Astronomy Rough Notes - Copernican Revolution
BRING:
Powerpoint
Software for sky
Geocentric model
Heliocentric model
Earth globe
DISCLAIMER: These notes do NOT cover everything you need to know. You may need to look up some
item or concept online or in a text. Test questions are not exact copies of the OBJECTIVES but if you
know the OBJECTIVES thoroughly, you should do well on the exams.
HANDOUTS:
None
OBJECTIVES:
Describe the geocentric and the heliocentric models of the solar system.
The word planet derives from the Greek "planetes" which means wanderer. Why were the planets called
wanderers?
Explain Aristotle’s main argument against the heliocentric model.
What was Aristotle’s other (and weaker) argument against the heliocentric model?
Who gave the first evidence in favor of the heliocentric model? What was the evidence?
Who added the mathematical calculations to the geocentric model?
When a planet undergoes retrograde motion, how does it look?
How did the geocentric model explain retrograde motion.
How did the heliocentric model explain retrograde motion.
What did Brahe contribute to the heliocentric vs. geocentric debate?
Define perihelion and aphelion.
Considering Kepler's three laws of planetary motion (you do not have to memorize them):
What shape orbit does a planet have?
When a satellite orbits the Earth, does it move faster at perigee or at apogee? When a comet orbits
the Sun, does it orbit faster at perihelion or at aphelion?
What is meant by the period of a planet. In our solar system, what planet has the longest period?
The shortest? Which planet orbits with the highest speed? Slowest?
What did Galileo notice about the Moon, the Sun, Jupiter, Venus? What did he conclude from these
observations? How did the church/state react?
Why does Venus exhibit phases but Mars does not. (Hint: Draw the Sun as well as Venus, Earth and Mars
in their orbits. Shade in Venus and Mars. What do they look like from Earth’s view?)
State Newton's Universal Law of Gravity. If the mass of one of two objects increases, what happens to
the gravitational force between the objects? If the distance between the two increases, what happens to
the force?
How can you tell if a process is science? (See earlier lecture on the hallmarks of science.)
When science uses the term “theory” such as the “Theory of Evolution”, is that a weak or a strong
statement? What backs up any theory in science?
****************Basic outline for the next two days:**********************************
 Heliocentric solar system vs. Geocentric solar system
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Pre-Copernican Astronomy
Copernican Revolution
Comments on science
*****************Heliocentric solar system vs. Geocentric solar system*******************
Heliocentric = Moon orbits Earth but Earth and planets orbit the Sun
Geocentric = Everything orbits Earth (Earth at the center)
More details added in a bit.
Heliocentric model
Geocentric model
Heliocentric image from: http://www.physics.uc.edu/~sitko/Fall2002/2Gravity/history_files/image014.gif
Geocentric image from: http://farside.ph.utexas.edu/teaching/301/lectures/img1903.png
************************PRE-COPERNICAN ASTRONOMY (Brief)********************
Greeks ~Fifth Century BCE
What is the sun? How is it related to fire?
What are those tiny lights in the sky?
Why do some lights wander among the others?
(7 Planetes* - Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn)
Pythagoras of Samos ~500 BCE
Earth is a sphere
Mathematical perfection (Note: separation of ideal from reality)
All complex phenomena must result from basic simple ones
Crystal spheres for Sun, Moon, 5 planets (Egypt, form not substance)
Plato ~360 BCE
Intellectual philosophy to explain universe
From the mathematical perfection, the Heavens are perfect, unchanging
Therefore, heavenly motion must be in circles and uniform
Emphasized complex phenomena result from simple ones
Aristotle ~350 BCE
Earth: Corrupt, changeable, imperfect (Note: Separation of heavens and Earth)
Argued for Geocentric model of universe*
No parallax of stars*
Good argument
Couldn’t believe universe was so large that you could not see parallax of stars
Moon would be left behind* (bad physics)
Side comment: Knew Earth was round (eclipses, ships, southern stars)
Aristarcus ~300 BCE
First direct evidence for heliocentric solar system*
Measured size of Sun and knew that it was substantially larger than Earth*
Measured that Sun was much further away than Moon*
Hipparchus ~150 BCE
First comprehensive star catalog
Magnitude scale (1 to 6)*
Discovered precession of Earth
Ptolemy ~140 AD
Almagest
Mathematical model of geocentric universe*
Epicycles (retrograde motion)* (Demos/drawings here)
Dark Ages
Roman libraries sacked, plague, rise and fall of Islamic science, etc
**********************End Pre-Copernican Astronomy (Brief)***********************
**************PRE-COPERNICAN ASTRONOMY (More detail for those interested)*********
References:
http://www-gap.dcs.st-and.ac.uk/~kistory/HistTopics/Greek_astronomy.html#s55
The Great Copernicus Chase - Owen Gingerich
Seeds, (see syllabus)
(Note: Western only – Please investigate other cultures)
Greeks (influenced by Babylonian, Mesopotamian, Egyptian, Roman astronomy)
~700 BCE
Early astronomy was about time keeping (when to sow and reap crops)*
Needed better observations to keep the various calendars synchronized
Hesiad, Works and Days
…when the Pleiades rise it is time to use the sickle, but the plough when they are setting; 40 days they
stay away from heaven; when Arcturus ascends from the sea and, rising in the evening, remain visible for
the entire night, the grapes must be pruned;…
Greeks ~Fifth Century BCE
Difference between living and nonliving
What is the sun? How is it related to fire?
