chapter 4 gravity and the motion of planets
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Chapter 4 Gravity and the motion of Planets. Retrograde Motion – the apparent backwards motion of a planet across the sky. - PowerPoint PPT PresentationTRANSCRIPT
Chapter 4
Gravity and the motion of Planets
Retrograde Motion – the apparent backwards motion of a planetacross the sky
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!
Nicolaus Copernicus(1473 – 1543)
His great contribution to astronomy was to place the Sun atthe center of the Universe.
Now retrograde motion has a more rational explanation.
The two key concepts in explaining retrograde motion
1. The planets orbit the Sun (not the earth).
2. The orbital velocity decreases with increasing distance from the Sun
The full understanding of retrograde motion came with
important work by 3 more individuals
1. Tycho Brahe (1546 – 1601)2. Johannes Kepler (1571 – 1630)3. Isaac Newton (1642 – 1727)
Introducing….
Tycho Brahe
Introducing…..
Johannes Kepler
Introducing……..
Isaac Newton
The Law of Gravitation
F = G Mm/r2
Gravity provides the centripetal force that cause planetsto 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 theslowest velocities (small v).