Transcript Chpt1
Chapter 1 The Copernican Revolution
Chapter 1 The Copernican Revolution
Goals
• Explain how accurate observations led to Heliocentric model
• Review contributions of Galileo and Kepler
• Explain Kepler’s Laws
• Explain Newton’s laws of motion and gravity
Chapter 1 The Copernican Revolution
Our life has always depended on the stars. Seafarers needed
to navigate and farmers needed to know when to plant.
Our modern calendar was based on the changing heavens.
We have seven days of the week because there were seven
movable objects in the heavens.
We have 12 months in a year because there are about 12
lunar cycles in a year.
We have 360 degrees in a circle because it takes about 360
days for the Sun to return to the same spot among the stars.
Chapter 1 The Copernican Revolution
Planetary Motion
Early astronomers were abscessed with perfection and thought
the motions of the heavenly bodies should be circles.
They also thought the Earth was stationary because they could
not fell the Earth move beneath them.
This led to the Earth centered or Geocentric model of which
Ptolemy was the main supporter.
While observations were crude the model seemed to work, but
as observation improved it became clear that something was
wrong.
Chapter 1 The Copernican Revolution
In order to modify his theory, Ptolemy had to add circles to
circles (deferent and Epicycle) to explain retrograde motion.
Chapter 1 The Copernican Revolution
Retrograde motion is
defined as the brief
westward motion
that a planet appears
to take when a
slower outer planet
is passed by a faster
inner planet.
While the geocentric model required a complicated set of circles on
circles to explain retrograde motion, the Sun centered (Heliocentric)
model easily explained the observations. Copernicus was the main
supporter for the Heliocentric model.
Chapter 1 The Copernican Revolution
While the Copernican theory did explain
the observations better with a simpler
model, civilization at the time resisted the
model partly because the church of the
time had declared the geocentric model as
correct with out regard for scientific
observations.
What finally convinced everyone was the
observations of Galileo Galilei.
Chapter 1 The Copernican Revolution
Using his telescope, Galileo discovered:
• the Moon had creators and mountains like the Earth and was
not perfect.
• the Sun was imperfect (sunspots) and rotated on its own axis.
• Jupiter had four moons in orbit around it which showed things
could orbit other things besides the Earth.
• Venus showed complete set of phases only explainable if it
was in orbit around the Sun not the Earth.
Because of his insistence of the Heliocentric model based on his
observations, Galileo was excommunicated from the church and
was not forgiven until 1992.
Chapter 1 The Copernican Revolution
Chapter 1 The Copernican Revolution
Kepler
Now that we have the correct heliocentric model
we need to explain why it is the way it is.
• What causes the planets to orbit the Sun ?
• Why do the different planets have different
speeds?
• How can we predict their positions in the future?
Chapter 1 The Copernican Revolution
Kepler’s Laws of Motion
First Law: The orbits of the planets are in the shape of an
ellipse with the sun at one focus.
Chapter 1 The Copernican Revolution
Second Law: a vector from the sun to the planet sweeps out
equal areas in equal times (conservation of angular momentum).
Chapter 1 The Copernican Revolution
Third Law: the period of a planet and its semimajor axis
are related by.
P2 = a3
Chapter 1 The Copernican Revolution
Newton’s Laws
While Kepler discovered certain relationships and facts
concerning how the planets moved about the Sun he could
not explain why.
First Law: objects remain in motion or at rest unless acted
upon by a force.
Second Law: F = Ma: where F = force, M= mass, a =
acceleration
Third Law: Fg = GMm/d2 Where Fg is the force due to
gravity, G is a constant, and M and m are the mass of the
two objects, and d is the distance of their separation.
Chapter 1 The Copernican Revolution
Thus gravity provides the force which causes objects to orbit
other objects. Newton invented calculus to help him solve
his problems
Chapter 1 The Copernican Revolution
Newton’s third law allows us
to detect planets around other
stars and determine their mass.