Download Retrograde Motion

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