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Charles Hakes
Fort Lewis College
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Prologue
Seasons
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Fort Lewis College
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Outline
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Logistics
Review
HW
Seasons
Parallax
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Fort Lewis College
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Logistics
• Note Conversion practice page
• Get your folders here if you don’t have one yet.
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Fort Lewis College
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Scale of the Universe
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Fort Lewis College
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Powers of Ten
• http://www.wordwizz.com/pwrsof10.htm
• http://antwrp.gsfc.nasa.gov/apod/astropix.html
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Fort Lewis College
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Introduction
Scientific Theory
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Fort Lewis College
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Scientific Theory
• Theory - the framework of ideas and
assumptions used to explain some set of
observations and make predictions about
the real world.
• Can prove them wrong by a single bad
prediction.
• Can’t ever prove them “right.” They just get
more widely accepted.
• Eventually, some theories might be called
“law,” (e.g. gravity) but they are still just
scientific “theories.”
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Fort Lewis College
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Scientific Theory
• Must be testable.
• Must continuously be tested.
• They should be simple.
• Occam’s Razor - if two competing theories both explain
the facts, then the simpler one is better.
• KISS engineering - “Keep It Simple, Stupid.”
• They should be elegant.
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Fort Lewis College
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Discovery 1-1a
The Scientific Method
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Fort Lewis College
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Is it possible to prove or disprove a
scientific theory?
A) Yes prove, yes disprove
B) Yes prove, no disprove
C) No prove, yes disprove
D) No prove, no disprove
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Fort Lewis College
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Which statement about scientific theories
is true?
A) More complicated theories are usually better
B) A theory is proven true if it makes correct predictions
C) A theory must be continually tested
D) Both B and C
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Fort Lewis College
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SETI@home
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Fort Lewis College
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Chapter 18
Is There Intelligent Life Elsewhere in the Universe?
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Fort Lewis College
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Are we alone in the Milky Way?
A) definitely
B) probably
C) maybe
D) probably not
E) no way
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Fort Lewis College
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The Drake Equation
• In science, the complete lack of any useful data
does not always stop us. Instead, we proceed
anyway, with a specialized equation to
characterize our ignorance!
• The number of advanced civilizations in our
Galaxy with whom we can communicate is:
N = R fp np fl fi ft L
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Fort Lewis College
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Figure 18.7
Drake Equation
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Fort Lewis College
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The Drake Equation
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R is the average rate of star formation (~10 stars/year)
fp is the fraction of stars with planets (current evidence - nearly 1; that is,
~100% of stars form with planets)
np is the habitable planets per star with planets (the Chaisson textbook
estimates 0.1)
fl is the fraction of those planets with life (?!?)
fi is the fraction of those life-bearing planets with at least one intelligent
species (?!?)
ft is the fraction of planets with an intelligent species in which that species
develops technology capable of interstellar radio communication (?!?)
L is the average lifetime of a technological civilization (our society has been
capable of radio communication for only about 100 years; how much longer
will we remain so? Are we a good model for other possible civilizations?)
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Fort Lewis College
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The Drake Equation
• Make your own estimate for the
unknown values and calculate and
answer for the Drake equation.
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Fort Lewis College
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How many technological civilizations are there
in the Milky Way?
(N = R fp np fl fi ft L)
A) 0
B) 1-9
C) 10-99
D) 102-104
E) more than 104.
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Fort Lewis College
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The Drake Equation
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Suppose all the f terms that aren't specified above are nearly 1 (or
100%). (optimistic!) If we do that, then the lifetime of a civilization
(in years) roughly equals the number of societies in our Galaxy. So
if a technological society lasts for 100 years say, there would be
100 of them in our Galaxy.
However, even if there are 100 other civilizations capable of radio
communication in our Galaxy, and even if they're interested in
talking to us, the average distance between civilized worlds is about
10,000 light years - so it would take 20,000 years to get a response
to any message we send.
Even if we suppose that there are 1 million civilizations out there,
they'd still be separated by about 300 light years!
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Fort Lewis College
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The Drake Equation
• Go look for ET on your own!
• Run SETI@home on your computer, see:
http://faculty.fortlewis.edu/hakes_c/
• SETI@home is a scientific experiment that uses Internetconnected computers in the Search for Extraterrestrial
Intelligence (SETI). You can participate by running a free
program that downloads and analyzes radio telescope
data.
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Fort Lewis College
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Earth’s Orbital Motion
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Fort Lewis College
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Earth’s Orbital Motion
• Day to day changes
• Seasonal changes
• Long term changes
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Fort Lewis College
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Day to Day Changes
• Solar Day
• Time from one noon to the next
• 24hrs
• Sidereal Day
• Time that a star passes directly overhead until it
does so again.
• Less than 24 hrs.
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Fort Lewis College
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Figure P.5
Solar and Sidereal Days
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Fort Lewis College
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Solar vs. Sidereal day
• Edmund Scientific Star and Planet Finder
• The view of the night sky changes during the year.
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Fort Lewis College
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Figure P.6
The Zodiac
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Fort Lewis College
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Seasonal changes
• One sentence - why we have seasons.