What are those tiny lights in the sky?
Why do some lights wander among the others?
(7 Planetes* - Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn)
Pythagoras of Samos ~500 BCE
Earth is a sphere*
Mathematical perfection* (Note: separation of ideal from reality)
All complex phenomena must result from basic simple ones*
Crystal spheres for Sun, Moon, 5 planets (Egypt, form not substance)
Plato ~360 BCE
Intellectual philosophy to explain universe
From the mathematical perfection, the Heavens are perfect, unchanging*
Therefore, heavenly motion must be in circles and uniform*
Emphasized complex phenomena result from simple ones
 About perfection from Plato’s Phaedo : The instance taken there is the mathematical relation of
equality, and the contrast is drawn between the absolute equality we think of in mathematics and
the rough, approximate equality which is what we have to be content with in dealing with objects
with our senses.
 About heavenly motion in circles at uniform speed
Eudoxus – concentric spheres
(first model to explain complex heavenly motion with simple model)
Stars, planets, Sun and Moon move around the Earth in crystalline spheres.
The sphere of the Moon was closest to the Earth, followed in order by the spheres of the Sun, then
Mercury, Venus, Mars, Jupiter, Saturn, and furthest away was the sphere of the stars
 About separating thought from measurement
(Plato from the Encyclopedia Britanica)
... the exact sciences - arithmetic, plane and solid geometry, astronomy, and harmonics - would first be
studied for ten years to familiarize the mind with relations that can only be apprehended by thought. Observing the heavens lowers the spirits…
 About complex from simple: Theon of Smyrna wrote: The changing aspects of the revolution of
the planets is because, being fixed in their own circles or in their own spheres whose movements
they follow, they are carried across the zodiac, just as Pythagoras had first understood it, by a
regulated simple and equal revolution but which results by combination in a movement that
appears variable and unequal
 Which led Theon to conclude: It is natural and necessary that all the heavenly bodies have a
uniform and regular movement.
Aristotle ~350 BC
Earth: Corrupt, changeable, imperfect* (Note: Separation of heavens and Earth)
Geocentric model of universe*
Arguments for geocentric model*
No parallax of stars*
Moon would be left behind*
Side comment: Knew Earth was round (eclipses, ships, southern stars)*
Aristarcus ~300 BC
Heliocentric solar system*
Measured size of Sun and knew that it was substantially larger than Earth
Measured that Sun was much further away than Moon
Hipparchus ~150 BC
Discovered precession of Earth
Measured length of year to within 6.5 minutes
May have invented trigonometry
Turned astronomy into a practical, predictable science
First comprehensive star catalog
Magnitude scale (1 to 6)*
Ptolemy ~140 AD
Almagest
Geocentric model*
Mathematical model of universe*
Perfect Bodies - circles, spheres*
Uniform motion*
Epicycles (retrograde motion)* (Demos here)
Dark Ages
Roman libraries sacked, plague, rise and fall of Islamic science, etc
Some Observations (Any model of the solar system must explain)
Stars, Sun, Moon rise in east and set in west daily/nightly
Retrograde motion of planets
Venus never seen at midnight (always morning or evening)
**************************End of Detailed Pre-Copernican Section***********************
*************************COPERNICAN REVOLUTION (Outline)***************
Copernican Revolution (~1500 to ~1700) – SUMMARY
Copernicus ~1500
Heliocentric model of the solar system returns
Still circles, uniform
Brahe ~1570
Equipment
Predict better
Wanted to prove his geocentric model
Kepler
~1600
Mathematical model of the solar system
Galileo
~1600
Developed and used the telescope.
