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Unraveling Retrograde Motion: Key Astronomers and Gravity's Role in Planetary Orbits

Exploring the historical challenge of retrograde motion for early astronomers, including Copernicus, Brahe, Kepler, and Newton, and how gravity explains these planetary phenomena. Learn about the shift from Earth-centered to Sun-centered models and the gravitational principles behind planet orbits.

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Unraveling Retrograde Motion: Key Astronomers and Gravity's Role in Planetary Orbits

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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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