Transcript class 4, F10 (ch. 2c and 3)

Class 4 (Sept 2, 2010): End of Ch 2 and Ch. 3
Last time:
2.1….. celestial coordinates
2.2 Seasons,…Precession
2.2 (cont.). 2.3. The Moon and Eclipses
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Lunar Phases and Eclipses
Today:
2.4 Ancient Mystery of the Planets:
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Apparent Retrograde motion of planets
Parallax
Ch 3 (Histrory of Astronomy)
2.4 The Ancient Mystery of the Planets
• Why do planets sometimes seem to move
backwards relative to the stars?
Apparent retrograde motion —
try it yourself!
Retrograde Motion
• Planets usually appear to move
eastward relative to the stars.
• But as we pass them by in our
orbit, they move west relative to
the stars for a few weeks or
months.
 Noticeable over many nights
Explaining Apparent Retrograde
Motion
• Easy for us to explain: occurs when we
“lap” another planet (or when Mercury or
Venus lap us)
• But very difficult to explain if you think that
Earth is the center of the universe!
• In fact, ancients considered but rejected
the correct explanation…
Why did the ancient Greeks reject
the notion that the Earth orbits the
Sun?
• It ran contrary to their senses.
• Greeks knew that we should see stellar
parallax if we orbited the Sun – but they
could not detect it.
Parallax Angle
Apparent shift of a star’s position due to the
Earth’s orbiting of the Sun
The nearest stars are
much farther away than
the Greeks thought.
So the parallax angles of
the star are so small, that
you need a telescope to
observe them.
Possible reasons why ancient Greeks
could not detect stellar parallax :
1. Stars are so far away that stellar parallax is
too small for naked eye to notice
2. Earth does not orbit Sun; it is the center of
the universe
Unfortunately, with notable exceptions like Aristarchus, the
ancient Greeks did not think the stars could be that far
away, and therefore rejected the correct explanation
(1)…
Thus setting the stage for the long, historical showdown
between Earth-centered and Sun-centered systems.
Chapter 3
The Science of Astronomy
(I prefer to call it History of Astronomy)
Ch 3 (Histrory of Astronomy)
1.
2.
Ancient Roots of Science (Archeoastronomy)
Astronomy of Greece: good records & more rational
approach
a. Thales and Pithagoras: roots of science
b. Plato and Aristotle: Geocentric universe
c. Ptolemy: mathematical models of geocentric views
3.
4.
The Copernican revolution
The Nature of Science
Ch 3 (Histrory of Astronomy)
1.
2.
Ancient Roots of Science (Archeoastronomy)
Astronomy of Greece: good records & more rational
approach
a. Thales and Pithagoras: roots of science
b. Plato and Aristotle: Geocentric universe
c. Ptolemy: mathematical models of geocentric views
3.
4.
The Copernican revolution
The Nature of Science
Histrory of Astronomy is more like a Soap Opera than a Sitcom
Ch 3 (Histrory of Astronomy) “Does the World Turn?”
1.
2.
Ancient Roots of Science (Archeoastronomy)
Astronomy of Greece: good records & more rational
approach
a. Thales and Pithagoras: roots of science
b. Plato and Aristotle: Geocentric universe
c. Ptolemy: mathematical models of geocentric views
3.
4.
The Copernican revolution
The Nature of Science
Ch 3 (Histrory of Astronomy) “Does the World Turn?”
1.
2.
Ancient Roots of Science (Archeoastronomy)
Astronomy of Greece: good records & more rational
approach
a. Thales and Pithagoras: roots of science
b. Plato and Aristotle: Geocentric universe
c. Ptolemy: mathematical models of geocentric views
3.
4.
The Copernican revolution “The Church Strikes back”
The Nature of Science
3.1 The Ancient Roots of Science
(Archeoastronomy)
How did astronomical observations
benefit ancient societies?
• Keeping track of time and seasons
– for practical purposes, including
agriculture
– for religious and ceremonial purposes
• Aid to navigation
3.1 The Ancient Roots of Science
(Archeoastronomy)
What did ancient civilizations
achieve in astronomy?
• daily timekeeping
• tracking the seasons and calendar
• predicting eclipses
• and more…
Days of week were named for Sun, Moon, and visible planets
England: Stonehenge (completed around 1550 B.C.) was used as an
observatory
Yucatan, Mexico: Mayan Observatory at Chichen Itza
3.2 Ancient Greek Science
Our goals for learning:
• 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?
Why does modern science trace its roots to the
Greeks? (some not in book)
•
Greeks: good records and
more rational approach than
previous civilizations
•
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 BC)
Prominent Ancient Greeks (some not in book):
•
Thales and Pithagoras: roots of science
a.
b.
•
•
Plato
Thales: Universe is rational (our mind can understan it)
Pithagoras: nature is governed by musical (mathematical)
principles
Plato and Aristotle: Geocentric universe
⁻
Earth at center of universe
⁻
Earth is imperfect Heavens are perfect
⁻
“uniform circular motion” is perfect. Hence, heavenly bodies
must follow uniform circular motion
Ptolemy: mathematical models of geocentric views (sufficiently
accurate to remain in use for 1,500 years)
Aristotle
Ptolemy
Ptolemy: mathematical models of geocentric views,
but this made it difficult to explain apparent
retrograde motion of planets…
So how does the Ptolemaic model explain retrograde motion?
Planets do go backward in this model…
Ptolemy’s
Model
What have we learned?
•Why does modern science trace its roots to the Greeks?
Greeks: good records and more rational approach than
previous civilizations.
•How did the Greeks explain planetary motion?
The Ptolemaic model had each planet move on a small
circle whose center moves around Earth on a larger circle.
