Chapter 3:
Motion of
Astronomical
Bodies
Observed Motion of the Planets
At its core, astronomy is an observational science.
Thus, we must start by simply observing. After that,
we can try to explain why things move as they do.
The ancient
Babylonians
were the
first to make
records of
celestial
motions
The Ancient Greeks were
the first to try to Explain
Celestial Motions
Sun, Moon and stars are no
problem. Each rotates around
Earth on a giant crystal sphere.
This is known as the Geocentric
model.
The Physics of Aristotle
All celestial motions must
be circular
The motions of the planets
were difficult to explain
The Ptolemaic (ancient Greek) Model
Earth is at the center of
everything. All celestial
motions are circular (as
per Aristotle). Circles on
top of circles gives a
reasonably close
description of planetary
motions.
Watch ClassAction
Renaissance Astronomy
module Animation
Ptolemaic Orbit of Mars
Nicolaus Copernicus
1453 – 1543
Moved Earth out
of the center
and placed the
Sun there
instead. The
Earth was just
another planet
moving around
the Sun.
The Copernican
Solar System
De Revolutionibus Orbium
Coelestium was published in
1543, the year of his death.
The orbits were still circles so
he still needed epicycles but
they were very small. Still,
there were problems, it wasn’t
perfect.
Watch Museo Galileo
Copernican System video
The Copernican system
explained retrograde motion
in a much simpler way
Watch ClassAction Renaissance Astronomy module
Animation Retrograde Motion
Tycho Brahe: The Father of
Observational Astronomy
1546 – 1601
The
Observatory
at Uraniborg
In the late 1500’s, if
you wanted to study
astronomy with the
best, you went to
Uraniborg on the
Danish island of Hven
Johannes Kepler 1571 – 1630
Tycho hired Kepler to
come work for him as an
assistant. Kepler couldn’t
see very well so his job
was to make calculations.
When Tycho died Kepler
took the data from Tycho’s
heirs and used them to
develop his Laws of
Planetary Motion.
Kepler had pre-existing
ideas about how the solar
system worked
Kepler’s planetary
spheres were nested
inside the five perfect
geometrical solids
Once he started his work
on the data, he let the
data lead him to the
answer rather than
trying to force it to his
pre-existing idea
Kepler’s Laws
of Planetary
Motion are
empirical laws
The laws are based on
the observational data
of Tycho Brahe. They
were derived to fit the
data. They do not try to
explain why the planets
move as they do.
Kepler’s
st
1
Law
The Law of Ellipses
The planets
move in
elliptical
orbits with
the Sun
located at
one focus
Check out Eccentricity
Demonstrator In ClassAction
Renaissance Astronomy
module Animations
This was a major change since everyone that came
before had used circles, including Copernicus
Kepler’s
nd
2
Law
The Law of Areas
A line drawn
from a planet
to the Sun will
sweep out
equal areas
in equal time
periods
Check out Planetary
Orbit Simulator in
ClassAction
Renaissance
Astronomy module
Animations
Kepler’s 3rd Law:
The Law of harmonies
The ratio of the square of the orbital period to
the cube of the orbital semimajor axis (the
radius) is the same for all the planets
Galileo Galilei was a
contemporary of Kepler’s
Galileo preformed physics
experiments and developed
new Physics to replace the
old physics of Aristotle
Galileo discovered the Law of
Falling Bodies
Watch Apollo 15
Feather and the
Hammer video
Astronomical Discoveries
of Galileo
Lunar Observations
Sunspots
Saturn Drawing
Phases of Venus
Watch ClassAction Renaissance
Astronomy Phases of Venus animations
Galileo sent his
observations of
Jupiter to
Kepler to verify
Kepler’s Laws
The ratio of the square of the
orbital period to the cube of
the orbital radius was the
same for all four bodies
orbiting Jupiter but it wasn’t
the same constant as the
planets orbiting the Sun
It took 50
years after
Galileo and
Kepler before
Isaac Newton
explained why
things moved
as they did on
Earth and in
the heavens
1642 – 1727
Newton’s 1st Law of Motion
The Law of Inertia
An object in straight line uniform motion will
continue that motion unchanged unless some
external force acts on it
This law was based entirely on
the work of Galileo
Newton’s Second Law
The Force Law:
F = ma
The acceleration a body experiences is directly
proportional to the net force acting on it and inversely
proportional to its mass
Acceleration 
How much velocity changes
How long it takes to change
There can be an acceleration
even when there isn’t a
change in speed
Mass plays
an
important
part in
Newton’s
2nd Law
Newton’s Third Law
The Action-Reaction Law
For every force there is an equal and opposite
reaction force
The Action-Reaction forces
always apply to different objects
While the force on each object may be the same,
the acceleration (and thus damage) each
experiences depends inversely on their mass
according to Newton’s 2nd Law