Transcript Unit 01 Slides - Chapter 3

Chapter 3
The Science of
Astronomy
How is modern science rooted in ancient astronomy?
Macchu Pichu, Peru: Structures aligned with solstices.
© 2014 Pearson Education, Inc.
Cosmogony
• A cosmogony is theory about Earth’s
place in the universe.
• A geocentric cosmogony is a theory that
proposes Earth to be at the center of the
universe.
• A heliocentric cosmogony is a theory that
proposes the Sun to be at the center of the
universe.
Which is the geocentric cosmogony and
which is the heliocentric cosmogony?
geocentric (Earth-centered)
heliocentric (Sun-centered)
Planets were often called wandering stars
because they seem to move from one
constellation to the next.
Planets Known in Ancient Times
• Mercury
– difficult to see; always close to
Sun in sky
• Venus
– very bright when visible;
morning or evening “star”
• Mars
– noticeably red
• Jupiter
– very bright
• Saturn
– moderately bright
What was once so mysterious
about planetary motion in our sky?
• Planets usually move slightly eastward from night to
night relative to the stars.
• But sometimes they go westward relative to the stars for
a few weeks: apparent retrograde motion.
We see apparent retrograde motion
when we pass by a planet in its orbit.
Explaining Apparent Retrograde Motion
• Easy for us to explain: occurs when we
“lap” another planet (or when Mercury or
Venus laps 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
real explanation for planetary motion?
• Their inability to observe stellar parallax was a major factor.
Scientists use parallax to measure distances.
The Greeks knew that the lack of observable
parallax could mean one of two things:
1.
Stars are so far away that stellar parallax is too small
to notice with the naked eye.
2.
Earth does not orbit the Sun; it is the center of the
universe.
With rare exceptions such as Aristarchus, the Greeks
rejected the correct explanation (1) because they did
not think the stars could be that far away.
Thus, the stage was set for the long, historical showdown between Earthcentered and Sun-centered systems.
Ancient Science
• The ancient Greeks were the first to seek to explain
nature by means of logic and geometry, and not resort to
supernatural explanations.
• We must not forget that besides the Greeks, there were
many cultures that contributed to the advancement of
science and astronomy.
• Egyptian and Mayan measurements of time. Polynesian
navigation. Chinese and Islamic astronomers, to mention
a few!
• While astronomy and astrology both grew out of the
same source, astrology cannot stand up to the rigors of
modern science.
Our mathematical and scientific heritage originated with the
civilizations of the Middle East.
The Greeks advanced the concept of
Science, and moved away from
superstition and mythology. For 700
years, the Greeks postulated such ideas
as the Earth is a sphere, that the
elements are composed of basic bits,
called atoms, and that planets existed
and circled in the heavens.
Aristotle argued for a Earth-centered
universe.
Eratosthenes accurately estimated the
circumference of the Earth.
Around 150 AD, Ptolemy put forth the
first published work detailing a
geocentric model of the Universe.
Special Topic: Eratosthenes Measures Earth
(c. 240 B.C.)
Measurements:
Syene to Alexandria
distance ≈ 5000 stadia
angle = 7°
Calculate circumference of Earth:
7/360  (circum. Earth) = 5000 stadia
 circum. Earth = 5000  360/7 stadia ≈ 250,000 stadia
Compare to modern value (≈ 40,100 km):
Greek stadium ≈ 1/6 km  250,000 stadia ≈ 42,000 km
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
The Copernican Revolution
Copernicus devised the first
comprehensive heliocentric
cosmology to successfully explain
retrograde motion
sidereal period is
the time it takes a
planet to orbit the
Sun once.
synodic period is
the time that elapses
between two
successive identical
configurations as
seen from Earth
(e.g., time from
opposition to
opposition)
Tycho Brahe measured distances using parallax
that disproved ancient ideas about the heavens
A supernova in 1572 was shown to exist in the
distant heavens; this troubled scholars who
previously thought the heavens were
unchanging.
He showed that comets were objects that
occurred in the region of the planets, not in
Earth’s atmosphere.
• Compiled the most accurate (one arcminute)
naked eye measurements ever made of
planetary positions.
• Still could not detect stellar parallax, and
thus still thought Earth must be at center of
solar system (but recognized that other
planets go around Sun).
• Hired Kepler, who used Tycho’s
observations to discover the truth about
planetary motion.
Tycho Brahe (1546-1601)
• Kepler first tried to match Tycho’s
observations with circular orbits
• But an 8-arcminute discrepancy led him
eventually to ellipses.
“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.”
Johannes Kepler
(1571-1630)
Kepler’s First Law: The orbit of a planet about the Sun
is an ellipse with the Sun at one focus.
The distance between the two foci impact the eccentricity of the
ellipse’s shape.
Kepler’s Second Law: A line joining a planet
and the Sun sweeps out equal areas in equal
intervals of time.
