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History & Development of Astronomy
Retrograde motionThe Greek ModelsGeocentrism, Epicycles, & the ChurchCopernicusTycho BraheKeplerGalileo
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…
Days of the week were named for Sun, Moon, and the 5 visible planets.
Ancient people of central Africa (6500 B.C.) could predict seasons from the orientation of the crescent moon.
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).
China: earliest known records of supernova explosions (1400 B.C.)
Bone or tortoiseshell inscription from the 14th century B.C.
"On the Xinwei day the new star dwindled."
"On the Jisi day, the 7th day of the month, a big new star appeared in the company of the Ho star."
Two Different Early Models…
GEOCENTRICEarth is the center of everythingEarth doesn’t spin or move
The Geocentric Model as art…
Two Different Early Models…
HELIOCENTRIC
The Sun is the center of the solar system
Earth spins (rotates) to create day/night
Earth orbits (revolves)to create the year
Geocentric Models made “sense”
GEOCENTRIC: Earth doesn’t move If we did, we’d feel it! If we did, we’d lose the moon! If we did, the stars around us would shift!
THEREFORE:Sky (& Stars!) rotated around us
Sun & Moon & Planets actually move among key constellations of the Zodiac by design/choice
Two Different Early Models… HELIOCENTRIC: Earth moves about the Sun
So do all of the planetsThe Moon goes around us, tooEarth spins to create night and day
THEREFORE:Sky (& Stars!) just SEEM to rotate around us
Sun & Moon & Planets moved among key constellations of the Zodiac because of OUR motion
KEY IDEA: Retrograde motion of the planets
We make the observation that planets – and only the planets – “dance” in front of the stars. 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
Retrograde Motion Explanations
Ancient (and geocentric)The planets move on their own around usGod(s) control their motionsHeaven’s realm – doesn’t concern us!It just is…
Retrograde Motion Explanations
Modern (and heliocentric)
Earth and other planets orbit the sun at different rates
Earth “laps” slower-moving outer planets – and they appear to loop
Retrograde Motion Explanations
Modern (and heliocentric)
Earth and other planets orbit the sun at different rates
Inner planets speed between us & sun one way, then seem to reverse along far side
Venus’ Different Views
Venus September 2013 –
evening sky
Venus’ Different Views
Venus September 2013 –
evening sky
Venus’ Different Views
Venus January 2013 – morning sky
Why does modern science trace its roots to the Greeks?
How did the Greeks explain planetary motion?
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.
• 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. Greek geocentric
model (c. 400 B.C.)
Why does modern science trace its roots to the Greeks?
Eratosthenes measures the Earth (c. 240 B.C.)
Measurements:Syene to Alexandria • distance ≈ 5,000 stadia• angle = 7°
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
Underpinnings of the Greek geocentric model:
Plato
Aristotle
How did some Greeks explain planetary motion?
• Earth at the center of the universe
• Heavens must be “perfect”—objects move on perfect spheres or in perfect circles.
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.
Sun-centered models had been considered
Aristarchus
Archimedes
How did other Greeks explain planetary motion?
• Philolaus: Sun – the central “fire” at the center of the universe
• Aristarchus: Earth must be smaller
• Archimedes: Stars must be MUCH farther away!
The most sophisticated geocentric model was that of Ptolemy (A.D. 100–170) — the Ptolemaic model:
Ptolemy
• Sufficiently accurate to remain in use for 1,500 years
• Arabic translation of Ptolemy’s work named Almagest (“the greatest compilation”)
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?
TheologicallyEarth at the center of everything “fits” western
religious growth in Christianity
ScientificallyEarth rotating and revolving mysteriously through
“unseen” forces is hard to prove
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.
• Brahe compiled most accurate naked eye measurements of planetary positions ever made at the time.
• Precise to 1/60th of a degree!
• Still could not detect stellar parallax
• Thought Earth must be at center of solar system
• Recognized that other planets go around Sun.
