Jupiter Impact! Monday Sept. 10, 2012 at 11:35 UT Possible asteroid or comet Frame from a video recording made in Dallas, Texas by amateur astronomer

Transcription

1 Jupiter Impact! Monday Sept. 10, 2012 at 11:35 UT Possible asteroid or comet Frame from a video recording made in Dallas, Texas by amateur astronomer George Hall Read story in Space weather web site:

2 The Copernican revolution Chapters 2 and 3

3 The Ancient Mystery of the Planets Chapter 2, section 2.4 Topics we will explore: What was once so mysterious about planetary motion in our sky? Why did the ancient Greeks reject the real explanation for planetary motion?

4 3.2 Ancient Greek Science More topics to explore: Why does modern science trace its roots to the Greeks? How did the Greeks explain planetary motion? How was Greek knowledge preserved through history?

5 Planets Known in Ancient Times Mercury difficult to see; always close to Sun in sky Venus very bright when visible; morning or evening star. Mars noticeably red Jupiter very bright, Saturn moderately bright Picture of a grouping of five planets in the evening sky on April 23, Their positions trace a portion of the ecliptic. This is called in the media planetary alignment

6 What was once so mysterious about planetary motion in our sky? Planets usually move slightly eastward from night to night relative to the stars ( wanderers in the sky). But sometimes they go westward relative to the stars for a few weeks. This is called apparent retrograde motion. A composite of 29 individual pictures of Mars taken between June and November 2003 shows the apparent retrograde motion. Notice that Mars is brighter around August 27 when it is closer to Earth. Also note that the series of small dots to the right of the center is the planet Uranus which happens to be in that part of the sky

7 How did the Greeks explained the retrograde motion? According to the Ptolemaic model (based on Ptolemy A.D. model ) during the retrograde motion, the planets really go backward.

8 How the retrograde motion can be explained in the heliocentric model? We see apparent retrograde motion when the Earth passes by a planet such as Mars in its orbit.

9 Explaining Apparent Retrograde Motion Easy for us to explain: occurs when we lap another planet (or when Mercury or Venus laps us). But very difficult to explain if you think that Earth is the center of the universe! In fact, ancients considered but rejected the correct explanation.

10 Parallax concept

11 Why did the ancient Greeks reject the real explanation for planetary motion? Their inability to observe stellar parallax was a major factor. If the Earth was in orbit around the Sun we should see nearby stars changing position when the Earth move in its orbit

12 The Greeks knew that the lack of observable parallax could mean one of two things: 1. Stars are so far away that stellar parallax is too small to notice with the naked eye. 2. Earth does not orbit the Sun; it is the center of the universe. With rare exceptions such as Aristarchus ( B.C.), the Greeks rejected the correct explanation (1) because they did not think the stars could be that far away. Aristarchus is credited to be the first to suggest that the Earth goes around the Sun Thus, the stage was set for the long, historical showdown between Earthcentered and Sun-centered systems.

13 What have we learned? What was so mysterious about planetary motion in our sky? Like the Sun and Moon, planets usually drift eastward relative to the stars from night to night, but sometimes, for a few weeks or few months, a planet turns westward in its apparent retrograde motion. Why did the ancient Greeks reject the real explanation for planetary motion? Most Greeks concluded that Earth must be stationary, because they thought the stars could not be so far away as to make parallax undetectable.

14 Ancient Greek Science Chapter 3, section 3.2 Geocentric model: the Earth is the center of the solar system (and the universe). Heliocentric model: The Sun is the center of the solar system

15 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.). The Earth at the center of a series of nested spheres that contain the planets. The outermost sphere hold the stars.

16 Special Topic: Eratosthenes Measures Earth (c. 240 B.C.) He was able to measure the circumference of the Earth Measurements: Syene to Alexandria distance 5000 stadia At noon: Sun at Syene at zenith Sun at Alexandria ~angle = 7 Alexandria Syene Calculate circumference of Earth: 7/360 (circum. Earth) = 5000 stadia circum. Earth = /7 stadia 250,000 stadia Compare to modern value ( 40,100 km): Greek stadium 1/6 km 250,000 stadia 42,000 km

17 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. Plato Aristotle

18 The most sophisticated geocentric model was that of Ptolemy (A.D ) the Ptolemaic model: Sufficiently accurate to remain in use for 1,500 years, until the 1600 s when the heliocentric model was introduced. Ptolemy Arabic translation of Ptolemy s work named Almagest ( the greatest compilation )

19 Ptolemaic Universe Useful for predicting the positions of planets in the sky, but ultimately wrong. The large circle, called deferent is the path of a planet in its orbit around the Earth The small circle is called epicycle. It was necessary to introduce it to explain the retrograde motion

20 What have we learned? How was Greek knowledge preserved through history? While Europe was in its Dark Ages, Islamic scientists preserved and extended Greek science, later helping to ignite the European Renaissance

21 The Copernican Revolution Chapter 3, section 3.3 Some of the topic we will explore are: 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?

