Gravity

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Gravity again

• Solar System Astronomy
• Chapter 10

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More than Keplers Laws

• Gravity moves beyond orbital motion
• Internal forces
• Tides
• Orbital resonances
• Chaotic orbits

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Internal Forces

• Gravity works on every part of every body.
• Therefore self-gravity exists inside a planet
• This produces internal forces. These hold the
  planet together
• Theres a special case spherically symmetric
  bodies.
• Force from a spherically symmetric body is the
  same as from a point mass at the center

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Tidal Forces

• Tides are a consequence of gravity
• Consider Earth and Moon
• Parts of the Earth are closer to the Moon than
  other parts
• The force of gravity is stronger on the closer
  parts

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Tidal Forces

• Tides are a consequence of gravity
• Consider Earth and Moon
• Parts of the Earth are closer to the Moon than
  other parts
• The force of gravity is stronger on the closer
  parts

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Tidal Forces
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Tidal Forces

• Not until Newton could forces be analyzed
• Galileo tried
• Oceans have tidal bulge
• Elongated in direction nearly pointed at Moon
• (Earth rotates under tidal bulge)
• Two high and low tides per day
• As we are under, then opposite, the Moon
• There are both lunar tides and solar tides

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Tidal Forces

• Not until Newton could forces be analyzed
• Galileo tried
• friction of ocean with Earth slows rotation
• day grows by 0.0023 s per century
• 900 million years ago 18 hour days
• tidal flexing within moon dissipated rotational
  motion
• tidal locking
• Moon being pulled forward (sped up)
• 4 cm farther away each year

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Roche Limit

• aka tidal limit