The Scientific Method

Transcription

1 The Scientific Method Objectives: 1. Outline the scientific method. 2. Explain why the scientific method has been more successful than other approaches to understanding the universe. 3. Distinguish between a law and a theory. 4. Discuss the role of a model in formulating a scientific theory. 5. Give the reason why Polaris is the heavenly body that remains most nearly stationary in the sky. 6. Define constellation. 7. Tell how to distinguish planets from stars by observations of the night sky made several weeks or months apart. 8. Compare how the ptolemaic and copernican systems account for the observed motions of the sun, moon, planets, and stars across the sky. 9. Explain the significance of Kepler's laws. 10. State why the copernican system is considered correct. 11. Define day and year. 12. Define fundamental force. 13. Explain why the earth is round but not a perfect sphere. 14. Explain the origin of tides. 15. Explain in terms of the scientific method why the discovery of Neptune was so important in confirming the law of gravity. 10/1/2010 ASTR 3001 Dep. of Physics & Electronics 1

2 1 1. The Scientific Method The scientific method consists of five steps: 1. Formulation of a problem a. Documentation b. Observation c. Questions 2. An initial scientific interpretation is called the hypothesis 3. Observation and experiment 4. Interpretation 5. Testing the interpretation 10/1/2010 ASTR 3001 Dep. of Physics & Electronics 2

3 A. law describes a relationship or regularity between' naturally occurring phenomena. B. A theory explains why a phenomenon or a set of phenomena occurs. C. Scientists often use models to simplify complex situations. Example: The celestial sphere to describe the Universe Newton chose an oval called an ellipse as a model of the earth's orbit. 10/1/2010 ASTR 3001 Dep. of Physics & Electronics 3

4 1 2. Why Science Is Successful A. A scientific law or theory, if refuted by contrary evidence, must be modified or discarded. A. The work of scientists is open to review, test, and change. B. Science has provided an understanding of the natural world and a sophisticated technology. C. Scientific laws and theories are not accepted as "absolute truth" and therefore differ from belief systems. Voyager 2 spaceship have not the predicted position by calculation using gravity theory, so scientific is looking for a new expression for corrections to the Newton Law. But another physical phenomena should be responsible for the discrepancies. 10/1/2010 ASTR 3001 Dep. of Physics & Electronics 4

5 1 3. A Survey of the Sky To an observer north of the equator, the position of the North Star, or Polaris, changes very little, and the whole nighttime sky appears to revolve around Polaris. The constellations are easily recognized groups of stars and are useful as labels for regions of the sky. Polaris 10/1/2010 ASTR 3001 Dep. of Physics & Electronics 5

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8 PLANETS 378 días 399 días 116 días 584 días 780 días The planets visible to the naked eye (Mercury, Venus, Mars, Jupiter, and Saturn) appear to drift in a generally eastward motion relative to the stars; however, each planet at times appears to head westward briefly, and its path across the sky resembles a series of loops. The phase period in days are shown below. 10/1/2010 ASTR 3001 Dep. of Physics & Electronics 8

9 1 4. The Ptolemaic System A. Ptolemy of Alexandria (2nd century A.D.) described the universe in the Almagest. 1. The earth is the center of the universe. 2. The sun, stars, and planets revolve around the earth. 3. The orbits of the planets are circular. B. According to Ptolemy, the planets as they orbit the earth travel in a series of loops (epicycles). C. The ptolemaic system had the components of a valid theory: 1. It was based on observation. 2. It apparently accounted for known celestial motions. 3. It made predictions that could be tested. 10/1/2010 ASTR 3001 Dep. of Physics & Electronics 9

10 1 5. The Copernican System A. The ptolemaic system failed to make accurate predictions of planetary positions. B. Nicholaus Copernicus ( ) developed anew theory of the universe: 1. The earth and planets follow circular orbits around the sun. 2. The earth rotates on its axis once every 24 hours. 3. The earth's rotation explains the daily rising and setting of celestial bodies. 4. Irregular movements of the planets area result of the combination of their motions around the sun and the change in position of the earth in its orbit. C. The copernican system was attacked by religious leaders and by other supporters of the ptolemaic system. 10/1/2010 ASTR 3001 Dep. of Physics & Electronics 10

