
Figure Caption: Astronomical Unit. Simplified geometry of the orbits of Earth and Venus as they move around the Sun. The wavy blue lines represent the paths along which radar signals are transmitted toward Venus and received back at Earth at the particular moment (chosen for simplicity) when Venus is at its minimum distance from Earth. Because the radius of Earth’s orbit is 1 AU and that of Venus is about 0.7 AU, we know that this distance is 0.3 AU. Thus, radar measurements allow us to determine the astronomical unit in kilometers.

52. Galileo ( )
An Italian scientist, Galileo was renowned for his contributions to physics, astronomy, and scientific philosophy. He is regarded as the chief founder of modern science.
Galileo was condemned by the Catholic Church for his view of the cosmos based on the theory of Copernicus.

53. Galileo Galilei’s Telescope
Galileo used the telescope to observe spots on the sun and moon
Discredited the Ptolemaic notion that the heavenly bodies are smooth, immaculate, unchanging and perfectly spherical
Galileo’s drawing of the moon showing craters

54. Galileo Galilei Other achievements:
Noticed four of the moons that orbit Jupiter
Observed previously unknown distant stars
Meant universe is much larger than previously suspected
Showed that the velocity of falling bodies depends not on their weight but on the height from which they fall
Galileo’s telescope
However, Galileo was unable to clearly resolve Saturn’s rings!

55. Galileo’s Books
Galileo published his discoveries and support for the Copernican model in two books published in 1616 and 1632.
Galileo was unusual for the time because he wrote in Italian rather than Latin like most scholars.
Galileo also took great pains to make his books interesting often writing them in the form of dialogues rather than dry, boring dissertations.
After his first book, "Starry Messenger", was published he was warned by the Church not to publicly support Copernicism again.

56. Galileo Galilei
In 1632, he published his Dialogue Concerning the Two Chief World Systems which compares the Copernican and Ptolemaic systems
Found guilty of heresy by the Spanish Inquisition and spent the rest of his life under house arrest
Galileo’s Dialogue

57. Trial Before the Inquisition
Galileo abided by this edict until 1632 when he published "A Dialogue on the Two Chief World Systems". This book's outright support for the Copernican model and its ridiculing of the Ptolemaic model earned Galileo a trial before the Inquisition.
Galileo was accused of heresy and sentenced to house arrest for life. However, he got off easily compared to fellow Italian Giordano Bruno who was burned at the stake in 1600 for teaching Copernican ideas.

58. In 1992, the Roman Catholic Church finally repealed the ruling of the Inquisition against Galileo. The Church gave a pardon to Galileo and admitted that the heliocentric theory was correct. This pardon came 350 years after Galileo's death.