Transcript History of Astronomy

ASTRONOMY

The study of matter in outer space, in
particular the positions, dimensions,
distribution, motion, composition, energy,
and evolution of celestial bodies and
phenomena.
History of Astronomy
How have ideas about the solar system
and our place in it changed?
Egyptians
Saw patterns in the Sun, moon, and
Venus
 Called the sun “Ra” (Sun god) – rode in
his boat across the sky daily
 Developed 365 day year solar calendar
 Thought the milky way (The band of stars
in the sky) was a river that the dead
floated down to the afterlife.
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Ra (on right)
Sumerians/Mesopotamia
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The Sumerians
thought the night was
a blanket thrown over
them by the gods and
the stars were holes
in that blanket.
3300 BC to 100 BC
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Astrology and the
zodiac signs,
recognized
constellations
Greeks
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Eudoxus, born 400 B.C.
 saw objects that moved in sky
 called planets, sun, and moon “wanderers”
hought the planets were “Wandering gods”.
(Hermes, Aphrodite, Endymion, Ares, Zeus
and Kronos)
 Earth motionless and at the center of universe
Ptolemy, AD 140
 Leading astronomer
 “geocentric” theory – earth is center of
universe. Lasted for 1400 years.
Greeks
• Ptolemy created a model of the universe that
accounted for the movement of the planets.
• Retrograde motion is the apparent westward
motion of the planets with respect to the stars.
March
Feb.
Jan.
Dec.
April
Sept.
May
Aug.
East
June
July
Retrograde
motion of Mars
West
Romans
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The Romans also
thought the planets
were gods. (Mercury,
Venus, Terra, Mars,
Jupiter and Saturn)
Vikings
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8th to 11th century AD
Accomplished
seafarers, navigated
by stars and sun
Stars were spirits of
dead warriors
SUNSTONE
Claudius Ptolemy AD 100-170
“EPICYCLES” and
retrograde motion
The Copernican Revolution
Nicolaus Copernicus (1473 – 1543):
Heliocentric Universe (Sun in the Center)
Copernicus’ new (and correct) explanation
for retrograde motion of the planets
Retrograde
(westward)
motion of a
planet occurs
when the
Earth passes
the planet.
This made Ptolemy’s epicycles unnecessary.
Copernicus
from book De Revolutionibus
The English version…
What do you think?
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Why do you think it was difficult for people to
accept a heliocentric model (sun centered) over
the geocentric model (Earth centered)?
Geocentric model
Heliocentric model
Tycho Brahe 1546-1601
Collected data of position of planets for 20
years
 Able to make accurate predictions of positions
without telescopes
 Had own “Tychonic Universe” – combination
of Ptolemy and Copernicus
 Earth is stationary
 Believed in circular orbits
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Tychonic Universe
Why the Circles?
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Why do you think these astronomers
believed in circular orbits?
Kepler 1571-1630
Elliptical orbits
Kepler
Works with Tycho – after Tycho’s death,
Tycho’s family sued to recover instruments
and books of observations
 Why?
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Kepler was a Copernican, and they knew he
wouldn’t follow the Tychonic system
Kepler kept the books! Began to study
motion of Mars …
Kepler’s First Law
of Planetary Motion
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Orbits are elliptical (oval shaped),
sun is at the focus
Kepler’s Second Law
of Planetary Motion
A line from a planet to the sun sweeps
over equal areas in equal times
 Planets travel at different speeds – due to
pull of gravity
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Closer to Sun,
moves faster…
Why?
Same time,
moves slower
Kepler’s Third law
of Planetary Motion
Harmonic Law
 The larger the orbit, the slower the orbital
velocity
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Kepler’s Unanswered Question
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What keeps planets in orbit?
Galileo Galilei (1564 – 1642)
• Invented the modern view of science:
Transition from a faith-based “science” to
an observation-based science.
• Greatly improved on the newly invented
telescope technology. (But Galileo did
NOT invent the telescope!)
• Was the first to meticulously report
telescope observations of the sky to
support the Copernican Model of the
Universe.
Major Discoveries of Galileo
• Moons of Jupiter
(4 Galilean moons)
(What he really saw)
• Rings of Saturn
(What he really saw)
Major Discoveries of Galileo (2)
• Surface structures on the moon; first estimates
of the height of mountains on the moon
Major Discoveries of Galileo (3)
• Sun spots (proving that the
sun is not perfect!)
Major Discoveries of Galileo (4)
• Phases of Venus (including “full Venus”),
proving that Venus orbits the sun, not the Earth!
