Transcript Lecture 5 2014 Sept. 11 • The Nature of Scientific Theories • Giants of Scientific Thinking • Astrology The Planets Prof.

Lecture 5
2014 Sept. 11
• The Nature of Scientific Theories
• Giants of Scientific Thinking
• Astrology
The Planets
Prof. Geoff Marcy
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Announcements
• Read Chapter 3 !
Homework Assignment:
- Assignment Chapters 3: Due Fri at 6pm
Get your ABCD cards ready.
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Observing Project #1:
Monitoring the Motion of Mars
Chart the position of Mars in the night sky relative to Saturn and the star, Antares.
Observe Mars, Saturn, and Antares in the evening sky three times (at least once per week),
between now (Sept 9) and Sept 30.
Draw a sketch each evening (or take a photo), showing the positions of
Antares, Mars, and Saturn as dots. Record the time and date on the sketch.
After you have made your three (or more) sketches, write one to two paragraphs
explaining your observations of the position of Mars. Did it move?
If so, why did it move the way it did?
Due Thursday, October 2.
Sept. 5, 2014
Tycho Brahe (1546-1601)
• Best Observer of the position of
Mars (including retrograde
motion)!
• Danish King, Fredrick II, gave him the
entire island of Hveen for observations.
• Even better charts of planets positions.
• Johannes Kepler wanted Brahe’s data,
to explain Mars’s orbit.
Another astronomer wanted Brahe’s data:
Frank Tengnagel
Tycho Brahe
(1546-1601)
Body Exhumed in 2010
Johannes Kepler (1571-1630)
• Greatest theorist of
his day
• Imagined planets on
“heavenly spheres”
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Assuming a circular orbit for Mars, Kepler could not match Brahe’s
precise measurement of Mars’ orbit. A tiny difference of 8 arc minutes
(0.13o) remained. So he abandoned “uniform” and “circular” motion.
Each planet’s orbit around the Sun is an
ellipse, with the Sun at one focus.
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Kepler’s Laws
1. Each planet’s orbit around the Sun is an
ellipse, with the Sun at one focus.
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Eccentricity of an Ellipse
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Kepler’s 2nd Law
A planet’s speed changes predictably:
A line from the planet to the Sun sweeps out
equal areas in equal intervals of time.
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Kepler’s 3rd Law
The cube of a planet’s average distance from the Sun is equal
to the square of its orbital period.
Orbital period:
P
planet’s average distance: a
2
(measure in years)
(measure in AU)
P =a
3
a
More precisely: a is the semi major axis
Question:
An asteroid orbits the Sun at an average distance
a = 4 AU. How long does it take to orbit the Sun?
A.
B.
C.
D.
4 years
8 years
16 years
64 years
Hint: Remember that P2 ~ a3
Question:
An asteroid orbits the Sun at an average distance
a = 4 AU. How long does it take to orbit the Sun?
A.
B.
C.
D.
4 years
8 years
16 years
64 years
We need to find P so that P2 = a3.
Since a = 4, a3 = 43 = 64
Therefore P= 8, P2 = 82 = 64
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Calculate the orbital period of:
Planet
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto
a (AU)
0.4
0.7
1.0
1.5
5.0
10
20
30
50
Period (yrs)
92 days = 0.25 yr
214 days = 0.59 yrs
1.0
1.8
11.2
32
89
164
354
WHY do planets move on
elliptical orbits?
Astronomers Kepler, Halley, Wren, and
Hooke proposed an inverse square law of
gravity:
Force of gravity ~ 1 / distance2
In 1684, Sir Christopher Wren issued a small prize
(40 shillings) for the first person who could prove that
Kepler’s first law follows from an the gravity law
above.
In August of 1684, Halley met Isaac Newton in Cambridge…
How did Newton change our view of the Universe?
Sir Isaac Newton
(1642-1727)
• Realized the same physical
laws that operate on Earth
also operate in the heavens
 one universe
• Discovered laws of motion
and gravity
• Much more: experiments
with light; first reflecting
telescope (using mirrors
rather than lenses)
• Invented calculus in parallel
to Friedrich Leibnitz in
Germany
1. What determines the strength of
gravity?
