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Chapter 4 Gravity and the motion of Planets

Chapter 4 Gravity and the motion of Planets. Retrograde Motion – the apparent backwards motion of a planet across the sky.

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Chapter 4 Gravity and the motion of Planets

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  1. Chapter 4 Gravity and the motion of Planets

  2. Retrograde Motion – the apparent backwards motion of a planet across the sky

  3. Explaining Retrograde Motionbecame a real challenge for early astronomers, particularly those who believed that the earth was at the center of the universe – an idea that was popular until just 500 years ago!

  4. Nicolaus Copernicus(1473 – 1543) His great contribution to astronomy was to place the Sun at the center of the Universe. Now retrograde motion has a more rational explanation.

  5. The two key concepts in explaining retrograde motion • The planets orbit the Sun (not the earth). • The orbital velocity decreases with increasing distance from the Sun

  6. The full understanding of retrograde motion came with important work by 3 more individuals • Tycho Brahe (1546 – 1601) • Johannes Kepler (1571 – 1630) • Isaac Newton (1642 – 1727)

  7. Introducing…. Tycho Brahe

  8. Introducing….. Johannes Kepler

  9. Introducing…….. Isaac Newton

  10. The Law of Gravitation F = G Mm/r2 Gravity provides the centripetal force that cause planets to orbit the Sun F = mv2/r Equate these two equations and one finds that v =  G M/r Which is why planets furthest from the Sun (big r) orbit with the slowest velocities (small v).

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