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The Origin of Modern Astronomy
Outline
I. The Roots of Astronomy
A. Archaeoastronomy
B. The Astronomy of Greece
C. The Ptolemaic Universe
II. The Copernican Revolution
A. Copernicus the Revolutionary
B. Galileo the Defender
III. The Puzzle of Planetary Motion
A. Tycho the Observer
B. Tycho Brahe's Legacy
C. Kepler the Analyst
D. Kepler's Three Laws of Planetary Motion
E. The Rudolphine Tables
IV. Modern Astronomy
The Roots of Astronomy
• Already in the stone and bronze ages,
human cultures realized the cyclic
nature of motions in the sky.
• Monuments dating back to ~ 3000 B.C.
show alignments with astronomical
significance.
• Those monuments were probably used
as calendars or even to predict eclipses.
Stonehenge
Summer solstice
Heelstone
• Alignments with
locations of sunset,
sunrise, moonset
and moonrise at
summer and winter
solstices
• Probably used as
calendar.
• Constructed: 3000 – 1800 B.C.
Other Examples All Over the World
Big Horn Medicine Wheel (Wyoming)
Other Examples All Over the World (2)
Caracol (Maya culture, approx. A.D. 1000)
Ancient Greek Astronomers (1)
• Unfortunately, there are no written
documents about the significance of
stone and bronze age monuments.
• First preserved written documents
about ancient astronomy are from
ancient Greek philosophy.
• Greeks tried to understand the motions
of the sky and describe them in terms
of mathematical (not physical!) models.
Ancient Greek Astronomers (2)
Models were generally wrong because
they were based on wrong “first
principles”, believed to be “obvious” and
not questioned:
1. Geocentric Universe: Earth at the
Center of the Universe.
2. “Perfect Heavens”: Motions of all
celestial bodies described by motions
involving objects of “perfect” shape, i.e.,
spheres or circles.
Ancient Greek Astronomers (3)
• Eudoxus (409 – 356 B.C.):
Model of 27 nested spheres
• Aristotle (384 – 322 B.C.),
major authority of philosophy
until the late middle ages:
Universe can be divided in 2
parts:
1. Imperfect, changeable Earth,
2. Perfect Heavens (described
by spheres)
• He expanded Eudoxus’ Model to use 55 spheres.
Eratosthenes (~ 200 B.C.):
Calculation of the Earth’s radius
Angular distance between
Syene and Alexandria:
~ 70
Linear distance between
Syene and Alexandria:
~ 5,000 stadia
 Earth
Radius ~ 40,000
stadia (probably ~ 14 %
too large) – better than
any previous radius
estimate.
Eratosthenes’s Experiment
Later refinements (2nd century B.C.)
• Hipparchus: Placing the Earth away from the centers of the
“perfect spheres”
• Ptolemy: Further refinements, including epicycles
Epicycles
Introduced to explain retrograde
(westward) motion of planets
The Ptolemaic system was considered
the “standard model” of the Universe
until the Copernican Revolution.
Epicycles
The Copernican Revolution
Nicolaus Copernicus (1473 – 1543):
Heliocentric Universe (Sun in the Center)
Copernicus’ new (and correct) explanation
for retrograde motion of the planets
Retrograde
(westward)
motion of a
planet occurs
when the
Earth passes
the planet.
This made Ptolemy’s epicycles unnecessary.
Galileo Galilei (1594 – 1642)
• Invented the modern view of science:
Transition from a faith-based “science” to
an observation-based science.
• Greatly improved on the newly invented
telescope technology. (But Galileo did
NOT invent the telescope!)
• Was the first to meticulously report
telescope observations of the sky to
support the Copernican Model of the
Universe.
Major Discoveries of Galileo
• Moons of Jupiter
(4 Galilean moons)
(What he really saw)
• Rings of Saturn
(What he really saw)
Major Discoveries of Galileo (2)
• Surface structures on the moon; first estimates
of the height of mountains on the moon
Major Discoveries of Galileo (3)
• Sun spots (proving that the
sun is not perfect!)
Major Discoveries of Galileo (4)
• Phases of Venus (including “full Venus”),
proving that Venus orbits the sun, not the Earth!