11 Copernicus The hypothesis The Model

12 1.6 Kepler's Laws A. A. Johannes Kepler ( ), using Tycho Brahe's improved measurements of planetary motion, found fault with the copernican system. B. Kepler's calculations resulted in the discovery of three laws of planetary motion: 1. The paths of the planets around the sun are ellipses. 2. A planet moves so that its radius vector sweeps out equal areas in equal times. 3. Period and distance are related: 4 π P GM 3 2 C. Kepler's laws agreed with past observations of planetary positions and made accurate predictions of future planetary movements. a 10/1/2010 ASTR 3001 Dep. of Physics & Electronics 12

13 1.7 Why Copernicus was right? Tycho Brahe from a copy of an oil painting by Gemperlin (a painter hired by Tycho). The original was distroyed in a fire in 1859, but a copy by Jensen is preserved. This picture is from a card, 3.3 x 5.5 cm, that was massreproduced as a "cigarette card" during the nineteenth century, and with the original painting as model. Kepler visited Tycho because He had very precision positions of the planets Mars. Tycho was the first great optical observer. His primary contribution to astrophysics was the precise data he collected on the motions of the celestial bodies. 10/1/2010 ASTR 3001 Dep. of Physics & Electronics 13

14 His data were accurate to within 1/60 of one degree, i.e., to within 1 arc minute. (Comment A simple thing which Tycho did which greatly improved his accuracy was to make several measurements of quantities and to then average his results in order to arrive at his final answer. This is standard practice today, but was highly unusual during Tycho's time.) The most direct observational evidence for Earth's orbital motion is the apparent shift of nearby stars after six months, as the Earth moves from one side of its orbit to the other. Because of the large distance to even the nearest start, this parallax shift is too small to been seen without a telescope. 10/1/2010 ASTR 3001 Dep. of Physics & Electronics 14

15 However Copernicus Was Right because: A. There is direct evidence that the earth rotates and revolves around the sun. B. There is direct evidence for the motions of the moon and the other planets. The most direct evidence of daily rotation is via a Foucault pendulum, which swings in the same plane as the Earth rotates beneath it. At either pole, the swinging plane mirrors the Earth's 24 hour period. Some rotation is observed at all other locations on the Earth's surface as well, except for the equator. 10/1/2010 ASTR 3001 Dep. of Physics & Electronics 15

16 1.8 What Is Gravity? A. Isaac Newton's ( ) discovery of the law of gravity was dependent upon Copernicus's model of the solar system. B. Gravity is a fundamental force. 1. A fundamental force cannot be explained in terms of any other force. 2. There are four fundamental forces: a. Gravitational b. Electromagnetic c. Weak d. Strong C. Gravity is thought to be a universal force because: 1. Observed star systems and galaxies behave as if influenced by gravity. 2. Matter appears to be the same throughout the universe; therefore, gravitational attraction must also be universal. 3. There is no evidence that gravity is not universal. mm F G d /1/2010 ASTR 3001 Dep. of Physics & Electronics 16

17 1.9 Why the Earth Is Round? A. The theory of gravity accounts for the earth's shape; the earth is round because gravity squeezes it into a spherical shape. B. The earth is not a perfect sphere because its spinning motion causes it to bulge slightly at the equator and flatten slightly at the poles. The asteroid Ida is about 35 miles (55 kilometers) long. It is one of thousands of asteroids in the asteroid belt, a region between the orbits of Mars and Jupiter. Image credit: NASA Asteroids vary greatly in size. The largest and first known asteroid, Ceres, was discovered in It is 580 miles (933 kilometers) in diameter. Ceres is believed to contain about 1/3 the total mass of all the asteroids. One of the smallest, discovered in 1991 and named 1991 BA, is only about 20 feet (6 meters) across. 10/1/2010 ASTR 3001 Dep. of Physics & Electronics 17

18 1.10 The Tides A. The law of gravity successfully explains the occurrence of the tides. B. The earth's tides are the result of the gravitational attraction of the moon and the sun. C. Coastal areas experience two high tides and two low tides each day. D. The relative positions of the earth, sun and moon produce different tides. 1. Unusually high (and low) spring tides occur bimonthly when the moon and sun are aligned with the earth. 2. Weak neap tides occur bimonthly when the sun and moon pull at right angles to each other in regard to earth. 10/1/2010 ASTR 3001 Dep. of Physics & Electronics 18

