Transcript Document

The Moon and Planets
A Backyard View
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Goals
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How does the Moon move through the sky?
What causes its phases?
What really is the “dark side” of the moon?
What are eclipses?
What are occultations?
How do planets move through the sky?
What is retrograde motion?
How is the Universe ordered?
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The Moon and
Phases
• The Moon takes
27.3 days to orbit
the Earth.
• The Moon takes
27.3 days to turn on
its axis.
• Result: We always
see the same face.
• There is no “dark”
side of the Moon.
http://antwrp.gsfc.nasa.gov/apod/ap991108.html
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The Moon in the Sky
N
New
Full
S
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Lunar Eclipse
• Sometimes the Moon passes through the Earth’s
shadow as cast by the Sun.
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Composite photo of the January 2001 lunar eclipse.
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Solar
Eclipse
• Sometimes the
Moon passes
between the
Sun and the
Earth (and
casts a shadow
on the Earth).
August 1999 eclipse. Photo from crew of Mir.
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Composite photo of August 1999 solar eclipse.
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Types of Solar
Eclipses
• Partial
• Total
• Annular
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Occultations
• Eclipse: sometimes the moon passes in front of the
Sun.
• Occultation: sometimes the moon passes in front
of other things:
– Stars
– Asteroids
– Planets
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The Plane of the Solar System
• The Earth, Sun, Moon and Planets all lie in the
same plane.
• This is the plane of the solar system.
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Ecliptic
• As viewed from
inside the solar
system, the Sun,
Moon and planets
all move in a band
on the sky around
the Earth.
• This band is called
the ecliptic.
• The constellations
that the ecliptic
passes through are
the zodiac.
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Ecliptic and Occultations
• Result: Moon, Sun
and planets aren’t
just “anywhere” in
the sky.
• Always found
somewhere on the
ecliptic.
• Distant planets pass behind nearby objects and are occulted.
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The Ecliptic in the Sky
The Ecliptic
New
Full
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Retrograde Motion
June, 2000 to May 2001
Credit & Copyright: Tunc Tezel
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The Wanderers
• The planets (or “wanderers”) move
differently from stars
– They move with respect to the stars
– They exhibit strange retrograde motion
• What does all this mean?
• How can we explain these
movements?
• What does the universe
look like?
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Geocentric System
• The Earth is at the center of
Universe.
• The Sun, stars, and planets orbit the
Earth: explains daily movement.
• To account for unusual planetary
motion epicycles were introduced.
• Fit the Greek model of heavenly perfection – spheres are the
perfect shape, circular the perfect motion.
• The accepted model up until ~1630 AD.
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Heliocentric System
• What if?
• Sun at center (heliocentric).
• Moon orbits the Earth, the Earth
and planets orbit the Sun.
• Stars don’t move.
• The daily motion of the stars
results from the Earth’s spin.
• The annual motion of the stars
results from the Earth’s orbit.
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• In the heliocentric model, apparent retrograde
motion of the planets is a direct consequence of
the Earth’s motion
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Geocentric vs. Heliocentric
• How do we decide between two
theories?
• Use the Scientific method:
– These are both explanations based
on the observation of retrograde
motion
– What predictions do the models
make?
– How can these predictions be
tested?
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Phases of
Venus
• Heliocentric
predicts that
Venus should
show a full phase,
geocentric does
not
• Unfortunately, the
phases of Venus
cannot be
observed
with the
naked eye
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Moons of Jupiter
• In a geocentric philosophy,
there is only one center of
motion: the Earth.
• If one thing can be found
not to orbit the Earth, then
maybe nothing needs to
orbit the Earth.
• Using his new telescope, Galileo:
– discovered four moons of Jupiter  The Galilean Moons.
– Saw the phases of Venus.
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A Scientific Universe
• The heliocentric model fits the observations best.
• It’s the model of the solar system we have today.
• While our uncritical eyes tell us one thing, the
scientific method, and critical thought, reveals
another.
• Retrograde motion, phases of Venus, and moons
of Jupiter:
– We are the center of the Universe.
– We as a single part, in no way special.
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