History Development of Astronomy Ø Retrograde motion

History & Development of Astronomy Ø Retrograde motion Ø The Greek Models Ø Geocentrism, Epicycles, & the Church Ø Copernicus Ø Tycho Brahe Ø Kepler Ø Galileo

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…

Ancient people of central Africa (6500 B. C. ) could predict seasons from the orientation of the crescent moon.

Ø Egyptian obelisk: Shadows tell time of day.

England: Stonehenge (completed around 1550 B. C. )

Mexico: model of the Templo Mayor

New Mexico: Anasazi kiva aligned north–south

SW United States: “Sun Dagger” marks summer solstice

Scotland: 4, 000 -year-old stone circle; Moon rises as shown here every 18. 6 years.

Peru: lines and patterns, some aligned with stars

Macchu Pichu, Peru: structures aligned with solstices

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

France: Cave paintings from 18, 000 B. C. may suggest knowledge of lunar phases (29 dots).

"On the Jisi day, the 7 th day of the month, a big new star appeared in the company of the Ho star. " "On the Xinwei day the new star dwindled. " Bone or tortoiseshell inscription from the 14 th century B. C. China: earliest known records of supernova explosions (1400 B. C. )

Two Different Early Models… Ø GEOCENTRIC Ø Earth is the center of everything Ø Earth doesn’t spin or move

The Geocentric Model as art…

Two Different Early Models… Ø HELIOCENTRIC Ø The Sun is the center of the solar system Ø Earth spins (rotates) to create day/night Ø Earth orbits (revolves) to create the year

Geocentric Models made “sense” Ø GEOCENTRIC: Earth doesn’t move Ø If we did, we’d feel it! Ø If we did, we’d lose the moon! Ø If we did, the stars around us would shift! Ø THEREFORE: Ø Sky (& Stars!) MUST rotate around US Ø Sun & Moon & Planets actually move among key constellations of the Zodiac by design/choice

Heliocentric required motion… Ø HELIOCENTRIC: Earth moves about the Sun Ø So do all of the planets Ø The Moon goes around us, too Ø Earth spins to create night and day Ø THEREFORE: Ø Sky (& Stars!) just SEEM to rotate around us Ø Sun & Moon & Planets moved among key constellations of the Zodiac because of OUR motion

One additional observation to explain… Ø The (5) “Wandering” Planets! Ø Different patterns in front of zodiac stars Ø Different times of year

KEY IDEA: Retrograde motion of the planets Ø We make the observation that planets – and only the planets – “dance” in front of the stars. Ø How is this observation explained in each model?

The Motion of the Planets in the sky over time

The Motion of Mars In 2009 -2010

Geocentric Explanation Ø The planets move on their own around us ØGod(s) control their motions ØHeaven’s realm – doesn’t concern us! ØIt just is…

Heliocentric Explanation Ø Planets orbit sun at different rates Ø Earth “laps” slowermoving outer planets –they appear to loop

Heliocentric Explanation Ø Planets orbit sun at different rates Ø Earth “laps” slower-moving outer planets – they appear to loop Ø Inner planets speed between us & sun one way, then seem to reverse along far side Ø Check out neat Venus/Earth dance…(but it’s upside down!)

Venus’ Different Views Ø Venus September 2013 – evening sky

Venus’ Different Views Ø Venus September 2013 – evening sky

Venus’ Different Views Ø Venus January 2013 – morning sky

Venus’ Different Views Ø Venus in the evening sky

Why does modern science trace its roots to the Greeks? Ø How did the Greeks explain planetary motion? Ø How did Islamic scientists preserve and extend Greek science? Artist’s reconstruction of the Library of Alexandria

Our mathematical and scientific heritage originated with the civilizations of the Middle East.

Why does modern science trace its roots to the Greeks? • Greeks were the first people known to make models of nature. Greek geocentric model (c. 400 B. C. ) • They tried to explain patterns in nature without resorting to myth or the supernatural.

