1. Chapter 3: The Science of Astronomy
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2. 3.1 The Ancient Roots of Science
Our goals for learning:
In what ways do all humans use scientific thinking?
How is modern science rooted in ancient astronomy?
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3. In what ways do all humans employ scientific thinking?
Scientific thinking is based on everyday ideas of observation and trial-and-error experiments.
Emphasize that scientists don't "think different"—but we use formal procedures and principles that help organize our search for answers to scientific questions.
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4. How is modern science rooted in ancient astronomy?
Astronomy is the oldest of the sciences.
It was often practiced for practical reasons.
In keeping track of time and seasons
for practical purposes, including agriculture
for religious and ceremonial purposes
In aiding navigation
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5. 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.
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6. Days of the week were named for the Sun, Moon, and visible planets.
And here's an example that we still live with today…
Days of the week were named for the Sun, Moon, and visible planets.
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7. What did ancient civilizations achieve in astronomy?
Daily timekeeping
Tracking the seasons and calendar
Monitoring lunar cycles
Monitoring planets and stars
Predicting eclipses
And more…
Here we list a few examples of what ancient civilizations learned to do. The next several slides are a brief "slide show" of ancient structures.
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8. Egyptian obelisk: Shadows tell time of day.
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9. England: Stonehenge (completed around 1550 b.c.)
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10. Mexico: Model of the Templo Mayor
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11. New Mexico: Anasazi kiva aligned north–south
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12. SW United States: "Sun Dagger" marks summer solstice
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13. Scotland: 4000-year-old stone circle; Moon rises as shown here every 18.6 years.
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14. 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
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15. Machu Picchu, Peru: Structures aligned with solstices
Note: fun to discuss the claims that these had to have been made by "ancient astronauts"…
Machu Picchu, Peru: Structures aligned with solstices
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16. South Pacific: Polynesians were very skilled in the art of celestial navigation.
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17. This picture is not in the text, but very cool—possible evidence of astronomical observations by cave dwellers.
France: Cave paintings from 18,000 b.c. may suggest knowledge of lunar phases (29 dots).
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18. China: Earliest known records of supernova explosions (1400 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."
Another extra picture not in book…
Bone or tortoiseshell inscription from the 14th century b.c.
China: Earliest known records of supernova explosions (1400 b.c.)
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19. What have we learned?
In what ways do all humans employ scientific thinking?
Scientific thinking involves the same type of trial-and-error thinking that we use in our everyday lives, but in a carefully organized way.
How is modern science rooted in ancient astronomy?
The oldest of the sciences, as it has important practical applications for developing civilizations.
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20. 3.2 Ancient Greek Science Our goals for learning:
Why does modern science trace its roots to the Greeks?
How did the Greeks explain planetary motion?
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21. This map is not in the book, but may be useful for giving students some context on the locations of ancient civilizations and the way the Greek empire was able to be a crossroads at which the knowledge of many cultures was combined. Point out location of Alexandria for context on the library...
Our mathematical and scientific heritage originated with the civilizations of the Middle East.
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22. Artist's reconstruction of the Library of Alexandria
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23. Why does modern science trace its roots to the Greeks?
Greeks were the first people known to make models of nature.
They tried to explain patterns in nature without resorting to myth or the supernatural.
Greek geocentric model (c. 400 b.c.)
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24. Special Topic: Eratosthenes measures the Earth (c. 240 B.C.)
Measurements:
Syene to Alexandria
distance ≈ 5000 stadia
angle = 7°
Calculate circumference of Earth:
7/360 x (circum. Earth) = 5000 stadia
 circum. Earth = 5000 x 360/7 stadia ≈ 250,000 stadia
This slide is based on the Special Topic box showing Eratosthenes's calculation.
Compare to modern value (≈ 40,100 km):
Greek stadium ≈ 1/6 km  250,000 stadia ≈ 42,000 km
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25. How did the Greeks explain planetary motion?
Underpinnings of the Greek geocentric model:
Earth at the center of the universe
Heavens must be "perfect"—objects move on perfect spheres or in perfect circles.
