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).