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Fort Lewis College
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Seasonal changes
• What about seasonal changes in temperature?
• Let's propose a (wrong) theory: “the Earth is
closer to the Sun in summer”
• What testable predictions can we make? (E.g.,
what is life like on the equator? Durango? Alaska?
the North Pole? the southern hemisphere?) Any
successful theory will make correct predictions
regarding length of day and temperature, which
together define the seasons.
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Fort Lewis College
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Theory: “the Earth is closer to the
Sun in summer”
Where on Earth would it be warmer at
perihelion (closest approach to the Sun)?
A) Northern Hemisphere
B) Southern hemisphere
C) Everywhere at the same time.
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Fort Lewis College
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Seasonal changes
• Where on Earth would it be warmer at perihelion
(in January)? The answer is everywhere, unlike
the reality of our world's seasons which vary by
hemisphere.
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Fort Lewis College
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Seasonal changes
• Where on Earth would it be warmer at perihelion
(in January)? The answer is everywhere, unlike
the reality of our world's seasons which vary by
hemisphere.
• This is a huge piece of knowledge, which most
Americans get wrong! I absolutely insist that you
all do better.
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Fort Lewis College
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Seasonal changes
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Fort Lewis College
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Seasonal changes
• Let's look at a different view. The left frame shows
our initial theory, with no tilt. The right frame adds
a tilt to the Earth's rotation axis.
•
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Fort Lewis College
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Figure P.8
Seasons
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Fort Lewis College
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Seasonal changes
• When the sun is high, the light rays are more
concentrated - the sun feels hotter.
• When “your” hemisphere is pointed towards the
sun, it receives more daylight hours compared to
nighttime hours.
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Fort Lewis College
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Seasonal changes
• From the point of view of the Earth, the path of the
Sun in the sky appears tilted compared to Earth’s
equator.
• Ecliptic - The apparent path of the sun on the
celestial sphere during the year.
• Equinoxes - Two points where the ecliptic crosses
the celestial equator.
• Vernal equinox (first day of Spring ~Mar. 21)
• Autumnal equinox (first day of fall ~Sept 21)
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Fort Lewis College
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Figure P.7
Ecliptic
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Fort Lewis College
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Long Term Changes
• The Earth’s tilt wobbles
• Precession takes ~26,000 years.
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Fort Lewis College
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Figure P.9
Precession
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Fort Lewis College
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The angle that the North Star (Polaris) makes with the
horizon (its’ height above the horizon) changes noticeably
A) as hours go by during the night
B) as months go by during the year
C) as you change latitude on Earth
D) as you change longitude on Earth
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Fort Lewis College
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The angle that the North Star (Polaris) makes with the
horizon (its’ height above the horizon) changes noticeably
A) as hours go by during the night
B) as months go by during the year
C) as you change latitude on Earth
D) as you change longitude on Earth
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Fort Lewis College
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Northern spring (March 21 to June 21) and autumn (Sept
21 to Dec 21) are the hottest seasons of the year at
A) Nowhere.
B) The tropic of cancer.
C) The tropic of capricorn.
D) The equator.
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Fort Lewis College
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Northern spring (March 21 to June 21) and autumn (Sept
21 to Dec 21) are the hottest seasons of the year at
A) Nowhere.
B) The tropic of cancer.
C) The tropic of capricorn.
D) The equator.
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Fort Lewis College
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Where along the horizon does the Sun rise on
June 21 in Durango, Colorado?
A) North of east
B) Due east
C) South of east
D) Can’t tell with information given
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Fort Lewis College
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Where along the horizon does the Sun rise on
June 21 in Durango, Colorado?
A) North of east
B) Due east
C) South of east
D) Can’t tell with information given
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Fort Lewis College
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Where along the horizon does the Sun rise on
June 21 in Sydney, Australia?
A) North of east
B) Due east
C) South of east
D) Can’t tell with information given
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Fort Lewis College
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Where along the horizon does the Sun rise on
June 21 in Sydney, Australia?
A) North of east
B) Due east
C) South of east
D) Can’t tell with information given
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Fort Lewis College
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You carefully measure the height of Polaris from
Durango and from Grand Junction to the north.
A) Polaris appears higher in Durango
B) Polaris appears higher in Grand Junction
C) Polaris is the same height in both places
D) not enough information
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Fort Lewis College
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You carefully measure the height of Polaris from
Durango and from Grand Junction to the north.
A) Polaris appears higher in Durango
B) Polaris appears higher in Grand Junction
C) Polaris is the same height in both places
D) not enough information
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Fort Lewis College
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You carefully measure the height of the noon Sun
from Durango and from Grand Junction.
A) The Sun is higher in Durango
B) The Sun is higher in Grand Junction
C) Which is higher depends on the season.
D) Not enough information.
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Fort Lewis College
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You carefully measure the height of the noon Sun
from Durango and from Grand Junction.
A) The Sun is higher in Durango
B) The Sun is higher in Grand Junction
C) Which is higher depends on the season.
D) Not enough information.
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Fort Lewis College
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Three Minute Paper
• Write 1-3 sentences.
• What was the most important thing
you learned today?
• What questions do you still have
about today’s topics?
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Fort Lewis College
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