Eratosthenes measures the Earth (c. 240 B. C. ) Measurements: Syene to Alexandria • distance ≈ 5, 000 stadia • angle = 7°

How did some Greeks explain planetary motion? Underpinnings of the Greek geocentric model: • Earth at the center of the universe • Heavens must be “perfect”—objects move on perfect spheres or in perfect circles. Plato Aristotle

But this made it difficult to explain the apparent retrograde motion of planets… Review: Over a period of 10 weeks, Mars appears to stop, back up, then go forward again.

How did other Greeks explain planetary motion? Sun-centered models had been considered • Philolaus: Sun – the central “fire” at the center of the universe • Aristarchus: Earth must be smaller • Archimedes: Stars must be MUCH farther away! Aristarchus Archimedes

The most sophisticated geocentric model 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

So how does the Ptolemaic model explain retrograde motion? Planets really do go backward in this model.

Ptolemaic Retrograde Motion

Ptolemaic Retrograde Motion

What happened after Ptolemy’s model? Ø Theologically Ø Earth at the center of everything “fits” western religious growth in Christianity Ø Scientifically Ø Earth rotating and revolving mysteriously through “unseen” forces is hard to prove Ø Science was unnecessary…

The Roman Era

The Roman Era … Science should be “practical”

The Fall of Rome

The Looting of the Library of Alexandria

The Dark Ages…. .

The Dark Ages…. . In Europe…

The Dark Ages…. . In Europe… Not in the Middle East, China, The Yucatan, Polynesia….

The Dark Ages…. . In Europe… The Crusades!

The Crusades! Greek Records, Preserved, Translated, and advanced by Arabic Empires are brought back to Europe… Including works by Ptolemy, Eratosthenes, Aristarchus, others

The Crusades! Greek Records, Preserved, Translated, and advanced by Arabic Empires are brought back to Europe… …and used for navigation charts

The Crusades! Greek Records, Preserved, Translated, and advanced by Arabic Empires are brought back to Europe… …and Ptolemy’s model doesn’t quite work – especially for Mercury

The Crusades! Greek Records, Preserved, Translated, and advanced by Arabic Empires are brought back to Europe… …and they are copied and kept…

The Crusades! Greek Records, Preserved, Translated, and advanced by Arabic Empires are brought back to Europe… …and they are copied and kept… by the Church

How did Copernicus, Tycho, and Kepler challenge the Earth-centered idea? Copernicus (1473– 1543): • Proposed Sun-centered model (“heliocentric”) published 1543. • Used model to determine layout of solar system

How did Copernicus, Tycho, and Kepler challenge the Earth-centered idea? Copernicus (1473– 1543): • Sun-centered model • Determined layout of solar system (planetary distances in AU). But. . . • Assumed CIRCULAR orbits • Model was no more accurate than Ptolemaic model in predicting planetary positions.

Tycho Brahe (1546– 1601) • Brahe compiled most accurate naked eye measurements of planetary positions ever made at the time. • Precise to 1/60 th of a degree!

Tycho Brahe (1546– 1601) • Still could not detect stellar parallax • Thought Earth must be at center of solar system • Recognized that other planets go around Sun.

Parallax

• Parallax results from shift in viewing position • If CLOSE to Earth, a star would be seen in different locations (at different angles)

• Parallax results from shift in viewing position • If FAR from Earth, a star would NOT be seen in different locations (at different angles)

Tycho Brahe (1546– 1601) • Hired Johannes Kepler, who used Tycho’s observations to discover actual shape of planetary orbits and motions.

Johannes Kepler (1571– 1630) • Kepler first tried to match Tycho’s observations with circular orbits. • An 8 arc-minute discrepancy (about 13% of one degree) led him eventually to ellipses. • Developed 3 “laws” of orbits

Johannes Kepler (1571– 1630) • An 8 arc-minute discrepancy (about 13% of one degree) led him eventually to ellipses. 100 meters away! 8 arc-min

Johannes Kepler (1571– 1630) “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. ”

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

What are Kepler’s three laws of planetary motion? Ø Shape Ø Speed Ø Time

Kepler’s First Law: SHAPE The orbit of each planet around the Sun is an ellipse with the Sun at one focus.

