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Chapter 3 The Science of Astronomy

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1 Chapter 3 The Science of Astronomy

2 Early Astronomy All we know is what we read in the newspapers… and on the rocks , the scrolls, temples, and clay tablets… It is important to recognize parallel developments occurred in many parts of the world including Asia , India, and the Americas, as well as later Europe.

3 How did astronomical observations benefit ancient societies?
Keeping track of time and seasons for practical purposes, including agriculture for religious and ceremonial purposes Aid to navigation

4 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…

5 Bone or tortoise shell inscription from the 14th century BC.
"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 tortoise shell inscription from the 14th century BC. China: Earliest known records of supernova explosions (1400 B.C.)

6 North American North American Native Petroglyphs: Anasazi drawing on a ledge in Chaco Canyon, New Mexico is thought to represent the supernova of 1054 AD.

7 Plains Indians Pawnee Indian Sky Map A chart embossed on hide appears to depict constellations of the northern hemisphere. From “ When Stars come down to Earth: Cosmology of the Skidi Pawnee Indians of North America.

8 Babylonian Astronomy One of the earliest civilizations known to have written records developed along the Tigris and Euphrates rivers in what is now Iraq. King Hammurabi 1700-BC importance attributed to positions of the planets… astrology governed Babylon life. Detailed observations maintained over several centuries.

9 Babylonian Clock & Calendar
Babylonians Knew length of the year to accuracy of about 4 minutes. Divided year into 12 equal months of 30 days each. Babylonian number system based on 60. Angular system based on 360 degrees, 60 minutes to 1 degree, and 60 seconds to 1 minute. They had a 24 hour day they believed in symmetry of 12 hours each. 12 because of 12 lunar cycles per year. Later Chaldeans compiled records into tablets could predict solar, lunar and planetary positions As well as the occurrence of eclipses.

10 Egyptian obelisk: Shadows tell time of day.

11 Ancient people of central Africa (6500 BC) 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.

12 Days of week were named for Sun, Moon, and visible planets
And here’s an example that we still live with today… Days of week were named for Sun, Moon, and visible planets

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

14 England 1550 B.C.

15 Stonehenge Built in stages from 3000BC to 1800 BC late stone age into bronze age. One of many stone monuments found throughout Europe.



18 Mexico: model of the Templo Mayor
The sun rises between the temples on the equinoxes

19 Someone among the ancient Anasazi people carved a spiral known as the Sun Dagger on a vertical cliff face in New Mexico. There the Sun’s rays form a dagger of sunlight that pierces the center of a carved spiral only once each year - at noon on the summer solstice.

20 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

21 GREEK DEVELOPMENTS Greek culture started in Crete.. seafaring culture years ago. Source of the legends most of our constellations are named from.

22 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.)

23 Pythagoras (550 BC) Claimed that natural phenomena could be described by mathematics

24 Pythagoras (c BC) of Samos established a school teaching the idea that natural phenomenon can be described by numbers. This laid the foundation for modern trigonometry and geometry.   He is thought to have asserted the Earth is round and that all heavenly objects move in perfect circles. Anaxagoras ( BC) moon shines by reflected light, explains eclipses

25 Artist’s reconstruction of Library of Alexandria

26 Eratosthenes measures the Earth (c. 240 BC)
Measurements: Syene to Alexandria distance ≈ 5000 stadia angle = 7° Calculate circumference of Earth: 7/360  (circum. Earth) = 5000 stadia  circum. Earth = 5000  360/7 stadia ≈ 250,000 stadia This slide based on the special topic box to show Eratosthenes calculation. Compare to modern value (≈ 40,100 km): Greek stadium ≈ 1/6 km  250,000 stadia ≈ 42,000 km

27 Greek Mainland On mainland democratically ruled city-states arose, Athens, Sparta.5th century BC, Plato ( BC) teacher of Aristotle., what we see is an imperfect representation of a perfect creation. …we learn more by reason than observation. Dominated western thought for 2000 years.

28 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 moving on perfect spheres or in perfect circles.

29 Aristotle and Plato Aristotle was Plato’s most famous student.…first to adopt physical laws and use the laws to explain what we see.

