The Science of Astronomy
3.1 The Ancient Roots of Science
In what ways do all humans employ scientific thinking?
- Scientific thinking is based on everyday ideas of observation and trial-and-error experiments.
- In keeping track of time and seasons
- For practical purposes, including agriculture
- For religious purposes
- In aiding navigation
- Ancient people of central Africa (6500 BC) could predict seasons from the orientation of the crescent moon: Dry seasons / wet seasons based on angle of moon
- Days of the week named for visible planets in English, French, and Spanish
- Daily time keeping
- Tracking the seasons and calendar
- Monitoring lunar cycles
- Monitoring planets and stars
- Predicting eclipses
- Egyptian Obelisk: Shadows tell time of day
- England: Stonehenge (completed around 1550 BC)
- Mexico: Templo Mayor
- New Mexico: Anasazi kiva aligned north-south
- Southwestern United States: "sun dagger" marks summer solstice
- Scotland: 4000 years old- stone circle; Moon rises here every 18.6 years
- Peru: Lines and patterns, some aligned with stars
- Machu Picchu: Structures aligned with solstices
- Southern Pacific: Polynesians were very skilled in the art of celestial navigation.
- France: Cave paintings from 18,000 BC may suggest knowledge of lunar phases (29 dots).
- China: Earliest known records of supernova explosions (1400 BC): Tortoiseshell inscription
3.2 Ancient Greek Science
Why does modern science trace its roots to the Greeks?
- Our mathematical / scientific heritage originates with civilizations of the Middle East.
- Greeks were the first people to know to make models of nature.
- They tried to explain patterns in nature without resorting to myth or the supernatural.
- The Greek Geocentric model (400 BC)
- Eratosthenes measures the Earth (240 BC)
- Underpinnings of the Greek Geocentric model:
- Earth at the center of the universe
- Heavens must be perfect: objects move on perfect circles
- Plato and Aristotle
- This Geocentric model made it difficult to explain the retrograde motion of planets.
- Over 10 weeks, Mars appeared to stop, back up, then go forward again.
- The most sophisticated Geocentric model was from Ptolemy:
- Sufficiently accurate to remain in use for 1500 years.
- Arabic translation of Ptolemy's work named Almagest (the greatest compilation)
- The planets nearly do go backward in this model.
- Question: Which of the following is not a fundamental difference between the Geocentric model and the Sun-centered model of our solar system?
- A: Earth is stationary in the Geocentric model but moves around the Sun in the Sun-centered model.
- B: Retrograde motion is real in the Geocentric model but only apparent in the Sun-centered model.
- C: Stellar parallax is expected in the Sun-centered model but not in the Earth-centered model.
- D: The Geocentric model is useless for predicting planetary positions,
whereas even the earliest Sun-centered models worked almost perfectly.
- The Muslim world enhanced the knowledge they received from the Greeks while Europe was in the Dark Ages.
- House of Wisdom in Baghdad was a great center of learning around 800 AD.
- With the fall of Constantinople in 1453, Eastern scholars headed to Europe bringing their knowledge which helped to ignite the European Renaissance.
3.3 The Copernican Revolution
How did Copernicus, Tycho, and Kepler challenge the Earth-centered idea?
- Copernicus (1473-1543)
- Proposed the Sun-centered model
- Used the model to determine the layout of the solar system (planetary distances in AU)
- The model was no more accurate than the Ptolemaic model in predicting planetary positions because it still used perfect circles.
- Tycho Brahe (1546-1601)
- Compiled the most accurate naked eye measurements ever made of planetary positions (1 arcminute)
- He could not detect stellar parallax, thus still thought Earth was the center of the solar system but recognized the other planets go around the Sun.
- He hired Kepler, who used Tycho's observations to discover the truth about planetary motion
- Johannes Kepler (1571-1603)
- First tried to match Tycho's observations with circular orbits.
- An 8 arcminute error led him to ellipses: "A complete reformation in astronomy"
- First law: The orbit of each planet around the Sun is an ellipse with the Sun at one focus.
- Second law: As a planet moves around its orbit, it sweeps out equal areas in equal times. A planet travels faster when it is nearer to the Sun and slower when it is farther away from the Sun.
- Third law: More distant planets orbit the Sun at slower average speeds, obeying the relationship: p^2 = a^3
- p = orbital period in years
- a = average distance from Sun in AU
- Question: An asteroid orbits the Sun at an average distance of 4 AU. How long doe sit take to orbit the Sun?
- A: 4 years
- B: 8 years
- C: 16 years
- D: 64 years
- He overcame major objections to the Copernican view. Three key objections rooted in the Aristotelian view:
- Earth could not be moving because objects in the air would be left behind.
- Non-circular orbits are not perfect as the heavens should be.
- If Earth were really orbiting the Sun, we'd detect stellar parallax.
- Overcoming the first objection (nature of motion)
- Galileo's experiments showed that objects in air would stay moving with Earth.
- Aristotle thought that all objects naturally came to rest.
- Galileo showed that objects will stay in motion unless a force acts to slow them down (Newton's first law of motion).
- 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, mountains and valleys on the Moon (imperfections)
- 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 further than Tycho thought - in part by using his telescope to see the Milky Way.
- If stars were much farther away, then lack of detectable parallax was no longer so troubling.
- Galileo also saw four moons orbiting Jupiter, proving that not all objects orbit Earth.
- His observations of phases of Venus proved that it orbits the Sun and not Earth.
- Galileo Galilei
- 1633: The Catholic Church ordered him to recant his claim that Earth orbits the Sun.
- His books were removed from the Church's index of banned books in 1824.
- 1992: He was formally vindicated by the Church.
3.4 The Nature of Science
How can we distinguish science from non-science?
- 'Science' comes from Latin 'scientia' meaning knowledge.
- The idealized scientific method - based on proposing and testing hypothesis
- Hallmarks of science #1: Modern science seeks explanations for observed phenomena that rely on natural causes. (A scientific model cannot include divine intervention.)
- #2: Science progresses through the creation and testing of models of nature that explain the observations as simply as possible.
- #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
- In science, a theory is not the same as a hypothesis.
- A scientific theory must:
- Explain a wide variety of observations with few simple principles
- Be supported be a large, compelling body of evidence
- Not fail any crucial tests of validity
- Question: Darwin's theory of evolution meets all of the criteria of a scientific theory. This means:
- A: Scientific opinion is about evenly split as to whether evolution really happened.
- B: Scientific opinions run about 90% in favor of the theory of evolution and about 10% is opposed.
- C: After more than 100 years of testing, Darwin's theory stands stronger than ever, having met every scientific challenge of validity.
- D: There is no longer any doubts about the validity of Darwin's theory.
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