•How did Islamic scientists preserve and extend Greek
science?
While Europe was in its Dark Ages, Islamic scientists
preserved and extended Greek science, later helping to
ignite the European Renaissance.
Much of our mathematical and scientific heritage
originated with the civilizations of the Middle East
3.3 The Copernican Revolution
Histrory of Astronomy PART II: “The Church
Strikes Back”
1.
2.
3.
4.
5.
Problems with Ptolemy’s models
Copernicus: publishes heliocentric model and dies
Tycho observes planetary motions and dies
Kepler: uses Tycho’s observations & writes 3 laws (on
pages 67-69 of book):
a. Each planet moves in ellipse with the Sun at one focus.
b. The line between the Sun ……. (faster near Sun and vice versa)
c. p2 = a3
Galileo:
a.Uses telescope to discover moons of Jupiter, study
sunspots and phases of Venus
b.Supports Copernicus’ theory & gets in trouble with
Church (bad boy! go to your room and stay there till you die)
Copernicus’ Heliocentric Model
•With
better observations more problems
with the Ptolemaic model became
apparent…..and created more complex
fixes
• Copernucus’ solution was to put the Sun
at the center.
•The math was much simpler in
Copernucus’ solution
Copernicus’
Model
Ptolemy’s
Model
Copernicus’ Heliocentric Model
•Sun
is at center
•Earth orbits like any other planet
•Inferior planet orbits are smaller
•Retrograde motion occurs when we “lap”
Mars & the other planets
Occam’s Razor
• Among competing theories
(otherwise equal), the best theory is
the simplest theory-that is, the one
with the fewest assumptions.
• Heliocentric model is much simpler than
Ptolemy’s model
• Father of observational
astronomy.
• Danish nobleman and
Scientist. Lost his nose in a
Tycho Brahe
duel…. (ask me how he died)
• He had a young assistant,
Johannes Kepler
Johannes Kepler
• German astronomer and
mathematician
• Used Tycho’s observations to
write 3 laws of planetary
motion
• He was deeply religious and actually
strongly believed in the predictive power
of astrology (interesting characters in our
soap opera, don’t you think so?)
Kepler’s Laws (on page 67-69 of book)
1 Each planet’s orbit around the Sun is an
ellipse, with the Sun at one focus.
Kepler’s Laws
2 A planet moves along its orbit with a speed that
changes in such a way that a line from the planet to
the Sun sweeps out equal areas in equal intervals of
time. (the closer to the Sun, the faster it moves)
Kepler’s Laws
3 The ratio of the cube of a planet’s average distance
from the Sun to the square of its orbital period is
the same for each planet.
3
2
a /P = 1
3
a =P
a in AU
P in years
2
• Kepler first tried to match Tycho’s
observations with circular orbits
• But an 8 arcminute discrepancy led him
eventually to ellipses…
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.
Galileo Galilei (1564-1642)
• First man to point a
telescope at the sky
• wanted to connect
physics on Earth with
the heavens
• Wrote the book
“Dialogue Concerning
the Two Chief World
Systems” .
This book got him in trouble with the Church!
Go to your room!
Galileo’s Observations
• Galileo discovered
that Jupiter had four
moons of its own.
• Jupiter was the
center of its own
system.
• Heavenly bodies
existed which did not
orbit the earth.
Galileo’s observation of the phases of Venus was the
final evidence which buried the geocentric model.
Geocentric
No gibbous or full phases!
Heliocentric
All phases are seen!
Galileo observed all phases!
The Catholic Church ordered
Galileo to recant his claim
that Earth orbits the Sun in
1633
His book on the subject was
removed from the Church’s
index of banned books in
1824
Galileo Galilei
Galileo was formally
vindicated (pardoned) by the
Church in 1992 (ask me if
you want to know more juicy
details)
What have we learned?
• How did Copernicus, Tycho and Kepler challenge
the Earth-centered idea?
• Copernicus created a sun-centered model; Tycho
provided the data needed to improve this model;
Kepler found a model that fit Tycho’s data.
• What was Galileo’s role in the Copernican
revolution?
• His experiments and observations overcame the
remaining objections to the Sun-centered solar
system
Ch 3 (Histrory of Astronomy) “Does the World Turn?”
1.
2.
Archeoastronomy
Astronomy of Greece: good records & more rational approach
a. Thales and Pithagoras: roots of science
b. Plato and Aristotle: Geocentric universe
c. Ptolemy: mathematical models of geocentric views
3.
The Copernican revolution “The Church Strikes back”
4.
The Nature of Science
3.4 The Nature of Science
Our goals for learning:
How can we distinguish science from
nonscience?
Science and Pseudoscience
(not exactly like the book)
I.
Science:
1.
2.
3.
II.
Based on observations and theory
Open to criticism and constant challenge in light of
new evidence
Not perfect (human), not always right, but is the most
successful discipline at predicting the way nature
works.
Pseudoscience
1.
2.
3.
Generally holds “absolute truth”
Considers only part of evidence available
Tends to play on emotions and fears instead of logic
Hallmarks of Good Science
• Science seeks explanations that rely solely on natural
causes.
• Science progresses by creating and testing models of
nature that explain observations as simply as possible:
Occam’s Razor
• A scientific model must make testable predictions that
could force us to revise or abandon the model.
• Scientific Theory: a model which survives repeated
testing
Astrology (not exactly like book)
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Ancient Astronomers were also Astrologers
(e.g., Kepler)
Astrology is not a science
Empirical Discipline (no theoretical explanation)
Easily abused by charlatans
Big problem: astrologers do not agree on what
any given celestial configuration means
In other words, who will you believe if one
astrologer tells you this is a great time to travel
and another tells you to stay home?