Ancient Planetary Astronomy

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Ancient Planetary Astronomy
Ptolemy
Ptolemy
Geocentric Solar System
Galileo
Copernicus
Heliocentric Solar System
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Announcements

• The first homework assignment is on the course
  website
• Due January 25
• Start early!
• Starry Night Backyard software program
• The CD is in the back of the book it is a blue
  CD, right behind the yellow one (there are 2 CDs
  that accompany the textbook!)
• Preceptors
• Please fill out a preceptor application.
• It would be very helpful if you would also give
  me your schedule on the back of the application
• Mission updates
• Extra credit (tbd) for an informal and brief (5
  minute) presentation of a current (or past)
  mission or other topic related to the class
• LPL Public Lecture Series
• There will be some extra credit (tbd) given to
  attend these
• This is a great opportunity to learn about
  cutting-edge research at the UofA!
• See www.lpl.arizona.edu/COLPL for details

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Todays Topics

• Preliminary info
• Powers-of-ten notation
• Units and unit conversion
• Speed, time, velocity
• Ancient Planetary Astronomy
• Ancient Greeks
• Retrograde motion
• Ptolemaic model of the solar system
• Parallax
• The Copernican revolution
• A long-standing Earth-centered view of the solar
  system is turned on its head with the invention
  of the telescope
• Phases of Venus

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Powers-of-ten notation is a useful shorthand
system for writing numbers
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Common Prefixes
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Powers of ten manipulation

• 128 Billion miles 128x109 miles 1.28x1011
  miles
• 20 milliseconds 20 x 10-3 seconds 2x10-2
  seconds
• 6000 x 20000 (6x103) x (2x104) 12x107
  1.2x108

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Some common units used by planetary scientists

• Distance
• Between the planets ? AU
• 1AU mean distance between Earth and Sun
• Size of craters and other features on planets ?
  km
• Mass ? kg
• Note that mass is different from weight
• Weight is the force exerted by gravity
• Will depend on where you are (you would weigh
  less on the Moon and Mars)
• Speed ? km/s
• Temperature ? K (Kelvin), C (Centigrade), or F
  (Fahrenheit)
• Kelvin (K) is the most common

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Speed, Distance, and Time

• Distance traveled speed x time
• Basic algebra gives two other formulas from this
• Average Speed total distance traveled / time
• Time it takes total distance traveled / speed

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An Example

• When Voyager 2 spacecraft sent back pictures of
  Neptune (the most distant planet in our solar
  system) in 1989, the radio signals took 4 hours
  to reach Earth. How far away was the spacecraft?

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An Example

• When Voyager 2 spacecraft sent back pictures of
  Neptune (the most distant planet in our solar
  system) in 1989, the radio signals took 4 hours
  to reach Earth. How far away was the spacecraft?
• The correct formula to use is

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An Example

• When Voyager 2 spacecraft sent back pictures of
  Neptune (the most distant planet in our solar
  system) in 1989, the radio signals took 4 hours
  to reach Earth. How far away was the spacecraft?
• The correct formula to use is

(3x108 m/s) x (4 hours)
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An Example

• When Voyager 2 spacecraft sent back pictures of
  Neptune (the most distant planet in our solar
  system) in 1989, the radio signals took 4 hours
  to reach Earth. How far away was the spacecraft?
• The correct formula to use is

(3x108 m/s) x (4 hours)
(3x108 m/s) x ( 0.001 km/m) x (4 hours) x
(3600 s/hour)
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An Example

• When Voyager 2 spacecraft sent back pictures of
  Neptune (the most distant planet in our solar
  system) in 1989, the radio signals took 4 hours
  to reach Earth. How far away was the spacecraft?
• The correct formula to use is

(3x108 m/s) x (4 hours)
(3x108 m/s) x ( 0.001 km/m) x (4 hours) x
(3600 s/hour)
4.32 x 109 km
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Revolutions in Planetary Science

• Greek Philosophy (500 BC-200 AD)
• Greece, Turkey, Egypt, Syria
• Systematic philosophy
• Knew that Earth is a sphere

• Copernican Revolution (1500-1700 AD)
• Europe
• Modern telescopes

• Age of spacecraft (1960-present)

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Ancient Greek Astronomy

• The word planet is derived from a Greek word
  meaning wanderer
• They recognized that the planets did not stay
  stationary relative to the background of stars
• Only 5 were visible to the naked eye
• Mercury, Venus, Mars, Jupiter, and Saturn
• They were able to measure the size of the Earth
  and the relative distances between the Earth,
  Moon, and Sun
• They knew the Earth to be a sphere

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The Ancient Greeks were able to measure the size
of the Earth

• Erotosthenes (276-197 BC) Accurately measured
  the size of the Earth using simple geometry and
  the assumption of a spherical Earth

LIGHT RAYS FROM SUN
Deep Well at Syrene
post at Alexandria (which casts a shadow)
Erotosthenes method
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Ancient Greek Astronomy

• Ancient astronomers believed the Earth was the
  center of the Universe.
• Platos views of celestial bodies moving in
  perfect circles guided much of this thinking
• Some did consider the helio- (or Sun-) centered
  system.
• Aristarchus advocated the sun-centered system
  because the Sun was so big, it had to be at the
  center! His views were not accepted, however.
• "At the centre, they the Pythagoreans say, is
  fire, and the earth is one of the stars, creating
  night and day by its circular motion about the
  centre." -- in Aristotles Of the Heavens

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Aristotle dismissed the heliocentric system he
saw no parallax
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Parallax Shift
Only small apparent shifts can be seen using the
Earths rotation to change the observing
point. Larger shifts can be seen using Earths
orbit around the Sun (but still to small to be
noticed without a telescope)
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Retrograde motion of planets

• When observed from one night to the next, a
  planet appears to move from West to East against
  the background stars most of the time.
• Sometimes it will appear to reverse direction.
  For a short time, it moves from East to West
  against the background constellations.

• This reversal is known as retrograde motion. All
  planets exhibit this behavior as seen from Earth.
  It is due to the relative motion of Earth and
  the planet.

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Ptolemy

• Egypt (127-145 AD)
• Not much known about his life.
• Used the concept of Epicycles to explain the
  motion of the Sun and planets

crater Ptolemeaus Consolidated Lunar Atlas
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(No Transcript)
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Ptolemys Universe

• The epicycle picture explains the retrograde
  motion of the planets
• This picture lasted 1000 years!
• Newtons physics has only been around for about
  400 years and Einstein has already corrected it !
• But, as we now know, it is flawed.

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The inner planets

• Venus is only seen easily (when it is dark
  outside) in the morning or evening
• The ancients first thought that the
  morning/evening stars were different objects
• Pythagoras was the first to note that they were
  the same object -- Venus
• Their orbits must be between that of the Earth
  and Sun

The Same is True for Mercury
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(No Transcript)
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The Copernican RevolutionA short list of key
people
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Nicholas Copernicus

• 1473-1543, Polish
• Re-proposed heliocentric theory
• Put the Sun at the center, but still believed the
  orbits of the planets were circles epicycles
• He felt that this was a more natural explanation
  of the solar system

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Ptolemy vs. Copernicus Solar Systems
Path of Mars in the sky (Ptolemys epicyles)
Path of Mars in the sky (Copernicuss system)
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Todays in-class activity

• You may work together in small (2-3 people)
  groups
• You must hand in your own work
• Note the scoring system
• After the activity is turned in, we will toss a
  coin to decide if it will be graded
• Heads it is graded
• Tails it will not be graded