Plato
Aristotle
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26. But this made it difficult to explain the apparent retrograde motion of planets…
Review: Over a period of 10 weeks, Mars appears to stop, back up, then go forward again.
You may wish to review what we mean by apparent retrograde motion before showing the Greek explanation…
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27. Sufficiently accurate to remain in use for 1500 years
The most sophisticated geocentric model was that of Ptolemy (a.d. 100–170)—the Ptolemaic model:
Sufficiently accurate to remain in use for 1500 years
Arabic translation of Ptolemy's work named Almagest ("the greatest compilation")
Ptolemy
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28. So how does the Ptolemaic model explain retrograde motion
So how does the Ptolemaic model explain retrograde motion? Planets really do go backward in this model.
Note that we describe "circle upon circle" motion, avoiding use of "epicycle" and "deferent" in order to keep jargon to a minimum. You might wish to discuss the complexity of this model to help set stage for Copernican revolution.
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29. Thought Question
Which of the following is NOT a fundamental difference between the geocentric and Sun-centered models of the solar system?
Earth is stationary in the geocentric model but moves around the Sun in Sun-centered model.
Retrograde motion is real (planets really go backward) in the geocentric model but only apparent (planets don't really turn around) in the Sun-centered model.
Stellar parallax is expected in the Sun-centered model but not in the Earth-centered model.
The geocentric model is useless for predicting planetary positions in the sky, whereas even the earliest Sun-centered models worked almost perfectly.
Optional thought question to check student understanding…
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30. Thought Question
Which of the following is NOT a fundamental difference between the geocentric and Sun-centered models of the solar system?
Earth is stationary in the geocentric model but moves around the Sun in Sun-centered model.
Retrograde motion is real (planets really go backward) in the geocentric model but only apparent (planets don't really turn around) in the Sun-centered model.
Stellar parallax is expected in the Sun-centered model but not in the Earth-centered model.
The geocentric model is useless for predicting planetary positions in the sky, whereas even the earliest Sun-centered models worked almost perfectly.
Optional thought question to check student understanding…
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31. Preserving the ideas of the Greeks
The Muslim world preserved and enhanced the knowledge they received from the Greeks while Europe was in its Dark Ages.
Al-Mamun's House of Wisdom in Baghdad was a great center of learning around A.D. 800.
With the fall of Constantinople (Istanbul) in 1453, Eastern scholars headed west to Europe, carrying knowledge that helped ignite the European Renaissance.
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32. What have we learned?
Why does modern science trace its roots to the Greeks?
They developed models of nature and emphasized that the predictions of models should agree with observations.
How did the Greeks explain planetary motion?
The Ptolemaic model had each planet move on a small circle whose center moves around Earth on a larger circle.
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33. 3.3 The Copernican Revolution
Our goals for learning:
How did Copernicus, Tycho, and Kepler challenge the Earth-centered model?
What are Kepler's three laws of planetary motion?
How did Galileo solidify the Copernican revolution?
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34. How did Copernicus, Tycho, and Kepler challenge the Earth-centered model?
Copernicus proposed the Sun-centered model (published 1543).
He used the model to determine the layout of the solar system (planetary distances in AU).
But . . .
The model was no more accurate than the Ptolemaic model in predicting planetary positions, because it still used perfect circles.
Model was also nearly as complex as the Ptolemaic model because he still used circles upon circles (epicycles) to try to get better matches to data.
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35. Tycho Brahe (1546–1601)
Brahe compiled the most accurate (1 arcminute) naked eye measurements ever made of planetary positions.
He still could not detect stellar parallax, and thus still thought Earth must be at the center of the solar system (but recognized that other planets go around the Sun).
He hired Kepler, who used Tycho's observations to discover the truth about planetary motion.
Remind students that 1 arcminute is equivalent to the width of a fingernail at arm's length…
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36. Kepler first tried to match Tycho's observations with circular orbits.
But an 8-arcminute discrepancy led him eventually to ellipses.
"If I had believed that we could ignore these eight minutes [of arc], I would have patched up my hypothesis accordingly. But, since it was not permissible to ignore, those eight minutes pointed the road to a complete reformation in astronomy."
