ISS FLYBY

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ISS FLYBY

• THIS FRIDAY 600 PM
• ON ROOF OF TATE

(FREE COFFEE)
Public Nights are presented by U of M Astronomy
Department for more info see our website at
www.astro.umn.edu
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Simplifying the Homework

• Many students do not have access.
• Thus
• All future homework will be shorter and only
  contain book problems.
• If you do not have the online material, just send
  me an email w/ your answers, print them out, etc.
• Online Homework
• Register in ASTRO1001AUSTIN
• Ill add additional problem sets that take
  advantage of the online tutorials, but they are
  not required or graded.
• Assignment 2 (Due 2/4 at noon)

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Planetary MotionReadings 2.4
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Wandering stars

• Planets were seen to move through the sky along
  the zodiac
• Nearly always moved eastwards across the stars.

Uranus
Mars
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Wandering stars

• Apparent retrograde motion.
• A period where the planet moves westward across
  the sky.
• Takes place over weeks or months.

Uranus
Mars
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Retrograde motion

• This is when the planets appear to move backwards
  w.r.t. the stars, i.e. westward.
• It is caused by Earth passing (or being passed)
  in its orbit.
• 02_MarsRetrogradeMotion.swf
• Retrograde demo

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• Last night I saw Mars move westward through the
  sky in its apparent retrograde motion.
• Yes, this occurs during certain times of the year
  when Earth overtakes Mars in its orbit.
• Yes, this is a well studied phenomenon and its
  explanation proved a challenge to ancient
  astronomers.
• All planets (and stars) move westward because of
  Earths rotation, so this is not unusual.
• No, apparent retrograde motion is only noticeable
  over many nights, not a single night.
• No, because Mars lies further from the Sun than
  Earth, it does not undergo retrograde motion.

4. No, apparent retrograde motion is only
noticeable over many nights, not a single night.
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The beginning of modern science and astronomy

• Readings 3.3 (the beginnings of modern astronomy)

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• Many ancient cultures studied astronomy
  Chinese, Mayan, Native Americans, and Egyptians
  are just a few.
• Astronomy was studied to predict seasons,
  eclipses, understand the heavens, tell time of
  day, etc.
• Earliest astronomical record a lunar calendar
  etched on bone from 6500 B.C. Uganda.

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• Stonehenge (3100 BC - 1500 BC) is aligned such
  that the sun rises along the main axis on the
  summer solstice.

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• Chinese Oracle bones
• Contain records of novae and comets.
• 1100 BC

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Ancient Greeks

• By 500 BC, the Greeks had constructed a model
  that describes the motions of the sky.
• Geocentric model
• Spherical Earth at the center of the universe
• Sun, Moon, and planets surrounded us on perfect
  spheres.

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Ancient Greeks

• We often say that modern science began with the
  Ancient Greeks.
• They would debate and challenge ideas
• Their ideas could only be correct if they agreed
  with observations.
• Used mathematic models to explain behavior of
  phenomena
• A scientific model is a conceptual representation
  of observed phenomena.
• It may be simpler than reality.
• Is used to predict future occurrences or
  observations.

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Pythagoras 500 BC

• Teaching that the Earth was round
• Believe Earth was at the center of a great
  celestial sphere.

Aristotle 350 BC

• Earth was round due to gravity
• All heavy things sank, thus the stars and planets
  had to be lighter than the air.
• Cited the Earths curved shadow during a lunar
  eclipse as evidence for a spherical Earth.

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The Geocentric Model

• Earth is at the center of the Universe.
• Each planet, the sun, and the moon all moved on
  their own spheres around the Earth.

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Ptolemaic Model

• Common belief heavenly objects could only move
  in perfect circles.
• Model agreed with planetary positions and could
  predict future positions within a few degrees.
• Problem kept adding epicycles for better
  accuracy.
• 03_PtolemaicModel.swf

Ptolemy (100 - 170 AD)
Ptolemys Universe
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Copernican Revolution

• First heliocentric model by Aristarchus about 250
  BC
• The Earth and planets orbit the Sun.
• Revived by Copernicus early 1500s
• Motivation Ptolemaic model had been growing
  inaccurate and was very complex.
• Feared persecution for his beliefs.

Copernicus
Copernican Model
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Copernicus 1500s

• Still believed all motions were on perfect
  circles.
• Thus needed to add epicycles to accurately
  explain the planets positions.
• His model did
• Not make better predictions than the Geocentric
  model.
• Not simplify the calculations from the Geocentric
  model.
• Not convince many people!

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Tycho Brahe late 1500s

• Was frustrated with the current planetary
  predictions being inaccurate.
• The geocentric model started out with small
  errors, but over time, they had built up to a
  couple of days.
• Made very accurate (1 arc minute) observations of
  the motions in the sky.
• These are still the most accurate naked-eye
  observations the telescope was invented soon
  after.