30 Aristotle becomes Da’ Man
Aristotle wrote and taught on philosophy, history, politics, poetry, ethics, drama and science. He did this well. Because of his success his works became the great authority for the next 2000 years. Astronomers (indeed all scholars) cited his work as the authority. If a thought was in conflict with his works it must be wrong… Ideas: The Universe is divided into two parts. The earth corrupt and changeable ..and the heavens perfect and immutable. ( Notice the similarity with some theology.) Earth at the center of the universe. Geocentric. 56 crystalline spheres. Physics: Earth , water, air, fire. Seek natural order. Circular motion expected.

31 But this made it difficult to explain apparent retrograde motion of planets…
You may wish to review what we mean by apparent retrograde motion before showing the Greek explanation… Review: Over a period of 10 weeks, Mars appears to stop, back up, then go forward again.

32 The most sophisticated geocentric model was that of Ptolemy (A. D
The most sophisticated geocentric model was that of Ptolemy (A.D ) — the Ptolemaic model: Sufficiently accurate to remain in use for 1,500 years. Arabic translation of Ptolemy’s work named Almagest (“the greatest compilation”) Ptolemy

33 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.

34 Introduced by Ptolemy (ca. A.D. 140)
The Ptolemaic system was considered the “standard model” of the universe until the Copernican Revolution.

35 How was Greek knowledge preserved through history?
Muslim world preserved and enhanced the knowledge they received from the Greeks 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.

36 Copernicus, Galileo, Tycho, and Kepler challenge the Earth-centered idea
Proposed Sun-centered model (published 1543) Used model to determine layout of solar system (planetary distances in AU) But . . . Model was no more accurate than 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.

37 key objections rooted in Aristotelian view were:
Galileo ( ) overcame major objections to Copernican view. Three key objections rooted in Aristotelian view were: Earth could not be moving because objects in air would be left behind. Non-circular 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.

38 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).

39 Overcoming the second objection (heavenly perfection):
Tycho’s observations of comet and supernova already challenged this idea. Using his telescope, Galileo saw: Sunspots on Sun (“imperfections”) Mountains and valleys on the Moon (proving it is not a perfect sphere)

40 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 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.

41 Galileo also saw four moons orbiting Jupiter, proving that not all objects orbit the Earth

42 Galileo’s observations of phases of Venus proved that it orbits the Sun and not Earth.

43 GG3 In the Ptolemaic model Venus should always be a crescent. .


45 Galileo was formally vindicated by the Church in 1992
The Catholic Church ordered Galileo to recant his claim that Earth orbits the Sun in 1633 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

46 Tycho Brahe ( ) Compiled the most accurate (one arcminute) naked eye measurements ever made of planetary positions. Still could not detect stellar parallax, and thus still thought Earth must be at center of solar system (but recognized that other planets go around Sun) Hired Kepler, who used Tycho’s observations to discover the truth about planetary motion. Remind students that one arcminute is equivalent to the width of a fingernail at arm’s length…

47 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.” Johannes Kepler ( ) Kepler quote offers a good opportunity to talk about the nature of science, and how failure to match observations should force a change in hour hypotheses…

48 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.

49 An ellipse looks like an elongated circle
What is an ellipse? Use this slide to review ellipses and the definition of eccentricity. An ellipse looks like an elongated circle

50 Kepler’s Second Law: As a planet moves around its orbit, it sweeps out equal areas in equal times.
means that a planet travels faster when it is nearer to the Sun and slower when it is farther from the Sun.

51 Kepler’s Third Law p = orbital period in years
More distant planets orbit the Sun at slower average speeds, obeying the relationship p2 = a3 p = orbital period in years a = avg. distance from Sun in AU

52 Kepler’s Third Law

53 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).

54 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

55 An asteroid orbits the Sun at an average distance a = 4 AU
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 Since a = 4, a3 = 43 = 64 Therefore p2 = 82 = 64, p = 8

56 99 years of astronomy

57 List Of Greek Achievements

58 How can we distinguish science from non-science?
Defining science can be surprisingly difficult. Science from the Latin scientia, meaning “knowledge.” But not all knowledge comes from science…

59 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.

