Motion

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Motion

• Dr Bryce

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Class Notices

• This weeks observing at Clear Sky Patrol for
  students with ID numbers ending in a 2 or a 3

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The Suns annual motion
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In summary

• The Sun appears to move Eastwards along the
  Ecliptic.
• At midnight the stars on a North to South
  imaginary line are opposite the Sun.
• The constellations that lie along the ecliptic
  are collectively called the zodiac.
• The constellations you can see depend on the time
  of year.

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Planetary Motion

• We see Venus (and it we are very fortunate
  Mercury) just after Sunset/Sunrise.
• Morning/Evening Star
• Mars, Jupiter, Saturn etc move Eastwards from
  night to night.
• Planets move along the Ecliptic

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Westwards motion
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The explanation
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In ancient times

• It was believed that the Earth was at the centre
  of the Universe.
• The Sun, Moon and Planets moved on perfect
  circles
• Made it difficult to explain retrograde motion.

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How distant are the stars?

• Possibly one of mans first questions about
  astronomy.
• We have developed many different techniques for
  measuring the distances to celestial objects.
• The technique for measuring the distances to our
  nearest stellar neighbours is called parallax

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Difficult to observe

• The angles that even the closest stars move by
  are very small.
• As parallax was not observed by ancient
  astronomers they had great difficulty believing
  that the Earth was orbiting the Sun.
• They did not believe that the stars were so far
  away as to not observably move!

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Remember
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Parallax

• Gives us depth perception
• Hold a finger up at arms length, look at the
  finger with just your right eye and then just
  your left.
• You will observe that your fingers position
  relative to the background has moved.

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Parsec

• Stars dont move by large amounts relative to the
  background.
• Typically close by stars move by fractions of arc
  seconds.
• The further away a star is the smaller the angle.
• A star subtending one second of arc when observed
  at positions differing by 1 AU is 1 parsec
  distant.

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Modern Science and the Greeks

• Greeks were the first people known to make
  models of nature.
• They tried to explain patterns in nature without
  resorting to myth or the supernatural.

Greek geocentric model (c. 400 B.C.)
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Eratosthenes measures the Earth (c. 240 BC)
Measurements Syene to Alexandria distance
5000 stadia angle 7
Calculate circumference of Earth 7/360 ?
(circum. Earth) 5000 stadia ? circum. Earth
5000 ? 360/7 stadia 250,000 stadia
Compare to modern value ( 40,100 km) Greek
stadium 1/6 km ? 250,000 stadia 42,000 km
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• The most sophisticated geocentric model was that
  of Ptolemy (A.D. 100-170) the Ptolemaic model
• Sufficiently accurate to remain in use for 1,500
  years.
• Arabic translation of Ptolemys work named
  Almagest (the greatest compilation)

Ptolemy
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The Ptolemaic model explanation of retrograde
motion. Planets really do go backward in this
model