Tycho, Kepler and Newton
Great Astronomers
Tycho Brahe - An Observer
• Tycho Brahe was a prominent scholar
and aristocrat in Denmark in the midlate 1500's
• He made a huge number of
observations of the stars and planets,
all with the naked eye
– Even without a telescope, he was
very accurate in his measurements
• Also recorded the appearance of
comets and supernovae
– The Tycho supernova remnant is still
visible today
Tycho (1546-1601)
Johannes Kepler - A Theorist
• Shortly before his death, Tycho
began working with another
scientist named Kepler
• Kepler was put to the task of
creating a model to fit all of
Tycho's planetary data
• Kepler spent the remainder of
his life formulating a set of
laws that explained the motion
of the planets
Kepler (1571 - 1630)
Kepler's First Law
• Kepler first noted that the orbital
path of a planet around the Sun
is an ellipse, not a perfect circle
• The Sun lies at one of the foci of
the ellipse
• The eccentricity of an ellipse is a
measure of how 'squished' from
a circle the shape is
• Most planets in the Solar System
are very close to a perfect circle
Focus
Focus
Kepler's 1st Law: The orbital
paths of the planets are elliptical
with the Sun at one focus.
Kepler's Second Law
• Kepler also noticed that the
planets sweep out equal areas in
their orbit over equal times
• Notice that this means the
planet must speed up and slow
down at different points
• If it takes the same amount of
time to go through A as it does
C, at what point is it moving
faster?
– C, when it is closest to the Sun
Kepler's 2nd Law: An imaginary line
connecting the Sun to any planet
sweeps out equal areas of the
ellipse over equal intervals of time.
Kepler's Third Law
• Finally, Kepler noticed that
the period of planet's orbit
squared is proportional to the
cube of its semi major axis
• This law allowed the orbits of
all the planets to be calculated
• It also allowed for the
prediction of the location of
other possible planets
Kepler's 3rd Law Simplified
P a
2
3
NOTE: In order to use the
equation as shown, you must be
talking about a planet in the Solar
System, P must be in years, and
a must be in A.U. !!!
Isaac Newton
• Kepler's Laws were a revolution in
regards to understanding planetary
motion, but there was no explanation
why they worked
• That explanation would have to wait
until Isaac Newton formulated his
laws of motion and the concept of
gravity
• Newton's discoveries were important
because they applied to actions on
Earth and in space
• Besides motion and gravity, Newton
also developed calculus
Newton (1642-1727)
Newton's First Law
• Newton's first law states: An object at rest will remain at rest, an
object in motion will stay in motion - UNLESS acted upon by an
outside force
Outside Force
• This is why you should always wear a seat belt!
Newton's Second Law
• Acceleration is created whenever there is a change in
velocity
– Remember, this can mean a change in magnitude AND/OR
direction
• Newton's Second Law:
F  ma
• Notice how this equation works:
– The bigger the mass, the larger the force
– The bigger the acceleration, the larger the force
Newton's Third Law
• Newton's Third Law states: For
every action, there is an equal
and opposite reaction
• Simply put, if body A exerts a
force on body B, body B will
react with a force that is equal
in magnitude but opposite
direction
• This will be important in
astronomy in terms of gravity
– The Sun pulls on the Earth and
the Earth pulls on the Sun
Newton and the Apple - Gravity
• After formulating his three laws
of motion, Newton realized that
there must be some force
governing the motion of the
planets around the Sun
• Amazingly, Newton was able to
connect the motion of the planets
to motions here on Earth through
gravity
• Gravity is the attractive force two
objects place upon one another
Revisions to Kepler's Laws
• Newton's law of gravity
required some slight
modifications to
Kepler's laws