19 1 11. The Discovery of Neptune A. Discrepancies in the predicted orbit of Uranus led to two hypotheses: 1. The law of gravity is wrong. 2. An unknown body is exerting a gravitational pull on Uranus. B. Calculations based on the law of gravity predicted the position of an unknown body. C. The prediction was tested, resulting in the discovery of Neptune. John Couch Adams ( ) John Couch Adams (5 June January 1892) was a British mathematician and astronomer. His most famous achievement was predicting the existence and position of Neptune, using only mathematics. The calculations were made to explain discrepancies with Uranus's orbit and the laws of Kepler and Newton. At the same time, but unknown to each other, the same calculations were made by Urbain Le Verrier. Le Verrier would assist Berlin Observatory astronomer Johann Gottfried Galle in locating the planet on 23 September 1846, which was found within 1 of its predicted location, a point in Aquarius. (There was, and to some extent still is, some controversy over the apportionment of credit for the discovery; see Discovery of Neptune.) 10/1/2010 ASTR 3001 Dep. of Physics & Electronics 19

20 Supposedly, Adams communicated his work to James Challis, director of the Cambridge Observatory, in mid September 1845 but there is some controversy as to how. On 21 October 1845, Adams, returning from a Cornwall vacation, without appointment, twice called on Astronomer Royal George Biddell Airy in Greenwich. Failing to find him at home, Adams reputedly left a manuscript of his solution, again without the detailed calculations. Airy responded with a letter to Adams asking for some clarification. It appears that Adams did not regard the question as "trivial", as is often alleged, but he failed to complete a response. Various theories have been discussed as to Adams's failure to reply, such as his general nervousness, procrastination and disorganization. Adams learned of the irregularities while still an undergraduate and became convinced of the "perturbation" hypothesis. Adams believed, in the face of anything that had been attempted before, that he could use the observed data on Uranus, and utilizing nothing more than Newton's law of gravitation, deduce the mass, position and orbit of the perturbing body. 10/1/2010 ASTR 3001 Dep. of Physics & Electronics 20

21 Meanwhile, Urbain Le Verrier, on November 10, 1845, presented to the Académie des sciences in Paris a memoir on Uranus, showing that the pre existing theory failed to account for its motion. Unaware of Adams's work, he attempted a similar investigation, and on June 1, 1846, in a second memoir, gave the position, but not the mass or orbit, of the proposed perturbing body. Le Verrier located Neptune within one degree of its predicted position. Urbain Le Verrier ( ) On September 23 (1846) Le Verrier asked to Johann Gottfried Galle to look at a certain region of sky to find a predicted new planet, which would explain the perturbations of the planet Uranus. Johann Gottfried Galle ( ) 10/1/2010 ASTR 3001 Dep. of Physics & Electronics 21

22 Relative positions calculated by Adams and Leverrier, respect to the actual position. Neptuno Planet 10/1/2010 ASTR 3001 Dep. of Physics & Electronics 22

23 In fact Neptune could have been discovered without the mathematical arguments. It seen by Galileo, the first person who could possibly have discovered a new planet. Galileo turned his telescope on the planets and was immediately fascinated by the system of Jupiter and its moons which he observed. While he was observing the Jupiter system on 28 December 1612 he recorded Neptune as an 8th magnitude star. Just over a year later the same alignment of Neptune and Jupiter occurred and Galileo, on 27 January 1613, recorded two stars in his field of view. One was Neptune and, remarkably, Galileo observed it again the following night when he noted that the two stars appeared to be further apart. How close he was at that point to discovering that one of the stars was the planet Neptune. Neptune was to be recorded several more times, without being recognized as a planet, over the following years. Galileo s record, 1612 Neptune is the only planet in the Solar System whose existence was mathematically predicted before it was directly observed. By 1846, the planet Uranus had completed nearly one full orbit since its discovery by William Herschel in 1781, and astronomers had detected a series of irregularities in its path which could not be entirely explained by Newton's law of gravitation. 10/1/2010 ASTR 3001 Dep. of Physics & Electronics 23