60 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.

61 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.

62 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 early 1600s, we 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…

63 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…

64 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, rather: A scientific theory must: Explain a wide variety of observations with a few simple principles, AND Must be supported by a large, compelling body of evidence. Must NOT have failed any crucial test of its validity.

65 Celestial Timekeeping and Navigation
Chapter S1 Celestial Timekeeping and Navigation

66 Much of this chapter has already been covered
Sidereal and solar day Sidereal and synodic month Leap years RA and dec Motion of the stars in the sky Motion of the sun through the sky Tropic of Cancer and Capricorn

67 Special Locations of Planets

68 Opposition: Planet is opposite of the sun in the sky.
Conjunction: Planet is in the same part of the sky as the sun. Inferior conjunction: inferior planet is between the earth and sun (transits can occur here) Superior conjunction: the sun is between the earth and planet. Greatest Elongation: Planet is farthest east or west from the sun in the sky.

69 Venus Transiting the Sun

70 Exam I Chapter 1 - Our Place in the Universe universe light year
observable universe Scale of the universe astronomical unit ecliptic vernal and autumnal equinox summer and winter solstice What causes seasons? precession

71 Chapter 2 - Discovering the Universe for Yourself
constellations & Asterisms celestial sphere north and south celestial poles celestial equator & ecliptic zenith altitude right ascension & declination Approximate angualr measurements latitude & longitude Celestial prime meridian circumpolar zodiac lunar phases lunar eclipses solar eclipses direct motion and retrograde motion parallax Reason for the seasons Solar vs sidereal day

72 Chapter 3 - The Science of Astronomy
Day Month Year How does Eratosthenes measure the Earth? Pythagoras Aristotle Ptolemy epicycle geocentric model Copernicus heliocentric model Tycho Brahe Kepler Kepler’s laws (including calculations) ellipse focus semimajor axis period eccentricity Galileo

73 S1- Celestial Timekeeping and Navigation
Sidereal and solar day Sidereal and synodic month Leap years RA and dec Motion of the stars in the sky Motion of the sun through the sky Tropic of Cancer and Capricorn Opposition Conjunction Greatest eastern and western elongation

74 Chapter 6 Telescopes Reflector vs Refractor
Types, advantages, disadvantage, optics involved. Telescope mounts Light Gathering Power (calculation) Resolution (Calculation) Magnification (Calculation) Maximum magnification (calculation) Adaptive optics Interferometry CCD imaging Atmospheric absorption of light Hubble

1. The point in the sky directly overhead. 2. The circle dividing the sky into eastern and western halves. 3. Locate the North Celestial Pole precisely in the Bryan sky. 4. Over what point on Earth is the North Celestial Pole? 5. The Celestial Equator is a circle on the sky that crosses the horizon at which two points? 6. How far from the zenith is the Celestial Equator when it crosses the Celestial Meridian for an observer in Bryan? 7. The yearly path of the Sun. 8. The points where the path of the Sun crosses the Celestial Equator. 9. The origin of the RA, DEC coordinate system. 10. The RA of Canopus ( Carini).

76 11. The RA of Spica ( Viginis).
12. Between Spica and Canopus the star farther east. 13. Compared to the terrestrial system of latitude and longitude, RA corresponds to___. 14. The DEC of Arcturus ( Boötis). 15. The DEC of Antares ( Scorpii). 16. Between Arcturus and Antares the star farther north. 17. Compared to the terrestrial system of latitude and longitude, DEC corresponds to___. 18. Name four points on the Celestial Meridian (these can be compass points, special positions on the celestial sphere, but not star names, coordinates, etc.) 19. The most northerly position of the Sun is called the ___ and occurs about ___ each year.

77 20. The coordinates of the Autumnal Equinox.
21. The DEC of the zenith. 22. The RA of the winter solstice. 23. The DEC of the North Celestial Pole. 24. The second brightest star in the constellation of Orion has the Bayer designation ___ and the common name ___. 25. Which constellation is the Sun in on the day of the Vernal Equinox?

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