Kepler quote offers a good opportunity to talk about the nature of science, and how failure to match observations should force a change in our hypotheses…
Johannes Kepler (1571–1630)
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37. An ellipse looks like an elongated circle.
What is an ellipse?
An ellipse looks like an elongated circle.
Use this slide to review ellipses and the definition of eccentricity.
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38. Eccentricity of an Ellipse
You can also use this tool to explain eccentricity (from the tutorial on Orbits and Kepler's Laws).
Eccentricity and Semimajor Axis of an Ellipse
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39. What are Kepler's three laws of planetary motion?
Kepler's First Law: The orbit of each planet around the Sun is an ellipse with the Sun at one focus.
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40. Kepler's Second Law: As a planet moves around its orbit, it sweeps out equal areas in equal times.
This means that a planet travels faster when it is nearer to the Sun and slower when it is farther from the Sun.
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41. You can use this tool to demonstrate Kepler's first two laws.
Kepler's 2nd Law
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42. Kepler's Third Law
More distant planets orbit the Sun at slower average speeds, obeying the relationship p2 = a3 p = orbital period in years a = average distance from Sun in AU
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43. Kepler's Third Law
Kepler's Third Law
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44. Graphical version of Kepler's third law
Use these graphs to show the meaning of the equation for Kepler's third law. Note: if your students are not too afraid of the math, show them why a planet's average speed is 2πa/p (circumference of orbit divided by orbital period), then substitute from Kepler's third law to show that speed is proportional to 1/√a so that they can understand the shape of the curve in (b).
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45. Thought Question
An asteroid orbits the Sun at an average distance a = 4 AU. How long does it take to orbit the Sun?
4 years
8 years
16 years
64 years
(Hint: Remember that p2 = a3.)
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46. Thought Question
An asteroid orbits the Sun at an average distance a = 4 AU. How long does it take to orbit the Sun?
4 years
8 years
16 years
64 years
We need to find p so that p2 = a3.
Because a = 4, a3 = 43 = 64.
Therefore, p = 8, p2 = 82 = 64.
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47. How did Galileo solidify the Copernican revolution?
Galileo (1564–1642) overcame major objections to the Copernican view. Three key objections rooted in the Aristotelian view were the following:
Earth could not be moving because objects in air would be left behind.
Noncircular orbits are not "perfect" as heavens should be.
If Earth were really orbiting Sun, we'd detect stellar parallax.
We think it is worth going over these three objections so that students can see how the scientific process works. E.g., the doubters were not being unreasonable, and it took evidence to overcome their doubts.
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48. Overcoming the first objection (nature of motion):
Galileo's experiments showed that objects in air would stay with a moving Earth.
Aristotle thought that all objects naturally come to rest.
Galileo showed that objects will stay in motion unless a force acts to slow them down (Newton's first law of motion).
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49. Overcoming the second objection (heavenly perfection):
Tycho's observations of comet and supernova already challenged this idea.
Using his telescope, Galileo saw:
Sunspots on the Sun ("imperfections")
Mountains and valleys on the Moon (proving it is not a perfect sphere)
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50. Overcoming the third objection (parallax):
Tycho thought he had measured stellar distances, so lack of parallax seemed to rule out an orbiting Earth.
Galileo showed stars must be much farther than Tycho thought—in part by using his telescope to see that the Milky Way is countless individual stars.
If stars were much farther away, then lack of detectable parallax was no longer so troubling.
Remind students that lack of detectable parallax has two possible explanations: (1) stars are so far away that we can't measure it or (2) Earth is center of universe. Greeks and Tycho had rejected (1), but Galileo offered evidence that it was in fact the correct explanation.
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51. Galileo also saw four moons orbiting Jupiter, proving that not all objects orbit Earth.
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52. Galileo's observations of phases of Venus proved that it orbits the Sun and not Earth.
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53. Galileo was formally vindicated by the Church in 1992.
In 1633 the Catholic Church ordered Galileo to recant his claim that Earth orbits the Sun.
His book on the subject was removed from the Church's index of banned books in 1824.