Kepler’s Second Law: SPEED As a planet moves around its orbit, it sweeps out equal areas in equal times.

Kepler’s Second Law: SPEED This means that a planet travels faster when it is nearer to Sun and slower when it is farther from the Sun.

Kepler’s Second Law Simulation at Mastering Astronomy

Kepler’s Third Law: Time More distant planets orbit the Sun at slower average speeds, obeying the relationship p 2 ~ a 3 p = orbital period (years or days) a = average distance from Sun

Kepler’s Third Law Simulation at Mastering Astronomy

Graphical version of Kepler’s Third Law

Kepler’s Data Planet Orbit “a” (miles) Period “P” a 3 P 2 (days) Mercury 3. 596 x 107 86. 96 46. 49 x 1021 7734 6. 009 x 1018 Venus 6. 716 x 107 224. 7 303. 3 x 1021 50490 6. 008 x 1018 Earth 9. 290 x 107 365. 3 801. 7 x 1021 133500 6. 009 x 1018 Mars 14. 16 x 107 687. 1 2836 x 1021 472100 6. 008 x 1018 Jupiter 48. 33 x 107 4323 112900 x 1021 18780000 6. 012 x 1018 Saturn 88. 61 x 10760 695800 x 1021 115800000 6. 011 x 1018

Graphical version of Kepler’s Third Law

The Aristotelian Beliefs of Galileo’s Time: Heliocentrism was impossible! 1. Earth was the center of all celestial motions, & everything orbited us. 2. Noncircular orbits are not “perfect” as heavens should be. 3. Earth could not be moving because objects in air would be left behind. 4. If Earth were really orbiting Sun, we’d detect stellar parallax.

Galileo’s Telescopic Observations 1. 2. 3. 4. 5. 6. The Moon had mountains & craters The Sun had spots Jupiter had moons Venus had phases & shape changes Saturn had “ears” The Milky Way had countless stars

1. The Moon had mountains & craters

2. The Sun had spots

3. Jupiter had four moons in orbit around the planet!

Jupiter’s Moons

4. Venus had phases & shape changes

5. Saturn had “ears”

6. The Milky Way had countless stars

The Importance of Galileo’s Telescopic Observations 1. 2. 3. 4. 5. 6. The Moon had mountains & craters The Sun had spots Jupiter had moons Venus had phases & shape changes Saturn had “ears” The Milky Way had countless stars

The Heavens were NOT “perfect” 1. 2. 3. 4. 5. 6. The Moon had mountains & craters The Sun had spots Jupiter had moons Venus had phases & shape changes Saturn had “ears” The Milky Way had countless stars

The Earth was NOT the only center of motion 1. 2. 3. 4. 5. 6. The Moon had mountains & craters The Sun had spots Jupiter had moons Venus had phases & shape changes Saturn had “ears” The Milky Way had countless stars

Earth could “keep” its moon if it orbited the Sun 1. 2. 3. 4. 5. 6. The Moon had mountains & craters The Sun had spots Jupiter had moons Venus had phases & shape changes Saturn had “ears” The Milky Way had countless stars

Venus HAD to orbit the Sun, not Earth 1. 2. 3. 4. 5. 6. The Moon had mountains & craters The Sun had spots Jupiter had moons Venus had phases & shape changes Saturn had “ears” The Milky Way had countless stars

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

Geocentric system: Venus always seen as crescent About the same size Heliocentric system: Venus changes phase Distance varies so SIZE varies too

Photograph: ©Roger Hutchinson/2017 Astronomy Photographer of the Year

Stars are so far away, we can’t measure parallax even if the Earth moved! 1. 2. 3. 4. 5. 6. The Moon had mountains & craters The Sun had spots Jupiter had moons Venus had phases & shape changes Saturn had “ears” The Milky Way had countless stars

Galileo’s observations destroyed Aristotelian beliefs held to be true: 1. Noncircular orbits are not “perfect” as heavens should be. 2. Earth was the center of all celestial motions, & everything orbited us. 3. Earth could not be moving because objects in air would be left behind. 4. If Earth were really orbiting Sun, we’d detect stellar parallax.