Tycho Brahe (1546–1601)
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)
• Hired Johannes Kepler, who used Tycho’s observations to discover actual shape of planetary orbits and motions.
Tycho Brahe (1546–1601)
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.”
An ellipse looks like an elongated circle.
What is an ellipse?
Shape Speed Time
What are Kepler’s three laws of planetary motion?
The orbit of each planet around the Sun is an ellipse with the Sun at one focus.
Kepler’s First Law: SHAPE
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
Kepler’s Second Law: SPEED
and slower when it is farther from the Sun.
Kepler’s Second Law Simulation at Mastering Astronomy
More distant planets orbit the Sun at slower average speeds, obeying the relationship
p2 ~ a3
p = orbital period (years or days) a = average distance from Sun
Kepler’s Third Law:
Time
Kepler’s Third Law Simulation at Mastering Astronomy
Graphical version of Kepler’s Third Law
Kepler’s DataPlanet Orbit
“a” (miles)
Period“P”
(days)
a3 P2 a3
P2
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 107 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. The Moon had mountains & craters2. The Sun had spots3. Jupiter had moons4. Venus had phases & shape changes5. Saturn had “ears”6. 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. The Moon had mountains & craters2. The Sun had spots3. Jupiter had moons4. Venus had phases & shape changes5. Saturn had “ears”6. The Milky Way had countless stars
The Heavens were NOT “perfect”
1. The Moon had mountains & craters2. The Sun had spots3. Jupiter had moons4. Venus had phases & shape changes5. Saturn had “ears”6. The Milky Way had countless stars
The Earth was NOT the only
center of motion
1. The Moon had mountains & craters2. The Sun had spots3. Jupiter had moons4. Venus had phases & shape changes5. Saturn had “ears”6. The Milky Way had countless stars
Earth could “keep” its moon if it
orbited the Sun
1. The Moon had mountains & craters2. The Sun had spots3. Jupiter had moons4. Venus had phases & shape changes5. Saturn had “ears”6. The Milky Way had countless stars
Venus HAD to orbit the Sun, not Earth
1. The Moon had mountains & craters2. The Sun had spots3. Jupiter had moons4. Venus had phases & shape changes5. Saturn had “ears”6. 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
Stars are so far away, we can’t measure
parallax even if the Earth moved!
1. The Moon had mountains & craters2. The Sun had spots3. Jupiter had moons4. Venus had phases & shape changes5. Saturn had “ears”6. 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.
• Moons of Jupiter clearly orbited Jupiter, not Earth
• Venus’ Phases and size changes showed it orbited the Sun, not Earth.
• NOTE!
• He didn’t see proof of EARTH orbiting the sun
Overcoming the first objection (Earth at center of solar system):
Galileo’s experiments showed that objects in air would stay with a moving Earth.
Overcoming the second objection (nature of motion):
• Aristotle thought that all objects naturally come to rest.
• Galileo showed that objects will stay in motion unlessa force acts to slow them down (Newton’s first law of motion).
• The planets COULD move about the Sun and not stop!
Overcoming the third objection (heavenly perfection):
• Using his telescope, Galileo saw:
— Sunspots on Sun (“imperfections”)
— Mountains and valleys on the Moon (proving it is not a perfect sphere)
— “Ears” of Saturn
• Tycho thought 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 see that the Milky Way is countless individual stars.
If stars were much farther away, then lack of detectable parallax was no longer so troubling.
Overcoming the fourth objection (parallax):
Galileo Galilei
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.
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?
What makes a theory scientific? 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?
What is the shape of Earth’s orbit around the Sun?
All planets have elliptical orbits around the Sun.
WHAT DID YOU THINK?
Do the planets orbit the Sun at constant speeds?
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?
Do all of the planets orbit the Sun at the same speed?
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?
How much force does it take to keep an object moving in a straight line at a constant speed?
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?
How does an object’s mass differ when measured on Earth and on the Moon?
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?
Do astronauts orbiting the Earth feel the force of gravity from our planet?
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.
