Observing the Solar System

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Imagine a time before the telescope ...
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A Brief History ofObserving the Heavens
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• Consider the night sky
• What objects might you see?
• Do some objects move? Hourly? Daily?
  Seasonally?

Since the beginning of recorded history,
humankind has watched the motions of the heavens.
Some early philosophers attempted to explain the
motions of celestial objects...
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Files\Sienna\Starry Nights Deluxe\Example
Files\Others\Many Celestial Objects.snf
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Winter Sky to the South
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Spring Sky to the South
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• Observation Reveals
• the background of stars seem to move slowly
  across the sky, but stars do not move with
  respect to each other.
• the sun moves across the sky in an arc
• the moon moves across the sky in the same arc
• some wondering stars seem to move, with
  varying speed, across the sky in the same arc
  followed by the moon and the sun

wondering stars planets
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• Explanation Steeped in Bias
• The Earth was considered to be the center of the
  known universe.
• Celestial objects appeared to the naked eye to
  be perfect spheres.
• Circular motion and perfect spheres seemed to be
  a reserved for the wonders of the heavens. Less
  perfect shapes and motions were reserved for
  Earthly existance.

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Explanation Steeped in Bias Some planets
exhibited retrograde motion
Viewed from Earth, Mars appears to go backward
during its orbit. The effect is due to Earth
catching up and passing Mars.
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Explanation Steeped in Bias Retrograde motion
did not fit nicely into the idea of perfect
spheres and circular motion. It begged for an
explanation.
However, people in the early times were no
different than people of today, and they did not
want an explanaiton that destroyed their current
understandings ...
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• Ptolemys Geocentric Model
• 200 CE (CE common era formerly 200 AD)
• Ptolemy - a Greek philospher - developed a model
  to explain the observed celestial motions,
  including retrograde motion, observed in the
  night sky
• Ptolemys model imagined the planets on small
  circular obrits, called epicycles. The center of
  each small orbit moved around Earth on a larger
  circular orbit called a deferent.
• The model predicted ONLY some of the observed
  retrograde motion - but not all.

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• Copernicus Heliocentric Model
• 1500s
• Copernicus proposed a radical new model to
  explain the observed celestial motion - a sun
  centered model.
• Copernicuss model suggested that the Earth was
  a planet, that it rotated, and that Earth and
  other planets revolved around the sun.
• The Heliocentric model is the basis of our
  current understanding.

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• Our Solar System
• Our solar system consists of nine planets
  orbiting one central star.
• Planets orbit the sun in nearly circular
  ellipses.
• The sun is located at one of the foci of these
  ellipses.
• The solar system appears as a flat disk. Each
  planets orbit shares the same plane, like
  marbles moving around a flat dinner plate.
• This plane is called the ecliptic plane.

ellipse
Sun at foci
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Our Solar System
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Files\Others\Solar System2.snf C\Program
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• Homework
• Summarize Keplers Laws (page 578-579)
• Summarize Gravitation (page 580)
• Answer page 580 1, 4, 5