Johannes Kepler (1571 – 1630)
• Used the precise observational tables of
Tycho Brahe (1546 – 1601) to study
planetary motion mathematically.
• Found a consistent description by
abandoning both
1. Circular motion and
2. Uniform motion.
• Planets move around the sun on elliptical
paths, with non-uniform velocities.
Kepler’s Laws of Planetary Motion
1. The orbits of the planets are ellipses with the
sun at one focus.
c
Eccentricity e = c/a
Eccentricities of Ellipses
1)
2)
e = 0.02
3)
e = 0.1
e = 0.2
5)
4)
e = 0.4
e = 0.6
Eccentricities of Planetary Orbits
Orbits of planets are virtually
indistinguishable from circles:
Most extreme example:
Earth: e = 0.0167
Pluto: e = 0.248
Planetary Orbits (2)
• A line from a planet to the sun sweeps
over equal areas in equal intervals of time.
• A planet’s orbital period (T) squared is
proportional to its average distance from the
sun (R) cubed:
Ty2
= RAU
3
(Ty = period in years;
RAU = distance in AU)
Historical Overview
New Terms
archaeoastronomy
eccentric
uniform circular motion
geocentric universe
parallax
retrograde motion
epicycle
deferent
equant
heliocentric universe
paradigm
ellipse
semimajor axis, a
eccentricity, e
hypothesis
theory
natural law
Quiz Questions
1. Why are Stonehenge and The Big Horn Medicine Wheel
thought to be ancient astronomical observatories?
a. Petroglyphs at each site describe how they were used to
make observations.
b. Ancient Greek writings list the important discoveries made at
each of these two sites.
c. Stones at each site aligned with significant rising and setting
positions.
d. Both a and c above.
e. All of the above.
Quiz Questions
2. Plato proposed that all heavenly motion is
a. constantly changing
b. circular
c. uniform
d. Answers a and b above.
e. Answers b and c above.
Quiz Questions
3. How did Claudius Ptolemaeus account for the retrograde
motion of the planets?
a. Planets slow down, stop, and then reverse their orbital
direction around the Sun.
b. Inner planets orbit the Sun faster and pass outer planets as
they orbit around the Sun.
c. Each planet moves on an epicycle, that in turn moves on a
deferent that circles around Earth.
d. The Sun and Moon orbit Earth, whereas all the other planets
orbit the Sun.
e. None of the above.
Quiz Questions
4. Who accurately determined the size of Earth by considering
Sun angles at Syene and Alexandria?
a. Thales of Miletus (c. 624-547 BC)
b. Pythagoras (c. 570-500 BC)
c. Eudoxus (409-356 BC)
d. Aristotle (384-322 BC)
e. Eratosthenes (c. 200 BC)
Quiz Questions
5. One of the first principles of ancient astronomy is that the
heavens beyond _____ are perfect, and the Earth is corrupt.
a. the atmosphere
b. the Sun
c. the Moon
d. Saturn
e. Pluto
Quiz Questions
6. Who taught that the Earth is stationary and at the center of
the universe with the Sun, the Moon, and the planets moving
around Earth in perfect circles?
a. Thales of Miletus (c. 624-547 BC)
b. Pythagoras (c. 570-500 BC)
c. Eudoxus (409-356 BC)
d. Aristotle (384-322 BC)
e. Eratosthenes (c. 200 BC)
Quiz Questions
7. How did Nicolaus Copernicus account for the retrograde
motion of the planets?
a. Planets slow down, stop, and then reverse their orbital
direction around the Earth.
b. Inner planets orbit the Sun faster and pass outer planets as
they orbit around the Sun.
c. Each planet moves on an epicycle, that in turn moves on a
deferent that circles around Earth.
d. The Sun and Moon orbit Earth, whereas all the other planets
orbit the Sun.
e. None of the above.
Quiz Questions
8. What feature of Aristotle's model of the universe was
included in the model proposed by Copernicus?
a. Earth is stationary and at the center.
b. Mercury and Venus move around the Sun.
c. Mars, Jupiter, and Saturn move around Earth.
d. Uniform circular motion.
e. Elliptical orbits.