Observed:
Sunspots
Craters and sunrise on the Moon
Moons of Jupiter
Phases of Venus
Newton ~1670
Recognized that gravity was universal
Developed law of gravity
Developed laws of motion that worked on Earth and in the heavens
******************End Copernican Revolution (Outline)********************************
******************Copernican Revolution (~1500 - ~1700) – MORE DETAIL**************
THE PEOPLE*
Copernicus ~1500
Brahe ~1570
Kepler ~1600
Galileo ~1600
Newton ~1670
THE ISSUE*
Geocentric vs Heliocentric
Which model fits the observations best
Stars, Sun, Moon rise in east and set in the west (both models explain it)
Planets exhibit retrograde motion (both models explain it)
Venus never seen at midnight, only in the morning or evening (both models explain it)
THE PEOPLE, THE CONTRIBUTIONS, AND THE SIGNIFICANCE
Copernicus ~1500
Portrait at http://micro.magnet.fsu.edu/optics/timeline/people/antiqueimages/copernicus.jpg
Resurrects heliocentric model of the solar system*
Still used circles and uniform motion so was worse at predictions than Ptolemy’s model
Brahe ~1570
Portrait at http://measure.igpp.ucla.edu/solar-terrestrial-luminaries/brahe.JPG
Built good equipment*
Made very accurate measurements*
Predict better
Wanted to prove his geocentric model
Kepler
~1600
Portrait at www.kepler.arc.nasa.gov/ johannes.html
Mathematical model of the solar system
Kepler’s Laws (mathematical laws derived from the data)
1. Planets orbit the sun in elliptical orbits*
Sun “centered”, Not perfect circles
Perihelion
Sun
Aphelion
2. Radius vector sweeps out equal areas in equal times
Meaning: Planet speeds up and slows down (non-uniform motion)*
Example: Earth speed at perihelion is 18.9 mi/s,
Earth speed at aphelion is 18.3 mi/s
3. T2 α R3
T is period, R is “average” radius of orbit
Meaning: Planet closer to Sun takes less time to revolve than a planet further from the Sun*
Mercury orbits in less time than Earth
Mercury
~0.4 AU
T = ~88 days
Earth
1 AU
T = ~365 days
Pluto
~40 AU
T = ~248 years
And speeds are slower for more distant planets*
Mercury ~30 mi/s
Earth ~18.6 mi/s
Pluto ~3 mi/s
Galileo
~1600
www.nd.edu/~hps/ galileo-front.html
Developed and used the telescope. Saw:
Earth's moon – Imperfect heavens, Earth-like heavens*
http://atropos.as.arizona.edu/aiz/teaching/nats102/lecture9.html
Sunspots - Imperfection, changing heavens, other centers of motion*
http://soho.nascom.nasa.gov/data/realtime-images.html
Jupiter's moons - Other centers of motion *
Phases of Venus – Strong evidence of heliocentric model
Heliocentric model predicted all phases and crescent would appear large and gibbous small*
Geocentric model predicted only crescent and not much change in size.*
For photos: http://antwrp.gsfc.nasa.gov/apod/ap040521.html
or http://www.calvin.edu/academic/phys/observatory/images/venus/
For demo http://galileoandeinstein.physics.virginia.edu/more_stuff/flashlets/PhasesofVenus.htm
NOT Handed out but useful if you want to think about the two models and the Phases of Venus:
Geocentric model vs Heliocentric model (Predict Phases of Venus)
K.I.S.S.* (sometimes referred to as Occam’s Razor)
Geocentric model had to add epicycle onto epicycle to match the actual motions of the planets so
that model also got too complex)
Newton ~1670
Mathematically unified objects that fall toward Earth with objects that orbit Earth*
(called the Newtonian synthesis)
Meaning:
Earth and heavens are alike*
Newton’s Three Laws of Motion (Read but don’t memorize)
NEWTON’S UNIVERSAL LAW OF GRAVITY
(This is different from Newton’s Three Laws of Motion)
Note “universal” (Laws here = laws elsewhere)*
Two masses (m and M)
F G
mM
D2
Every mass attracts every other mass*
Big mass means big force*
Big separation means small force*
Interesting side notes on Universal Law of Gravity:
Mechanistic or clockwork universe
Predictability (Cause and Effect)
Chaos (See for example, James Gleick, Chaos )
WHAT IS SCIENCE?
SCIENCE:*
Science - A way of knowing that tries to uncover the laws and processes of the universe.
Differs from theology or philosophy
Way to get passed the conflicts in various belief systems (by looking at evidence)
Sagan: “It doesn’t matter if it makes you feel good. What matters is, is it true?”
PROCESS OF SCIENCE*
Observe
Question
Model (hypothesize)
Predict
Test
Revise
You use the process all the time in daily life
Examples: Car stops….
Flashlight doesn’t work…
Cooking…
HALLMARKS OF SCIENCE* (How science stands apart. See Bennett - Cosmic Perspective )
Uses natural explanations (not supernatural)
Uses models that make testable, repeatable predictions
K.I.S.S. (often called Occam’s Razor)
Theory vs. Law*
!!!!Theory is stronger than Law in normal science terminology!!!!
Law - What happens
Predicts what will happen in specific cases
Ex - Universal Law of Gravity
If you drop a rock, it will fall down.
Can also calculate the force exerted on the rock.
Can also help calculate where the rock will fall if you throw it.
*Theory - Explains why things happen
Includes laws
Allows predictions of new tests of itself
* Note: Supported by many scientists
Supported by many testable predictions
Can never be proven but has not been disproven
Example of Theory vs Law
Universal Law of Gravity explains that gravity works
Can calculate gravitational force etc
General Theory of Relativity explains WHY gravity works
Includes the Law of Gravity BUT ALSO more
Explains the precession of Mercury’s perihelion
Predicts the existence of Black Holes
Note: Supported by many scientists
Supported by many testable predictions
Can never be proven but has not been disproven
These can always be modified, based on new data/information*
Homework:
Continue to make flash cards for the objectives.
Read notes and/or text and/or web
Review previous material and flash cards
Buy a pencil to bring to the exam.
Revised 5 January 2016