The Universal Law of Gravitation
1. Every mass attracts every other mass.
2. Attraction is directly proportional to the product of their
masses.
3. Attraction is inversely proportional to the square of the
distance between their centers..
Question:
Is the force the Earth exerts on you larger,
smaller, or the same as the force you exert
on it?
A. Earth exerts a larger force on you.
B. I exert a larger force on Earth.
C. Earth and I exert equal and opposite
forces on each other.
Question:
Is the force the Earth exerts on you larger,
smaller, or the same as the force you exert
on it?
A. Earth exerts a larger force on you.
B. I exert a larger force on Earth.
C. Earth and I exert equal and opposite
forces on each other.
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!
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The Scientific Method
1 Question
2 Hypothesis
– a tentative explanation
3 Prediction
4 Test
5 Result
– confirm, reject, or modify
should be the same no matter who conducts the test
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Hallmarks of Good Science
• Science seeks explanations for observed phenomena that
rely on natural causes.
• Science progresses through the construction and testing
of models of nature that explain the 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.
Theory
-- a concept or set of ideas (a “model”) that
survives repeated testing
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Bad Scientific Practice
• pseudoscience – beliefs that masquerade as
science, but do not change as new evidence
comes in that contradicts those beliefs.
• nonscience – establishes “truths” through
belief and faith. Tests are not done.
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Astrology
• Claims the positions of the Sun, Moon, & planets
determines each person’s personality and each
person’s future.
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Astrology
Theory:
The positions of the planets, sun, and moon
at the time of your birth determine your
personality and your future every day.
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Signs of the Zodiac
AQUARIUS January 19-February 17
PISCES February 18-March 19
ARIES March 20-April 18
TAURUS April 19-May 19
GEMINI May 20-June 19
CANCER June 20-July 21
LEO July 22-August 21
VIRGO August 22-September 21
LIBRA September 22-October 22
SCORPIO October 23-November 20
SAGITTARIUS November 21-December 20
CAPRICORN December 21-January 18
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Horoscope: Sept. 11, 2014
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Test of Astrology
• Write your “sign” of the zodiac
on one sheet of paper.
Signs of the Zodiac
AQUARIUS January 19-February 17
PISCES February 18-March 19
ARIES March 20-April 18
• Choose the horoscope number that
best describes your day today
TAURUS April 19-May 19
GEMINI May 20-June 19
CANCER June 20-July 21
LEO July 22-August 21
• Write that number on your paper.
VIRGO August 22-September 21
LIBRA September 22-October 22
SCORPIO October 23-November 20
SAGITTARIUS November 21-December 20
CAPRICORN December 21-January 18
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Horoscope: Sept. 11, 2014
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Existing Astrology Theory:
You can predict your future.
If Theory is True:
You will recognize your horoscope: It pertains to you!
If Theory is not true.
You will choose a random horoscope.
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Horoscope: Sept. 11, 2014
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Stand
up
Stand
up
Stand
up
Stand
up
ififyou
chose
#1
Stand
up
you
chose
#2
Stand
up
ififyou
chose
#3
Stand
up
you
chose
#4
Stand
up
ififyou
chose
#5
Stand
up
you
chose
Stand
up #6
ififyou
chose
#7
you
chose
#8
ififyou
chose
you chose#9
#10
#11
#12
Theory: Astrology predicts your day
The horoscope for your sign of the zodiac
allows you to predict your future.
If Theory is True:
People will recognize their horoscope.
If Theory is not true.
People will not recognize their own horoscope.
Only 1/10 of you recognized your horoscope!
Astrology, as a theory, didn’t match real world.
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Astrology:
What is the Mechanism
that makes it work?
What force makes the planets determine your entire life?
What is the “energy” that makes planets control your life?
What force exerted by planets acts on you at the
moment of birth to determine the rest of your life?
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Astrology:
Invent your own
Why do only planets and moons affect your life?
Other objects exert great gravitational forces on you, such as
nearby buildings, aircraft, mountains.
Quiz: Invent your own “astrology” based on the positions
of some other objects around you.
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