# The Copernican Revolution

## Presentation on theme: "The Copernican Revolution"— Presentation transcript:

The Copernican Revolution
Scotland: 4,000-year-old stone circle; the Moon rises as shown here every 18.6 years.

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… Mexico: model of the Templo Mayor England: 1550 BC Our mathematical and scientific heritage originated with the civilizations of the Middle East Mexico: model of the Templo Mayor

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. This is a very important step in understanding the natural world. Greek geocentric model (c. 400 B.C.)

What shape is the Earth?

Prove it!

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

APPARENT RETROGRADE MOTION OF THE PLANETS
We see apparent retrograde motion when we pass by a planet in its orbit. Easy for us to explain today: 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 … We also recommend that you encourage students to try the apparent retrograde motion demonstration shown in the book in Figure 2.33a, since seeing it for themselves really helps remove the mystery…

Why did the ancient Greeks reject the real explanation for planetary motion?
Their inability to observe stellar parallax was a major factor. p If the angle p = 1 second of arc then d = AU. We define this distance to be 1 parsec (pc). Can you show that 1 pc = 3.26 light-years? tan p = 1AU/d (AU) For small angles: p = 1/d d Here p is in radians, but there are arcseconds in 1 radian. 1 AU Not to scale

Earth does not orbit the Sun; the Earth is the center of the universe
The Greeks knew that the lack of observable parallax could mean one of two things: Stars are so far away that stellar parallax is too small to notice with the naked eye The Moon is 30 minutes of arc and the eye can separate objects about 1 minute of arc apart Earth does not orbit the Sun; the Earth is the center of the universe With rare exceptions such as Aristarchus, the Greeks rejected the correct explanation (1) because they did not think the stars could be that far away Thus setting the stage for the long, historical showdown between Earth-centered and Sun-centered systems. In fact, the nearest stars have parallax angles less than 1 arcsecond, far below what the naked eye can see. Indeed, we CAN detect parallax today, offering direct proof that Earth really does go around the Sun…

Explaining retrograde motion with the Geocentric model
Arabic translation of Ptolemy’s work named Almagest (“the greatest compilation”) 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. Ptolemy Most sophisticated geocentric model was that of Ptolemy (A.D ). Sufficiently accurate to remain in use for 1,500 years.

The Copernican Revolution
Our goals for learning: How did Copernicus, Tycho, and Kepler challenge the Earth-centered (geocentric) idea? What are Kepler’s three laws of planetary motion? How did Galileo solidify the Copernican revolution?

How did Copernicus, 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) 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. But . . . He had the right idea, yet Copernicus still assumed that the orbits of the planets were perfect circles and so his heliocentric model was no more accurate than Ptolemy’s geocentric model in predicting planetary positions!

Danish astronomer Tycho Brahe (1546-1601)
Compiled the most accurate (one arcminute) naked eye measurements ever made of planetary positions. He still could not detect stellar parallax, and thus still thought Earth must be at center of solar system (but recognized that other planets must go around the Sun) Hired Johannes Kepler, who later used these detailed 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… Brahe’s observatory was visual – before the telescope – and really a giant protractor for measuring angles.

Johannes Kepler ( ) 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 hour hypotheses…

What is an ellipse? An ellipse looks like an elongated circle
Use this slide to review ellipses and the definition of eccentricity. The semi-major axis is also the “average” distance from the focus. An ellipse looks like an elongated circle

Eccentricity and Planetary Orbits
Define c to be the distance from the center to either focus Define a to be the semi-major axis length The eccentricity e of an ellipse is then e = c/a [ For a circle e = 0 because c = 0] Perihelion distance = a(1 – e) Aphelion distance = a(1 + e) a c Earth: e = 0.017; a = million km; closest = million km and farthest = million km. Note that a is the average.

Eccentricity of an Ellipse
You can also use this tool to explain eccentricity (from the tutorial on Orbits and Kepler’s Laws).

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. NOTE: None of the planets have orbits as elliptical as this, only some comets.

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

You can use this tool to demonstrate Kepler’s first two laws.

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 If a = 1 AU then 13=1 and p2=1 therefore p=1 (year).

Graphical version of Kepler’s Third Law
p2 plotted against a3 v plotted against a 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). Planet’s average speed is v = 2πa/p (circumference of orbit divided by period), so v2 = (2π)2 a2/p2 but p2 = a3 so, v2 = (2π)2 a2/a3 = (2π)2/a, thus v = 2π/√a. Using v in km/s then v = 30/√a.

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

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 p = 8, p2 = 82 = 64

How did Galileo Galilei solidify the Copernican revolution?
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.

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 by experiment that an object will stay in motion unless a force acts to slow it down. Galileo proved this with countless experiments involving falling and rolling objects.

Overcoming the second objection (heavenly perfection):
Tycho’s observations of a comet and a supernova (new star) already challenged this idea. Using his telescope (1609), Galileo saw: Sunspots or “imperfections” on the Sun Mountains and valleys on the Moon (proving it is not a perfect sphere)

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 resolve the Milky Way into countless individual stars. If stars were much farther away, then lack of detectable parallax was no longer so troubling. The eventual discovery of the tiny angular shifts in a star’s position due to parallax as the Earth orbits the Sun required more than another century of technology development to make much bigger telescopes. 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.

Galileo’s observations of phases of Venus proved that it orbits the Sun and not Earth.
Galileo saw four moons orbiting Jupiter, proving that not all objects orbit the Earth

The Nature of Science Our goals for learning:
How can we distinguish science from nonscience? What is a scientific theory? The previous example of the evolution of ideas from the false geocentric model to the true heliocentric model leads us nicely to a discussion of the Scientific Method.

The idealized Scientific Method
Based on proposing and testing hypotheses through careful and repeated experiments hypothesis = educated guess Around the time of Galileo there was a fundamental shift away from mere speculation and towards tangible experimental evidence. You may wish to go through the flashlight example that appears in the text.

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. Example: Kepler sought a natural explanation for observations made by Tycho.

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… Occam’s principle states that “all things being equal, the simplest explanation tends to be the right one.” The heliocentric model is simpler than the geocentric one.

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… Example: each of the competing models offered predictions that were tested. Kepler’s model worked best. Kepler’s model can still be tested. In fact, slight discrepancies found at much later dates led to new discoveries, such as Einstein’s theories…

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, it is NOT a guess, 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.

Scientific Theories The theory of gravity
The theory of electromagnetism and light The theory of atoms The theory of evolution by natural selection The theory of relativity The theory of plate tectonics The quantum theory

Other modes of reasoning
Appeal to authority – weak because does not rely on experimental evidence and advance predictions, even if the authority figure is a famous scientist. Opposing advocates (the legal system) – good in principle if both sides present all the evidence and argue rationally for the truth. Fails in practice because each advocate presents only those facts that support their position. Myths, superstitions and divine intervention – weakest of all because there is no rational way to seek truth when explanations are attributed to unseen and unknowable forces. Today, after millennia of study, no belief systems based on superstitions stand the test of experiment. The Scientific Method, especially in modern times where many scientists of equal skill compete to verify facts by experiment, is the most powerful method of obtaining new knowledge.

What have we learned? How can we distinguish science from non-science?
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