22 How did Copernicus, Tycho, and Kepler challenge the Earth-centered model? Copernicus ( ) Copernicus: Proposed a Sun-centered model (published 1543). This idea was proposed by Aristarchus about 1700 years earlier. Used model to determine layout of 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.

23 Tycho Brahe: Compiled the most accurate (accurate to 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). Tycho Brahe ( ) He hired Kepler, who used Tycho s observations to discover the truth about planetary motion. Kepler was a mathematician, not an observational astronomer.

24 Kepler first tried to match Tycho s observations with circular orbits But an 8-arcminute discrepancy led him eventually to propose elliptical orbits. The discrepancy he found was about ¼ the diameter of the moon. (Remember that the Moon has a diameter about 30 arcminutes or ½ of a degrees.) He proposed his three laws of planetary motions, now knows as Kepler s laws Johannes Kepler ( ) 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.

25 What is an ellipse? An ellipse looks like an elongated circle.

26 Important parameters in the ellipse For a circle, the position of the focus coincide with the center and the distance c is zero and the eccentricity is zero

27 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.

28 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. Perihelion: minimum distance from the sun Aphelion: maximum distance from the Sun

29 Kepler s Second Law Planets sweep out equal areas in equal intervals of time. They move fastest at perihelion and slowest at aphelion. area A = area B = area C

30 Kepler s Third Law The square of a planet's orbital period is proportional to the cube of its semi-major axis. p = orbital period in years a = avg. distance from Sun in AU p 2 = a 3 Important: The period needs to be in years and the distance in AU Kepler s third law predicts that more distant planets (larger a) orbit the Sun at slower average speeds (longer p) His laws of planetary motions are empirical (Based on fitting this equation to the existing data)

31 Graphical version of Kepler s Third Law

32 Question An asteroid orbits the Sun at an average distance a = 4 AU. How long does it take to orbit the Sun? A. 4 years B. 8 years C. 16 years D. 64 years Hint: Remember that p 2 = a 3

33 Question An asteroid orbits the Sun at an average distance a = 4 AU. How long does it take to orbit the Sun? A. 4 years B. 8 years C. 16 years D. 64 years We need to find p so that p 2 = a 3. Since a = 4, a 3 = 4 3 = 4x4x4 = 64. Therefore, p 2 = 64 = 8 2 Then p= 8.

34 Galileo ( ) began constructing and using a telescope for astronomical observations around He did not invented or patented the telescope. Hans Lippershey patented the telescope in His telescope was very simple and by today standards very rudimentary. But he used an instrument that then it was a new device (But now is can be considered a toy) and was able to discover sunspots, lunar craters and mountains, the phases of Venus and the presence of many stars in the milky way. He turned the telescope into a scientific instrument. Galileo telescope

35 How did Galileo solidify the Copernican revolution? Galileo overcame major objections to the Copernican view. Three key objections rooted in Aristotelian view were: 1. Earth could not be moving because objects in air would be left behind. 2. Non-circular orbits are not perfect as heavens should be. 3. If Earth were really orbiting Sun, we d detect stellar parallax.

36 Overcoming the first objection (nature of motion): Galileo s experiments showed that objects in air would stay with Earth as it moves. 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 ( This became later Newton s first law of motion).

37 Overcoming the second objection (heavenly perfection): Tycho s observations of comet and supernova already challenged this idea by showing that the heavens could change Using his telescope, Galileo saw: Sunspots on the Sun (The Sun is not perfect, it has imperfections ) Mountains and valleys on the Moon (proving it is not a perfect sphere)

38 Overcoming the third objection (parallax): Tycho thought that his naked eye observations were precise enough to detect stellar parallax. Since he didn t detect parallax, lack of parallax seemed to rule out an orbiting Earth. The fact is that his observations were good to a few arc minutes but stellar parallax are smaller, around a few arc seconds. Galileo showed that the stars must be much farther than Tycho thought in part by using his telescope to see the Milky Way and be able to resolve into countless individual stars. If stars were much farther away, then lack of detectable parallax was no longer so troubling.

39 Galileo discovered two even more important facts Galileo also saw four moons orbiting Jupiter, proving that not all objects orbit the Earth. The figure shows Galileo s records of Jupiter (~1610) and the position of its four brightest satellites (now called Galilean satellites or moons). Their names are: Io, Europa, Ganymede and Callisto

40 Galileo s observations of the phases of Venus proved that Venus could not be in orbit around the Earth. His observations were consistent with Venus orbiting the Sun and not the Earth.

41 The Catholic Church ordered Galileo to recant his claim that Earth orbits the Sun in At that time he was around 70 years old so he did as he was ordered. His book on the subject was removed from the Church s index of banned books in And finally in 1835 all books on the heliocentric model were finally removed from the Index of prohibited books Galileo was formally vindicated by the Church in 1992.

