Transcript The Night Sky

Path of Mars
Discussion
Suppose Mars is moving in retrograde
motion and will rise at midnight.
Since Mars is moving with retrograde
motion, that means that during the night it
will be moving west-to-east rather than
east-to-west. So at midnight it will rise in
the west and move across the sky and then
set in the east
Do you agree or disagree?
Two types of planets
Superior planets – Mars, Jupiter, and Saturn
Superior planets can appear in the sky at
any time of the night.
Inferior planets – Mercury and Venus
Inferior planets always stay close to the Sun.
Discussion
If we consider the Moon to be a planet,
what type of planet would it be?
The simplest universe
Nothing ever changes!
The Sun and Moon do not move in the sky
There are no seasons
The stars do not move in the sky
The planets do not move in the sky
Second simplest universe
• Stars unchanging and fixed to the celestial
sphere
• Celestial sphere rotates about Earth
moving east to west once a day carrying
the Sun, Moon and planets with it.
• The Earth is at the center of the universe
and does not move, geocentric model
Third Simplest Universe
• Sun, Moon, and planets are carried east to
west along with rotation of celestial sphere
• Sun, Moon and planets move more slowly
west to east against celestial sphere
Ptolemy’s explanation for
retrograde motion
• Each planet moves on a small circle called
and epicycle.
• The center of each epicycle moves along a
larger circle centered near the Earth called
a deferent.
Ptolemaic system
• Very successful at predicting positions of
the planets but was not perfect
• Offered no explanation of why the planets
moved on deferents and epicycles
• There was no relationship between period
of revolution and epicycle size
Alternatives to Ptolemy’s model
Aristarchus proposed a heliocentric model of
the Solar System in the 3rd century B.C.E.
Reintroduced in the 16th century by
Copernicus
Heliocentric model
• Sun at the center
• Diurnal motion explained by rotation of the
Earth
• All the planets including Earth revolve
about the Sun in circular orbits with
different speeds
Disadvantages of the heliocentric
model
• Still required epicycles
• Was no better at predicting planetary
positions
• No stellar parallax observed
Advantages of heliocentric
model
• Provides natural explanation of retrograde
motion.
• Provides natural explanation for the motion of
mercury and Venus as inferior planets, i.e. their
orbits are interior to that of the Earth.
• Provides a relationship between distance from
Sun and orbital period. Planets farther from the
Sun took longer to complete an orbit.
Retrograde motion
Inferior planets
Discussion
How could you explain the the motion of
the inferior planets Mercury and Venus
with deferents and epicycles in the
geocentric model?
Tyco Brahe
Tycho Brahe’s Epilepsy Medicine:
The basic substance is the head of a person who
has been hanged or otherwise executed. The
head should be dried and crushed together with
peony seeds to a powder. This medicine should
not be taken at the full moon.
Tycho Brahe showed that the
celestial sphere could change
• Tycho’s supernova of 1572 – showed that
this new star had no parallax and thus was
more distant than the Moon
• Comet of 1577 – showed that it too was
beyond the distance of the Moon
Sextant
Tycho Brahe
Carefully tracked the position of the
planets for 20 years to unprecedented
accuracy in an attempt to disprove the
ideas of Copernicus.
Tycho’s model of
the solar system
Galileo
Galileo
Did not invent the telescope, but was
the first to publish astronomical
observations made with a telescope
Galileo’s Observations
• The Sun had spots which were considered
imperfections
• The Moon had mountains and valleys
• The Milky Way resolved into countless stars
• Jupiter had four moons that clearly orbited
it and not the Earth
• Venus had phases
Discussion
Explain why the observations of the phases
of Venus prove that Venus must orbit the
Sun. Why is this different than the Moon,
which also has phases but orbits the Earth?
Discussion
Why did Galileo's observations of the orbits
of the moons of Jupiter convince him that
the Copernican model of the solar dydtem
had to be correct?
Jupiter acted like a smaller
version of the Solar System
• Jupiter is bigger than its four moons and
the moons orbit it
• Jupiter’s moons orbit with periods that are
longer for those moons that are furthest
from the planet
Galileo is arrested
Galileo claimed that his observations proved
the Earth must revolve about the Sun which
was at odds with the teaching of the Church.