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Tycho Brahe late 1500s

• With his excellent observations, he could not
  detect stellar parallax.
• Parallax is caused by the Earths motion around
  the Sun.
• Effect Close stars appear to move. Stars that
  are further away do not show this effect.
• 03_ParallaxNearbyStar.swf
• Thus he believed the Earth did not move.
• He advocated a model where the Sun orbited the
  Earth and other planets orbit the sun -- not
  widely accepted.

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Johannes Kepler 1600

• Was asked by Tycho to find a model to describe
  his observations so his life was not in vain.
• Using the data and circular orbits, the planets
  positions were only 8 arc minutes off.
• Thus abandoned circles and found that the planets
  orbits are ellipses!

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Keplers 1st law

• All planets orbit the Sun on an ellipse, with the
  sun at one focus.

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Keplers 2nd Law

• As a planet moves through its orbit, it sweeps
  out equal areas in equal times.
• 03_Kepler2VvsOrbitR.swf

• Planets move
• Fast at perihelion
• Slow at aphelion

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Keplers 3rd law

• More distant planets orbit the Sun at slower
  speeds than closer ones.

Works for asteroids, comets, and everything
orbiting the Sun!
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Keplers Laws.

• Kepler provided strong evidence that the Sun was
  the center of the solar system, though there were
  still 3 main objections
• The Earth cannot be moving, or we would leave
  behind the clouds, etc.
• The heavens should be perfect and unchanging, and
  ellipses are not perfect.
• Stellar parallax had not been detected yet.

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Galileo (1600s) and the main objections

• The Earth cannot be moving, or we would leave
  behind the clouds, etc.
• Demonstrated that an object remains in motion
  until a force acts on it. This contradicted the
  view that the natural tendency is for an object
  to come to rest.
• The heavens should be perfect and unchanging, and
  ellipses are not perfect.
• Built a telescope (invented in 1608 by
  Lippershey) and showed that the sun had sun
  spots.
• Stellar parallax had not been detected yet.
• Showed that there were many, many stars in the
  galaxy which argued that they were too far away
  to detect parallax.

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Galileos other proof for a Sun-centric system.

• He found 4 moons orbiting Jupiter, showing that
  not everything orbited Earth.
• Showed that Venus had a full range of phases, and
  thus could not orbit the earth.

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Why the sun-centered model became dominant!

• It was the simplest explanation for the
  observations.
• It made predictions, and they were correct.
• It has not yet been proven wrong, unlike the
  geocentric model.

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Why do many scientists consider Galileo the
originator of modern science?

• He invented the telescope
• He proved that Copernicus was right
• He emphasized how important it is to test ideas
  through experiment

3. He emphasized how important it is to test
ideas through experiment
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Modern science

• Readings 3.4 (modern science)

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(No Transcript)
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Modern science

• Main process of science
• A hypothesis, or educated guess, is made about a
  natural phenomena.
• Test this hypothesis with new observations and
  making predictions.
• If the predictions are correct, great! If not,
  time to make a new hypothesis.
• A scientific model must make testable predictions
• These tests can be verified by anyone!

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Scientific Theories
Its just a theory.

• Theory
• An explanation of a group of occurrences in
  nature that has been confirmed by a substantial
  number of experiments and observations.
• Can never be proved beyond all doubt better
  observations may disagree.

Scientific theory is not mere conjecture.
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What characterizes a scientific explanation?

• It is based on the ideas of the smartest people
• The more ancient the wisdom, the better the
  explanation
• It is based on observations
• It is tested through prediction and experiments
• 3 and 4

5. 3 and 4
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Why is a scientific theory supposed to predict,
and not just explain after the fact?

• The scientific method is supposed to be followed
  in order
• People can usually think up explanations after
  something happens, but they may not be right
• Predicting things before they happen requires
  more understandingyou are more likely to have
  the correct explanation
• 2 and 3

4. 2 and 3
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What is the difference between the word theory
asused in everyday speech, and the word theory
as used in science?

• Theory, in common speech, is something uncertain
  (Its just a theory)
• A scientific theory is different. It has been
  thoroughly tested
• A scientific theory must be discarded if it fails
  to explain what is observed in any experiment
• All of the above

4. All of the above
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Do you think scientists follow the steps of the
scientific method given in the previous slide in
order (step 1, then 2, then 3) ?

• Yesthats the scientific method
• No
• Often, but not always

3. Often, but not always
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If scientists skip a step in the scientific
methodfor instance, intuitively guessing the
answer even before doing the experimentwhat
happens?

• They flunk out of science
• Real scientists never skip steps
• They go back later and do the steps they didnt
  do
• Other scientists repeat the experiment
• 3 and 4 often happen

5. 3 and 4 often happen
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Do you think that the scientific method involves
much creativity?

• Yes
• No

1. Yes