42 Galileo used the Scientific Method when studying objects in the sky. Observation Explanation Prediction

43 Relative positions of a planet respect to the Sun and Earth

44 A summary of what we learned? How did Copernicus, Tycho and Kepler challenge the Earth-centered idea? 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? 1. The orbit of each planet is an ellipse with the Sun at one focus. 2. As a planet moves around its orbit it sweeps out equal areas in equal times. 3. Planets orbit the Sun following the equation: p 2 = a 3

45 A summary of what we learned? How did Galileo contributed to the Sun-centered model? Galileo provided for the first time the observational evidence to support the Sun-centered model. Before the invention of the telescope there was no way to learn what Galileo discovered. The facts provided by the observations and supported by Kepler equations provided support to the heliocentric model and allowed to discard the geocentric model.

46 What have we learned? What was Galileo s role in solidifying the Copernican revolution? His experiments and observations overcame the remaining objections to the Sun-centered solar system model.

47 What have we learned? In the geocentric model the order of the bodies in the solar system are (in increasing distances): Earth, Moon, Mercury, Venus, Sun, Mars, Jupiter and Saturn Geocentric model In the heliocentric model the order of the bodies in the solar system (in increasing distance) are: Sun, Mercury, Venus, Earth (Moon), Mars, Jupiter and Saturn.

48 1 st Law 2 nd Law 3 rd Law

49 The Nature of Science Chapter 2, section 3.4 Our goals for learning: How can we distinguish science from nonscience? What is a scientific theory?

50 How can we distinguish science from non-science? Defining science can be surprisingly difficult. Science from the Latin scientia, meaning knowledge. Science is a quest for knowledge and an understanding of the Universe and all that is within it Individual scientist learn from those that have proceeded them and their work guide those that follow them As Newton said: If I have seen further it is by standing on the shoulders of giants

51 The idealized scientific method Based on proposing and testing hypotheses hypothesis = educated guess

52 But science rarely proceeds in this idealized way. For example: Sometimes we start by just looking then we discovered something and we need to come up with possible explanations for what we observed Serendipitous discoveries. Many of the most important discoveries came after somebody was investigating a completely different phenomenon. One example is the discovery of the 2.7 K cosmic background emission. Penzias and Wilson were investigating a different kind of antenna. They found an additional noise that they could not account for. Sometimes we follow our intuition rather than a particular line of evidence. An example is Kepler following an intuition to find a way to make his heliocentric model works.

53 Hallmark of Science: #1 Modern science seeks explanations for observed phenomena that rely solely on natural causes. (A scientific model cannot include divine intervention)

54 Hallmark of Science: #2 Science progresses through the creation and testing of models of nature that explain the observations as simply as possible. An example is the transition from the geocentric model to the heliocentric model. It eliminate the epicycles (Simplicity = Occam s razor )

55 Hallmark 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.

56 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 (or a theory in everyday life), 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.

57 Can a scientific theory be improved? A scientific theory can be improved. For example Newton gravitational theory is still valid. But 300 years later Einstein relativity theory proved to be more general and applies to extreme cases where Newton theory fail. But trying to use relativity in some simple cases is similar to trying to cross the street using a jet airplane! Another example is Kepler s law of planetary motion. It is limited and applies to planets in orbit around the Sun. Newton laws are more general and can be applied to any two bodies in orbit around each other.

58 Question Darwin s theory of evolution meets all the criteria of a scientific theory. This means: A. Scientific opinion is about evenly split as to whether evolution really happened. B. Scientific opinion runs about 90% in favor of the theory of evolution and about 10% opposed. C. After more than 100 years of testing, Darwin s theory stands stronger than ever, having successfully met every scientific challenge to its validity. D. There is no longer any doubt that the theory of evolution is absolutely true.

59 Question Darwin s theory of evolution meets all the criteria of a scientific theory. This means: A. Scientific opinion is about evenly split as to whether evolution really happened. B. Scientific opinion runs about 90% in favor of the theory of evolution and about 10% opposed. C. After more than 100 years of testing, Darwin s theory stands stronger than ever, having successfully met every scientific challenge to its validity. D. There is no longer any doubt that the theory of evolution is absolutely true.

60 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

61 Astrology Chapter 3, section 3.5 Our goals for learning: How is astrology different from astronomy? Does astrology have any scientific validity?

62 How is astrology different from astronomy? Astronomy is a science focused on learning about how stars, planets, galaxies and other celestial objects work and evolve. Astronomy make use of physics principles, theories and math. It can make testable predictions. Astrology is a search for hidden influences on human lives based on the positions of the Sun, Moon and planets among the stars in the sky, mainly the Zodiac constellations.

63 Does astrology have any scientific validity? Scientific tests have shown that astrological predictions are no more accurate than we should expect from pure chance. Casting horoscopes by astronomers in the 1600 s was a way to survive! A horoscope by Kepler

64 How is astrology different from astronomy? Astronomy is the scientific study of the universe and the celestial objects within it. Astrology assumes that the positions of celestial objects influence human events. Does astrology have any scientific validity? Scientific tests show that the predictions of astrology are no more accurate than pure chance. Astrology offers only vague advice rather than testable predictions