Quiz Questions
9. Why did the model of the universe proposed by Copernicus
gain support soon after its publication?
a. It more accurately predicted the position of planets.
b. It gave a better explanation for the phases of the Moon.
c. It was a more elegant explanation of retrograde motion.
d. The old system of Ptolemy was never very popular.
e. It displaced Earth from the center of the universe.
Quiz Questions
10. When Tycho observed the new star of 1572, he could
detect no parallax. Why did that result undermine belief in the
Ptolemaic system?
a. This star is closer than the Moon, and thus stars are not all
at the same distance.
b. This star is closer than the Moon, and thus smaller than
other stars.
c. This star is farther away than the Moon, and thus the
heavens are perfect and unchanging.
d. This star is farther away than the Moon, and thus the
heavens are not perfect and unchanging.
e. This star is planet-like.
Quiz Questions
11. What was the most important contribution of Tycho Brahe to
modern astronomy?
a. The invention of the optical telescope.
b. The discovery of four moons orbiting Jupiter.
c. A model of the universe that was part Aristotelian and part
Copernican.
d. The study of the Supernova of 1572.
e. Twenty years of accurate measurements of planetary
positions.
Quiz Questions
12. How was Tycho Brahe able to make more accurate
astronomical measurements than had been made before his
time?
a. He used a telescope to magnify the image and spacing of
celestial objects.
b. He designed and used large devices to measure small
angles.
c. His island observatory was hundreds of miles offshore,
under very dark skies.
d. His observatory was at high elevation and thus above much
of Earth's atmosphere.
e. All of the above.
Quiz Questions
13. How did Kepler's first law of planetary motion alter the
Copernican system?
a. It changed the perfect circles to ellipses.
b. It added epicycles to the perfect circles.
c. It placed the Sun at one focus of each orbit.
d. Answers a and c above.
e. Answers b and c above.
Quiz Questions
14. Which of the "First Principles of Ancient Astronomy" did
Kepler's laws contradict?
a. Earth is at the center of the universe.
b. The heavens are perfect and Earth is imperfect.
c. All heavenly motion is uniform and circular.
d. Both a and b above.
e. Both a and c above.
Quiz Questions
15. What does Kepler's second law indicate about the orbital
speed of a planet?
a. The orbital speed of each planet is constant.
b. A planet moves at its slowest when it is closest to the Sun.
c. A planet moves at its fastest when it is closest to the Sun.
d. The orbital speed of a planet varies in no predictable way.
e. None of the above.
Quiz Questions
16. If the semimajor axis of a planet is 4 AU, what is its orbital
period?
a. 4 years.
b. 8 years.
c. 16 years.
d. 64 years.
e. It cannot be determined from the given information.
Quiz Questions
17. Galileo's discovery of four moons orbiting Jupiter showed
that planetary bodies could move and carry moons. This
supports the model of the universe presented by
a. Aristotle
b. Claudius Ptolemaeus
c. Nicolaus Copernicus
d. Both a and b above.
e. All of the above.
Quiz Questions
18. What phases of Venus are predicted by the Ptolemaic
system?
a. New and Crescent phases only.
b. Quarter and Gibbous phases only.
c. Gibbous and Full phases only.
d. Crescent and Gibbous phases only.
e. New, Crescent, Quarter, Gibbous, and Full phases.
Quiz Questions
19. What phases of Venus were observed by Galileo?
a. New and Crescent phases only.
b. Quarter and Gibbous phases only.
c. Gibbous and Full phases only.
d. Crescent and Gibbous phases only.
e. New, Crescent, Quarter, Gibbous, and Full phases.
Quiz Questions
20. The phases of Venus observed by Galileo support the
model of the universe presented by
a. Aristotle
b. Claudius Ptolemaeus
c. Nicolaus Copernicus
d. Both a and b above.
e. All of the above.
Answers
1.
2.
3.
4.
5.
6.
7.
8.
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10.
c
e
c
e
c
d
b
d
c
d
11.
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17.
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19.
20.
e
b
d
e
c
b
c
a
e
c