History & Development of
Astronomy
Retrograde motion
The Greek Models
Geocentrism, Epicycles, & the Church
Copernicus
Tycho Brahe
Kepler
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.
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 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."
Bone or tortoiseshell inscription from the 14th century B.C.
China: earliest known records of supernova explosions (1400 B.C.)
Two Different Early Models…
 GEOCENTRIC
 Earth is the center of everything
 Earth 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!) MUST rotate around US
 Sun & Moon & Planets actually move among key
constellations of the Zodiac by design/choice
Heliocentric required motion…
 HELIOCENTRIC: Earth moves about the Sun
 So do all of the planets
 The Moon goes around us, too
 Earth 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
One additional observation to
explain…
 The (5) “Wandering” Planets!
 Different patterns in front of zodiac stars
 Different times of year
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
Geocentric Explanation
The planets move on their own around us
God(s) control their motions
Heaven’s realm – doesn’t concern us!
It just is…
Heliocentric Explanation
Planets orbit sun at
different rates
Earth “laps” slowermoving outer planets
–they appear to loop
Heliocentric Explanation
Planets orbit sun at different rates
Earth “laps” slower-moving outer planets –
they appear to loop
Inner planets speed between us & sun one
way, then seem to reverse along far side
 Check out neat Venus/Earth dance…(but it’s upside down!)
Venus’ Different Views
Venus
September
2013 –
evening
sky
Venus’ Different Views
Venus
September
2013 –
evening
sky
Venus’ Different Views
Venus January 2013 –
morning sky
Venus’ Different Views
Venus in the evening 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.
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?
 Theologically
 Earth at the center of everything “fits” western
religious growth in Christianity
 Scientifically
 Earth 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!
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?
 Shape
 Speed
 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
p2 ~ a3
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”
a3
P2
a3
P2
(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 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.
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
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.
Overcoming the first objection
(Earth at center of solar system):
• 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 second 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 that objects will stay in motion unless
a 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
Overcoming the fourth objection
(parallax):
• 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.
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
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.
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