In reality, his observations merely proved
that Mercury and Venus orbited the Sun and
not the Earth.
Galileo’s Physics
• The Earth’s gravity accelerates all objects,
regardless of weight, by the same amount
• A moving object will stay in motion in a
straight line at a constant speed unless
acted upon by a force
Kepler’s first law of planetary
motion
The orbit of a planet about Sun is an ellipses
with the Sun at one focus.
Semimajor axis
½ the long axis of an ellipse
The distance between the Sun and the
planet averaged over the entire orbit
Kepler’s Second law of planetary
motion
A line drawn from the planet to the Sun
sweeps out equal areas in equal intervals of
time.
Kepler’s third law of planetary
motion
The square of the sidereal period is equal to
the cube of the semimajor axis of the orbit.
Example
p 2 = a3
An asteroid has a period of 8 years.
8  8 = 64 = 4  4  4
So the semimajor axis of this asteroid’s
orbit is 4 AU.
Galileo’s Physics
• Inertia: A moving object will stay in motion
unless acted upon by a force.
• Rate of falling: Gravity accelerates all
objects, regardless of weight, by the same
amount
Speed
distance traveled
Speed 
time
Example: a car moving at 60 miles/hour
Velocity
Velocity refers to not only how fast an object is
moving but its direction as well.
Example: a car moving 60 miles/hour due west.
Acceleration
Acceleration refers to any rate of change in
the velocity of an object.
An acceleration can mean a speeding up, a
slowing down, or simply a change in the
direction of motion with no change in
speed.
Units of acceleration
velocity distance
Accelerati on 

2
time
time
Example
A car accelerates from a stop light at 10 m/sec2
following a straight path. So, at time t = 0 the
car’s speed is 0 m/sec.
After one second of acceleration, the car’s
speed is 10 m/sec (velocity 10 m/sec south).
After two seconds, the car’s speed is 20 m/sec.
Discussion
After one minute of accelerating at 10 m/sec2
at what speed is the car moving?
Discussion
If I had two identical inclined planes placed
so that they faced each other and I rolled a
ball down one of the planes. How high up
the second inclined plane will the ball get
before it stops rolling? Explain.
Discussion
What if the second inclined plane is
replaced by one which is half as steep as the
first inclined plane. How high will the ball
reach on this plane?
Law of falling objects
In the absence of air resistance, all objects
fall with the same constant acceleration
regardless of the objects mass.
Newton
Newton’s first law of motion
The Law of inertia
A body remains at rest or moves in a straight
line at a constant speed unless acted on by a
net external force.
Inertia is the tendency of an object to resist a
change in velocity.
Discussion
Using Newton’s first law of motion why is
it a good idea to be wearing a seatbelt in
case of an car accident?
Newton’s second law of motion
Force = mass  acceleration
If the same force is applied to an object with half
the mass, the acceleration of that object will be
twice as much.
Discussion
Using Newton’s 2nd law of motion, explain why
you can throw a baseball farther than a shotput.
Discussion
If I pull on either side of the a pen as hard as I
can, what is the net force I exert on the pen?
Discussion
Which will do more damage to your car.
Hitting a brick wall at 60 miles per hour which
does little damage to the brick wall.
A head on collision with another car traveling at
60 miles per hour in the opposite direction with
the same mass such that both cars immediately
come to rest.
Newton’s third law of motion
For any force there is always an equal and
opposite reaction force.
Example: Walking
In order to walk, you have to push with
your foot back on the ground.
The Earth pushes your foot with an equal
and opposite force.
While the forces are equal and opposite the
response to that force is not, because the
masses are very different.
Discussion
If I put my car in neutral and try to push it
with a force F, according to Newton’s third law
my car pushes back with the same force.
Therefore, the car should never move. Is
Newton wrong? Why or why not?
Discussion
Your on a space walk outside the space station
using Nasa’s jetpack to move around when it
breaks down leaving you stranded 500 feet
from safety. All you have is a bag of tools.
How do you get back to the space station.