The motions of the Planets

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The motions of the Planets
Each planet follows the same diurnal motion as
the Sun, Moon and stars, rising in the east and
setting in the west each day. Like the Sun and
Moon, each planet moves west to east with respect
to the stars. This is called direct motion. In
addition, all the planets stay close to the
ecliptic.
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Retrograde Motion
Occasionally each of the planets will also move
east to west with respect to the stars for a
short time. This is called retrograde motion,
motion opposite that of direct motion.
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Path of Mars
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Discussion
Suppose Mars is moving in retrograde motion and
will rise at midnight. Since Mars is moving
with retrograde motion, that means that during
the night it will be moving west-to-east rather
than east-to-west. So at midnight it will rise
in the west and move across the sky and then set
in the east Do you agree or disagree?
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Two types of planets
Superior planets Mars, Jupiter, and
Saturn Superior planets can appear in the sky at
any time of the night. Inferior planets
Mercury and Venus Inferior planets always stay
close to the Sun.
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Discussion
If we consider the Moon to be a planet, what type
of planet would it be?
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Discussion
What is science? What is it scientists do?
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The Scientific Method

• Observations
• Formulate a hypothesis a conjecture that is
  used as a model for describing observations.
• Use hypothesis to make predictions a
  hypothesis which does not make testable
  predictions is of no use in science.
• Test predictions by further observations

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Occams Razor
If two or more models encompass the observations
equally well, the simplest model is preferred.
Why make things more complicated than
necessary?
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Science
A collection of explanations (theories) that
allow us to make predictions about the future.
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Fallacy 1
Scientists seek to uncover the truth. The
goals of science are actually more limited to
invent theories that make accurate predictions.
Example Creationism may be true, but it is
not science!
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Fallacy 2
Scientists have to always keep an open mind.
Scientists almost always work with a particular
hypothesis in mind. The question to answer is
always do the observations confirm their
hypothesis.
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Fallacy 3
Science has facts which have validity that cannot
be questioned. Instead the things we loosely
refer to as facts are merely inferences from the
observations. Examples The Universe is 13.7
billion years old.
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Imagine you are in charge of the Universe
What is the simplest possible Universe you could
come up with? But, it has to contain all the
objects we have already talked about.
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The Simplest Universe

• Nothing ever Changes!
• The Sun and Moon do not move in the sky
• There are no seasons
• The stars do not move in the sky
• The planets do not move in the sky

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Second Simplest Universe

• Stars unchanging and fixed to the celestial
  sphere
• Celestial sphere rotates about Earth moving east
  to west once a day carrying the Sun, Moon and
  planets with it.
• The Earth is at the center of the Universe and
  does not move, geocentric model

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Discussion
What is wrong with our second simplest
Universe? How can we fix it?
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Third Simplest Universe

• Sun, Moon, and planets are carried east to west
  along with rotation of celestial sphere
• Sun, Moon and planets move more slowly west to
  east against celestial sphere

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Discussion
What is wrong with our third simplest Universe?
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Ptolemys explanation for retrograde motion

• Each planet moves on a small circle called and
  epicycle.
• The center of each epicycle moves along a larger
  circle centered near the Earth called a deferent.
  

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Most of the time the motion along these circles
adds and we see direct motion. But eventually
the planet comes around the epicycle and the
motions subtract, and the planet moves backward
or in retrograde motion.
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Ptolemaic system

• Very successful at predicting positions of the
  planets but was not perfect
• Offered no explanation of why the planets moved
  on deferents and epicycles
• There was no relationship between period of
  revolution and epicycle size

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Alternatives to Ptolemys model

• Aristarchus proposed a heliocentric model of the
  Solar System in the 3rd century B.C.E.
• Reintroduced in the 16th century by Copernicus

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Heliocentric model

• Sun at the center
• Diurnal motion explained by rotation of the Earth
  
• All the planets including Earth revolve about the
  Sun in circular orbits with different speeds

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Disadvantages of the heliocentric model

• Still required epicycles
• Was no better at predicting planetary positions
• No stellar parallax observed

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Parallax
The change in an objects apparent position when
viewed from two different locations. The
Greek astronomers argued that if the Earth moved,
they should observe a parallax shift in the
positions of the stars. This was not observed.
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Advantages of heliocentric model

• Provides natural explanation of retrograde
  motion.
• Provides natural explanation for the motion of
  mercury and Venus as inferior planets, i.e. their
  orbits are interior to that of the Earth.
• Provides a relationship between distance from Sun
  and orbital period. Planets further from the Sun
  took longer to complete an orbit.

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Its just parallax again!
Retrograde motion is a consequence of Earths
motion. When a one planet overtakes another
planet, the planet appears to move backward
against the background for a short period of
time.
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Discussion
How could you explain the the motion of the
inferior planets Mercury and Venus with deferents
and epicycles in the geocentric model?
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Tycho Brahes Epilepsy Medicine
The basic substance is the head of a person who
has been hanged or otherwise executed. The head
should be dried and crushed together with peony
seeds to a powder. This medicine should not be
taken at the full moon.
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Tychos interest in astronomy
Tycho observed the solar eclipse of 1560 and was
amazed that astronomers could predict the exact
timing of the event. Later he observed a
conjunction of Jupiter and Saturn, noticing that
the prediction was off by several days from the
actual event.
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Tycho Brahe showed that the celestial sphere
could change

• Tychos supernova of 1572 showed that this new
  star had no parallax and thus was more distant
  than the Moon
• Comet of 1577 showed that it too was beyond the
  distance of the Moon

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Tycho Brahe

• Carefully tracked the position of the planets for
  20 years to unprecedented accuracy in an attempt
  to disprove the ideas of Copernicus.

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Tycho Brahes data proved that neither the
Ptolemy nor the Copernicus theories were ableto
predict the positions of the planets to the
accuracy of his measurements.
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Tycho Brahe concluded that all the planets
orbited the Sun and that the Sun and the Moon
orbited the Earth.
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Galileo
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