Kepler’s Third Law: The square of a planet’s sidereal
period is proportional to the cube of the length of its orbit’s
semimajor axis (p2=a3).
Italian
scientist
Galileo made
discoveries
that strongly
supported a
heliocentric
cosmology
Galileo’s telescope revealed phases of Venus
which could only occur IF Venus orbits the Sun.
Galileo’s telescope
revealed that Jupiter
had moons which
orbited Jupiter instead
of Earth.
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 was formally vindicated by the Church in 1992.
Isaac Newton
formulated three
laws that
describe
fundamental
properties of
physical reality
Newton’s Three Laws of Motion
• A body remains at rest or moves in a straight
line at a constant speed unless acted upon by
an unbalanced outside force.
• Force = mass x acceleration
• Whenever one body exerts a force on a second
body, the second body exerts an equal and
opposite force on the first body.
Newton’s description of gravity
accounts for Kepler’s laws
Newton’s universal law of gravitation states: Two
bodies attract each other with a force that is directly
proportional to the square of the distance between
them.
– This law mathematically proves Kepler’s Third Law (p2=a3).
– This law describes various orbits objects can take when
moving near the Sun.
– This law can be used to predict when comets, such as
Halley’s comet, will pass near Earth.
Video
• Early Astronomers
Science vs. Myth
An introduction to the scientific
method, skepticism, hoaxes
& hokum
The Scientific Method
Observing the Universe led to the scientific method. As
astronomers sought to explain the patterns of the motion of the
planets, they developed a methodology that set science apart
from pseudo-science.
Science: The discovery of natural explanations for what we
observe and measure in the natural world, and can experiment on
in the laboratory.
It is an amazing realization that the Universe is knowable! Our
understanding of nature is growing exponentially. There is much
we don’t know, but we no longer accept that it can’t be known. In
time, we will learn more, and we will fill in the gaps in our
knowledge. We no longer need to invent explanations, through
the application of the Scientific Method, we know that if it can be
explained it will be explained in time.
The Scientific Method
• The scientific method can be used to predict how physical systems
will behave.
• Scientists first create a scientific theory, based on mathematical
models or observation.
• Then scientists make predictions using this theory.
• Scientists then perform experiments and make observations that
either agree with their theory or disprove it.
• Scientific theories are based on independent and verifiable testing,
not on personal belief systems.
Remember The
Scientific Method
--Observation and
Experiments are
used to Support
or Reject a
Theory
Not Scientific
How can we tell?
1. Some is clearly made up.
2. Images on the moon are presumed to
be pyramids, without more than a grainy
picture and an elaborate “pyramid
scheme” has been concocted.
Occam’s Razor says the most likely
explanation is the simpler one, as long
as it agrees with all the evidence.
We don’t choose an explanation that
makes us happy or fits our
preconceived ideas, if another theory is
better. Peer review helps to remove this
type of bias.
Pseudoscience
• Pseudoscience includes astrology, myths and belief
systems that cannot be confirmed or disproven by
science.
• There is insufficient evidence to “prove” UFOs or ghosts
or demons exist. There is no scientific theory that can be
applied and tested. As soon as we have
measurable/testable evidence for one of these, we can
apply the scientific method.
Video
• Carl Sagan on Astrology
Science & Common Sense
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Science explains natural phenomena with formulae that are tested and
superior to non-scientific answers. But, scientific answers do not always
agree with “Midwestern Common Sense”.
Why? We have a lot of experience with the world we live in, but we don’t
have experience being the size of an atom, or as massive as a star or as
fast as a light beam. Quantum Mechanics is incredibly accurate at making
predictions, yet it is hard to reconcile with our common sense.
Just because something doesn’t agree with our gut instinct doesn’t mean
we should reject it out of hand.
Example: A child has a hard time understanding the law of gravity. When
told that people live on the other side of the world, and the world is shaped
like a beach ball, the small child might wonder why the people don’t just fall
off the Earth!
Our common sense is biased towards our experiences.
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Video
• Michael Shermer on Cognitive Biases
• Perception Test
Conclusions
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Do you understand the scientific method, and how astronomy contributed to
its development and human history?
Read the handouts and consider examples of pseudoscience and why they
are rejected by the scientific method.
Think of the arguments made by people to justify their non-scientific beliefs.
Can you use Carl Sagan’s “Baloney Detection” to recognize them? [Bring
an example to our next class.]
Consider your place in the cosmos. Do you have a good perspective of the
scale of the universe? The size of atoms, planets, stars, galaxies and the
vast distances between them?
Consider the 13.7 billion year history of the cosmos. Think of how that
history fits onto the Cosmic Calendar, and how short a span of time
humans have existed.
Do you understand how looking farther out in space allows us to look
farther back in time?
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References
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www.badastronomy.com
www.randi.org
www.skeptic.org
www.scicop.org
www.badastronomy.com/bad/misc/astrology.html
• Carl Sagan’s “The Demon Haunted World:
Science as a Candle in the Dark”, ISBN 9780345409461.
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