Galileo
Moon surface
Phases of Venus
A New Era of Science
Mathematics as a tool for
understanding physics
Isaac Newton (1643 - 1727)
• Building on the results of Galileo and Kepler
• Adding physics interpretations to the
mathematical descriptions of astronomy by
Copernicus, Galileo and Kepler
Major achievements:
1. Invented Calculus as a necessary tool to solve
mathematical problems related to motion
2. Discovered the three laws of motion
3. Discovered the universal law of mutual gravitation
Newton’s Laws of Motion (1)
1. A body continues at
rest or in uniform
motion in a straight
line unless acted
upon by some net
force.
An astronaut floating in space
will continue to float forever in
a straight line unless some
external force is accelerating
him/her.
Newton’s Laws of Motion (2)
2. The acceleration a
of a body is
inversely
proportional to its
mass m, directly
proportional to the
net force F, and in
the same direction
as the net force.
a = F/m  F = m a
Newton’s Laws of Motion (3)
3. To every action,
there is an equal
and opposite
reaction.
M = 70 kg
V=?
The same force that is
accelerating the boy
forward, is accelerating
the skateboard backward.
m = 1 kg
v = 7 m/s
Laws of Motion Simplified
I. If you don't push it, it won't move.
II. If you push it, it will move.
III. If you push it, it pushes back.
Newton answers
Kepler’s Question…
What keeps planets in orbit?
The Universal Law of Gravity
• Any two bodies are attracting each
other through gravitation, with a force
proportional to the product of their
masses and inversely proportional to
the square of their distance:
F=-G
Mm
r2
(G is the Universal constant of gravity.)
Albert Einstein
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noticed that Newton’s laws of motion are only correct in the limit of
low velocities, much less than the speed of light.
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Theory of Relativity.
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Basically, the laws of physics are the same everywhere
It is actually two theories.
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SPECIAL RELATIVITY and GENERAL RELATIVITY
THEORY OF RELATIVITY
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Special Relativity.
This theory states that it is impossible to
determine whether or not you are moving unless you can look at
another object.
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Think about that. If you were in the middle of outer space far from any other
objects, how would you know whether or not you were moving? All movement is
relative to other objects.
Relative to the Earth, most meteorites move at about 25,000 miles an hour
(40,233 km), but if you were standing on a meteorite looking at another meteorite
going in the same direction as you and at the same speed, it would not appear to
move at all.
Special Relativity also says that the speed of light is always
constant. This means that no matter what you do to light it will
always go the same speed.
Special Relativity
Some Effects of Special Relativity
• Relativity of simultaneity This shows
that events that appear simultaneous in
one reference frame, may not do so in
another, and that what we perceive as the
present only corresponds to what is
occurring simultaneously to us, in our
reference frame
• Time Dilation: the time that a clock
takes between ticks appears to be longer
when the clock is moving relative to us
Time dilation and the relativity of simultaneity lead to
the twin paradox. The twin paradox shows that if one
twin travelled away from the other at near to the
speed of light, and then turned around and came
back again, they would not have aged as much as
their sibling on Earth. This appears to be a paradox
because Galileo's relativity states that there is no
absolute state of rest. This means that the twin on
Earth could equally consider themselves to be
moving away from their sibling at this speed. The
paradox is resolved by the fact that the moving twin
turns around and travels back to Earth. As they turn
around, they are forced to accelerate, and this breaks
the symmetry.
Some Effects of Special Relativity
• Length contraction: an object will
appear to be longer if we measure it while
we are moving parallel to it than if we are
stationary.
• The energy of a body at rest
is not 0. Instead, we find
E0 = m c2
THEORY OF RELATIVITY
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General Relativity redefined the laws of gravity.
States: it
is impossible to tell the difference between gravity and the force of
inertia from a moving object.
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if you climb inside of a spinning spacecraft, the inertia will cause you to move
towards the outside walls in a way that would feel just like gravity. This is why
future spacecraft designs often have large spinning cylinders attached to them.
General Relativity also says that large objects cause outerspace to
bend in the same way a marble laid onto a large thin sheet of rubber
would cause the rubber to bend. The larger the object, the further
space bends. Just like a bowling ball would make the rubber sheet
bend much more than the marble would
General Relativity
General Relativity
A new description of gravity
Postulate:
Equivalence Principle:
“Observers can not
distinguish locally
between inertial forces
due to acceleration and
uniform gravitational
forces due to the
presence of massive
bodies.”
Thought Experiment
This bending of light by the gravitation of massive
bodies has indeed been observed:
During total solar
eclipses:
The positions of
stars apparently
close to the sun
are shifted away
from the position
of the sun.
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New description of gravity as
curvature of space-time!
Another manifestation of bending of light:
Gravitational lenses
A massive galaxy cluster is bending and
focusing the light from a background object.
Other Effects of General Relativity
• Perihelion advance
(in particular, of
Mercury)
• Gravitational red shift: Light from sources near
massive bodies seems shifted towards longer
wavelengths (red).
Historical Overview