Galileo’s observations destroyed Aristotelian beliefs held to be true: 1. Noncircular orbits are not “perfect” as heavens should be. BUT…Using his telescope, Galileo saw: —Sunspots on Sun “imperfections” —Moon’s mountains & craters (proving it is not a perfect sphere) —“Ears” of Saturn

Galileo’s observations destroyed Aristotelian beliefs held to be true: 2. Earth was the center of all celestial motions, & everything orbited us. BUT…. . • Moons of Jupiter clearly orbited Jupiter, not Earth • Venus’ phases and size changes showed it orbited the Sun, not Earth.

Galileo’s observations destroyed Aristotelian beliefs held to be true: 3. Earth could not be moving because objects in air would be left behind. But…. • Galileo’s experiments showed that objects in air would stay with a moving Earth. • The planets COULD move about the Sun and not stop!

Galileo’s observations destroyed Aristotelian beliefs held to be true: 4. If Earth were really orbiting Sun, we’d detect stellar parallax. But… Galileo showed stars must be much farther by seeing Milky Way has countless individual stars. ü If stars were much farther away, then lack of detectable parallax was no longer so troubling.

In 1633 the Catholic Church ordered Galileo to recant his claim that Earth orbits the Sun. 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. Galileo Galilei

Galileo’s Daughter by Dava Sobel

Summary of Key Ideas

The Scientific Method Make Observations Research/Consider Prior Theories Analyze Results

The Scientific Method Make Observations Research/Consider Prior Theories Analyze Results If pre-existing theories explain observation, propose new observations & experiments to extend theories.

The Scientific Method Make Observations Research/Consider Prior Theories Analyze Results If pre-existing theories explain observation, propose new observations//experiments to extend theories. If NO pre-existing theories explain observation, modify or develop new theory

The Scientific Method Make Observations Research/Consider Prior Theories Analyze Results If pre-existing theories explain observation, propose new observations//experiments to extend theories. If NO pre-existing theories explain observation, modify or develop new theory Make predictions from new/modified theory Do the Experiment! Analyze Results Submit for Peer Review & Publish

WHAT DID YOU THINK? Ø What makes a theory scientific? Ø A theory is an idea or set of ideas proposed to explain something about the natural world. A theory is scientific if it makes predictions that can be objectively tested and potentially disproved.

WHAT DID YOU THINK? Ø What is the shape of Earth’s orbit around the Sun? Ø All planets have elliptical orbits around the Sun.

WHAT DID YOU THINK? Ø Do the planets orbit the Sun at constant speeds? Ø No. The closer a planet is to the Sun in its elliptical orbit, the faster it is moving. The planet moves fastest at perihelion and slowest at aphelion.

WHAT DID YOU THINK? Ø Do all of the planets orbit the Sun at the same speed? Ø No. A planet’s speed depends on its average distance from the Sun. The closest planet moves fastest, the most distant planet moves slowest.

WHAT DID YOU THINK? Ø How much force does it take to keep an object moving in a straight line at a constant speed? Ø Unless an object is subject to an outside force, like friction, it takes no force at all to keep it moving in a straight line at a constant speed.

WHAT DID YOU THINK? Ø How does an object’s mass differ when measured on Earth and on the Moon? Ø Assuming the object doesn’t shed or collect pieces, its mass remains constant whether on Earth or on the Moon. Its weight, however, is less on the Moon.

WHAT DID YOU THINK? Ø Do astronauts orbiting the Earth feel the force of gravity from our planet? Ø Yes. They are continually pulled earthward by gravity, but they continually miss it because of their motion around it. Because they are continually in free-fall, they feel weightless.

Eratosthenes measures the Earth (c. 240 B. C. ) Calculate circumference of Earth: 7/360 (circum. Earth) = 5, 000 stadia circum. Earth = 5, 000 360/7 stadia ≈ 250, 000 stadia Compare to modern value (≈ 40, 100 km): Greek stadium ≈ 1/6 km 250, 000 stadia ≈ 42, 000 km