Chapter 3 The Science of Astronomy

Astronomy of ancient civilizations Monitoring lunar cycles Daily timekeeping Tracking the seasons and calendar 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…

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

Ancient people of central Africa (6500 BC) could predict seasons from the orientation of the crescent moon. This helped with planting and harvesting. 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.

Egyptian obelisk: Shadows tell time of day.

England: Stonehenge (completed around 1550 B. C England: Stonehenge (completed around 1550 B.C.) Sun shines on heel stone at summer solstice.

England: Stonehenge (1550 B.C.) Optional slide (not in book): shows some of the markers in Stonehenge. England: Stonehenge (1550 B.C.)

Templo Mayor: Mexico City—The Aztecs The sun's rays shine between the shrines of Tlaloc and Hutzilopochtli atop the Templo Mayor into Temple of Quetzalcoatl, at sunrise on March 21, vernal equinox.

At the House of the Great Anasazi Kiva in New Mexico, during the time of the summer solstice, the rising sun's first light beams through the door of the altar room which creates a bright door-shaped rectangle on the west wall. A Sun Priest* behind the altar would have been illuminated by the sunrise of the summer solstice.

Chaco Canyon: “Sun Dagger” marks summer solstice.

Peru: Lines and patterns, some aligned with stars. Note: fun to discuss the claims that these had to have been made by “ancient astronauts”… Peru: Lines and patterns, some aligned with stars.

Macchu Pichu, Peru: Structures aligned with solstices. Note: fun to discuss the claims that these had to have been made by “ancient astronauts”… Macchu Pichu, Peru: Structures aligned with solstices.

South Pacific: Polynesians were very skilled in art of celestial navigation

This picture is not in the text, but very cool -- possible evidence of astronomical observations by cave dwellers. France: Cave paintings from 18,000 B.C. may suggest knowledge of lunar phases (29 dots)

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.) Also recorded big one in 1054 AD—now Crab Nebula—read manuscripts at night!

Ancient Greek Astronomy Motion of planets Celestial sphere Earth centered perspective Scientific hypotheses

Modern science has roots with the Greek geeks... Greeks were the first people known to make models of nature. Pythagoras led the way. Aristotle expanded the theories. They tried to explain patterns in nature without resorting to myth or the supernatural. This was beginning of the scientific method. Greek geocentric model (c. 400 B.C.)

How did the Greeks explain planetary motion? Greek geocentric model started by Pythagoras about 500 BC: Earth at the center of the universe Heavens must be “perfect”: Objects moving on perfect spheres or in perfect circles. Stars on celestial sphere Pythagoras taught Socrates, who taught Plato, who taught Aristotle

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.

Artist’s reconstruction of Library of Alexandria: first founded to house scholars gathered by Alexander the Great (Built by Ptolemy II in 283 BC).

Eratosthenes (Alexandrian Library) determines circumference of the Earth (c. 240 BC) Measurements: Sun’s rays at Summer Solstice: Syene versus Alexandria distance ≈ 5000 stadia angle = 7° Calculate circumference of Earth: Dist. Alex. To Syene = 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/stadium  250,000 stadia ≈ 42,000 km

The most sophisticated geocentric model of the solar system was that of Ptolemy (A.D. 100-170) — 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

He uses epicycles, circles upon circles. So how does the Ptolemaic model explain retrograde motion? Planets really do go backward in this model. He uses epicycles, circles upon circles. But handles interior planets differently from exterior. 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.

Greek knowledge was 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.

The ‘Copernican’ Revolution (actually started by Aristarchus 1600 years earlier) Aristarchus, Copernicus challenged the Earth-centered idea with heliocentric model. With Brahe’s data for Mars we get Kepler’s three laws of planetary motion. Galileo solidified the Copernican revolution with first telescope observations.

How did Copernicus, Brahe, and Kepler challenge the Earth-centered idea? Proposed Sun-centered model (published 1543). (ARISTARCHUS Came up with it 1600 years earlier!) 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. Model was only a little more accurate than Ptolemaic model in predicting planetary positions, because it still used perfect circles & stars on a sphere, only a little bigger than Pythagoras claimed.

Tycho Brahe (1546-1601) 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…

Kepler first tried to match Tycho’s observations with circular orbits But an 8-arcminute discrepancy for Mars 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… Johannes Kepler (1571-1630)

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

Eccentricity or flatness 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.

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.

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

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).

How did Galileo solidify the Copernican revolution? Galileo (1564-1642) 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. He said air moves with earth. Non-circular orbits are not “perfect” as heavens should be. He found imperfections: sunspots, lunar features. If Earth were really orbiting Sun, we’d detect stellar parallax. Stars are too far away. 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.

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

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

Galileo was formally vindicated by the Church in 1992 (Pope). 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 (Pope). The scientific case was essentially settled, but the story was more complex in his own time as politics intervened… Galileo Galilei

‘Advancement of Learning’ FRANCIS BACON 1620 ‘Advancement of Learning’ 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.

How is astrology different from astronomy? Astronomy is a science focused on learning about how stars, planets, and other celestial objects work. Astrology is a search for hidden influences on human lives based on the positions of planets and stars in the sky.

Does astrology have any scientific validity? Scientific tests have shown that western astrological predictions begun by Ptolemy (400 AD) are no more accurate than we should expect from pure chance. Michel Guaquelin (1960) Precession of the equinoxes has shifted sun signs for 1/3 of you since then.

SKIP CHAPTER S1! Flash Cards? Geocentric system Heliocentric system Eratosthenes and size of Earth Retrograde motion Epicycles Kepler’s 3 Laws of Planetary Motion Galileo’s law of inertia Galileo’s telescope observations Francis Bacon’s Scientific Method SKIP CHAPTER S1!