Goal: To understand how Galileo
and Newton used
experimentation and theory to
further the fields of physics and
astronomy.
Objectives:
1) To learn about the discoveries of Galileo.
2) To understand what made Galileo great.
3) To understand how Galileo’s findings led to
the acceptance of Kepler’s Laws
4) To learn about the accomplishments of
Newton.
Galileo Intro (1564-1642)
• Kepler’s sun centered universe was still not
universally accepted.
• It was still untested after all.
• There were 3 main arguments against Kepler’s
model:
1) Aristotle held that if the earth DID move,
that birds, clouds, ect. would be left behind.
2) This contradicted Aristotle’s claim that the
heavens must be perfect and unchanging.
3) If the earth really did orbit the sun, then
you would be able to see stars move back and
forth over the course of the year due to stellar
parallax, which NOBODY had observed!
Which of the following does Galileo
get “credited” for?
For each a = yes, b = no
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1) Mountains on the Moon.
2) Sunspots
3) rotation of the sun
4) the phases of Venus
5) 4 moons of Jupiter
6) The Milky Way was in fact a collection of stars.
7) Invention of the telescope
8) First to see Neptune
9) Discovered that gravity accelerates all objects at the
same rate, regardless of mass.
1) Mountains (and craters) on the Moon.
• Galileo was the first to document the mountains
and craters on the moon (although someone
found them just earlier, but was unable to
document them).
• Galileo was able to spot the shadows the
mountains caused allowing him to know what
they were.
• Galileo’s ability to draw clearly and artistically
allowed everyone to see what he could see in
his telescope.
• Mountains and craters were seen as
imperfections.
Sunspots
• While the ancient Chinese reported
spotting Sunspots long ago, it was neither
documented nor known to Europe.
• Galileo was the first to document the
sunspots (by doing something you should
never ever do, look at the sun through an
unfiltered telescope).
• The cause was unknown, but the sunspots
were seen as blemishes on the sun.
Rotation of the sun
• By watching the sunspots, he would notice
them move around the sun in about a 26
day period.
• This was the rotation of the sun and 26
days is the rotational period!
Phases of Venus
• Venus was known to stay within about 40
degrees of the sun (and Mercury even closer).
• By this time, the phases of the moon were
understood to be caused by the sun’s orientation
to the moon in the sky.
• If Venus orbited the earth, you would only expect
crescent phases of Venus.
• Galileo discovered Venus went through ALL the
phases!
• This meant that Venus MUST be orbiting the
sun.
Galilean moons of Jupiter
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Discovered the 4 largest moons of Jupiter.
Observed that they orbit around Jupiter!
Jupiter was its own mini solar system!
Not everything orbits the earth!
The Milky Way
• Galileo turned his telescope towards the cloudy
patch known as the Milky Way.
• What he saw was a field of lots and lots of tiny
stars.
• He concluded that these stars had to be more
distant that the stars you can see with your
naked eye.
• Also, this could mean that the distance to the
known stars is probably much higher than
expected, and therefore too large to be
measurable with stellar parallax.
Invention of the telescope.
• Galileo was certainly the first to use the
telescope for astronomical purposes.
• Galileo in fact had a better model of
telescope than had been around.
• However, the telescope itself was invented
much earlier to use as a spyglass.
• Galileo did not invent the telescope, and
does not get credit for it although it is a
common misbelief that he does.
First to actually view Neptune.
• Neptune was discovered much later.
• However, one night Galileo was observing
Jupiter at a time when Neptune was very close
to it.
• Galileo reported a weird blue star that appeared
to move a little.
• However Galileo was more preoccupied with
Jupiter so did not go back to further inspect this
funny blue “star”.
• Therefore, Galileo made the first known
observation of Neptune.
Discovered that gravity accelerates
all objects at the same rate,
regardless of mass
• True!
• As long as you can remove wind resistance…
• He tested this by dropping things from the top of the
Leaning Tower of Pisa.
• He found that everything hit the ground at the same time.
• He dropped from differing heights, and measure the time
it took to hit the ground in order to calculate the
acceleration.
• Acceleration was constant!
• This made him the father of experimental physics!
• Other experiments showed that things also remained in
motion UNLESS some force slowed them – so birds
could remain with the earth.
Galileo’s reward:
• Catholic church shunned him.
• His book was banned.
• 6/22/1633 brought before an inquisition
where Galileo was forced, under penalty of
death if he did not, to recant his findings.
• Galileo’s Book was unbanned in 1824.
• Galileo vindicated in 1992.
Isaac Newton (1642-1727)
• Newton was the father of theoretical physics!
• Had some mathematical discoveries such as
calculus.
• Created “classical mechanics” – branch of
physics.
• Used a prism to prove that white light was
actually a combination of many colors of light –
lead to his book on optics.
• Came up with his 3 laws of motion.
Newton’s Laws:
• Law 1: Law of Inertia
A body in rest stays at rest until acted upon
by
an outside force, and a body in motion
remains in
motion until acted upon by an outside force.
• Law 2: Force Law
Force = mass * acceleration
• Law 3: the Law of Action and Reaction
Whenever one body exerts a force on a 2nd body,
the 2nd body exerts an equal and opposite force on
the first body.
Universal law of gravity
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Realized that gravity was universal!
F = G m M / r2
G is the gravitational constant.
This displaced the long standing believe that
physical laws on Earth and in the heavens were
different somehow.
• It explained the reason for the motions of the
planets around the sun.
• Was able to derive Kepler’s Laws, and in doing
so validating and verifying BOTH theories.
Universal law of gravity - questions
• F = G m M / r2
• How does the magnitude of the force of
gravity on me by the earth compare to the
force I exert on it?
• A) Same
• B) Earth does far more
• C) I do far more
Universal law of gravity - questions
• F = G m M / r2
• What is the gravitational acceleration on an
astronaut at low earth orbit (~200 km above the
surface or 6600 km above the center of the
earth) if the acceleration of the surface of the
earth is 9.8 m/s2?
• A) 0 m/s2
• B) 9.2 m/s2
• C) 10.4 m/s2
• D) None of the above (i.e. very tiny or really big)
Universal law of gravity - questions
• F = G m M / r2
• If you double your distance from an object
how does the force change?
• A) Doubled
• B) Halved
• C) Stays the same
• D) Quartered
• E) gets 4 times larger
Which of the following is true:
A) Apples fall faster than pears.
B) Gravity only exists on the surface of an
object.
C) Gravity extends outward far beyond the
earth.
D) Gravity at large distances creates a repulsive
force which keeps everything from crashing
into the sun.
Newtonian
Telescope!
• Created a far superior version of the telescope
(so superior it is the most widely used by
professionals 300 years later).
• This is a reflecting telescope.
• Previous telescopes were refracting (refract light
through a lens). This creates “chromatic
abrasion”.
Conclusion
• Where Kepler left off on his observational
astronomy Galileo picked up with his
experimental astronomy, and Newton
finished with his theoretical astrophysics.
• It took all 3 to put the notion of a Earth
centered system to rest at long last.