Galileo was formally vindicated by the Church in 1992.
The scientific case was essentially settled, but the story was more complex in his own time as politics intervened…
Galileo Galilei
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54. What have we learned?
How did Copernicus, Tycho, and Kepler challenge the Earth-centered model?
Copernicus created a Sun-centered model; Tycho provided the data needed to improve this model; Kepler found a model that fit Tycho's data.
What are Kepler's three laws of planetary motion?
The orbit of each planet is an ellipse with the Sun at one focus.
As a planet moves around its orbit it sweeps out equal areas in equal times.
More distant planets orbit the Sun at slower average speeds: p2 = a3.
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55. What have we learned?
How did Galileo solidify the Copernican revolution?
His experiments and observations overcame the remaining objections to the Sun-centered solar system.
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56. 3.4 The Nature of Science Our goals for learning:
How can we distinguish science from nonscience?
What is a scientific theory?
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57. How can we distinguish science from nonscience?
Defining science can be surprisingly difficult.
Science comes from the Latin scientia, meaning "knowledge."
But not all knowledge comes from science.
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58. The idealized scientific method:
Based on proposing and testing hypotheses
Hypothesis = educated guess
You may wish to go through the flashlight example that appears in the text.
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59. But science rarely proceeds in this idealized way. For example:
Sometimes we start by "just looking" then coming up with possible explanations.
Sometimes we follow our intuition rather than a particular line of evidence.
E.g., Copernicus and his early backers did NOT have a model that worked better than the Earth-centered model, but their intuition kept them working at it until Kepler finally found a way to make it work.
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60. Hallmarks of Science: #1
Modern science seeks explanations for observed phenomena that rely solely on natural causes.
(A scientific model cannot include divine intervention.)
Example: Kepler sought a natural explanation for observations made by Tycho.
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61. Hallmarks of Science: #2
Science progresses through the creation and testing of models of nature that explain the observations as simply as possible.
(Simplicity = "Occam's razor")
Example: By the early 1600s, there were several competing models of planetary motion, including those of Ptolemy, Copernicus, and Kepler. Kepler's gained acceptance because it worked the best. Also, in principle, we could make a geocentric model arbitrarily accurate with enough circles, but its lack of simplicity would still lead us to prefer Kepler's model…
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62. Hallmarks of Science: #3 A scientific model must make testable predictions about natural phenomena that would force us to revise or abandon the model if the predictions do not agree with observations.
Example: Each of the competing models offered predictions that were tested. Kepler's model can still be tested. In fact, slight discrepancies found at later dates led to new discoveries, such as Einstein's theories…
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63. What is a scientific theory?
The word theory has a different meaning in science than in everyday life.
In science, a theory is NOT the same as a hypothesis.
A scientific theory must:
Explain a wide variety of observations with a few simple principles
Be supported by a large, compelling body of evidence
NOT have failed any crucial test of its validity
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64. Thought Question
Darwin's theory of evolution meets all the criteria of a scientific theory. This means:
Scientific opinion is about evenly split as to whether evolution really happened.
Scientific opinion runs about 90% in favor of the theory of evolution and about 10% opposed.
After more than 100 years of testing, Darwin's theory stands stronger than ever, having successfully met every scientific challenge to its validity.
There is no longer any doubt that the theory of evolution is absolutely true.
Optional thought question to check student understanding…
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65. Thought Question
Darwin's theory of evolution meets all the criteria of a scientific theory. This means:
Scientific opinion is about evenly split as to whether evolution really happened.
Scientific opinion runs about 90% in favor of the theory of evolution and about 10% opposed.
After more than 100 years of testing, Darwin's theory stands stronger than ever, having successfully met every scientific challenge to its validity.
There is no longer any doubt that the theory of evolution is absolutely true.
Optional thought question to check student understanding…
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66. What have we learned? How can we distinguish science from nonscience?
Science: seeks explanations that rely solely on natural causes; progresses through the creation and testing of models of nature; models must make testable predictions
What is a scientific theory?
A model that explains a wide variety of observations in terms of a few general principles and that has survived repeated and varied testing
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