Transcript History & Astronomers

History of Astronomy:
Don’t memorize dates
Who were the scientists?
What did they contribute to what we know
today?
When (approximate) did they make this
contribution?
What is the significance of that time
frame?
Ancient History-Greeks
(400B.C.-A.D. 150)
Aristotle (384-322 B.C.)
Determined that the earth is round. He
observed shadows cast on the Moon during
lunar eclipses.
Before Christopher Columbus!
Greeks-Two models
Heliocentric Model - Aristarchus(312-230
B.C.) = first Greek to believe this model
Figured out the size of and distances to the
moon and sun.
Used that to determine the sun as the center of
the universe
Earth and other planets revolve around the sun.
Not accepted by most Greeks
Ancient History - Greeks
Eratosthenes (276-195 B.C.)
Determined circumference of the earth using
geometry and trigonometry.
Hipparchus (190-120 B.C.)
Determined the location of 850 “fixed” stars
and the length of the year.
Greeks-Two models
Geocentric Model
Earth is the center of the universe.
Celestial sphere = transparent bubble that
contained all the stars, planets, etc.
Fixed stars = all the heavenly bodies that
remain stationary on the celestial sphere
Wanderers = 7 bodies that move within the
fixed (background) stars.
Included the sun, moon, Mercury, Venus, Mars,
Jupiter, and Saturn
Greeks- Accepted Model
Claudius Ptolemy- Proposed the Ptolemaic
System (A.D. ~127-145)
Circular Orbits
Geocentric model
Unchallenged for 13 centuries because it
predicted the motion of the planets.
Explained Retrograde motion-planets move
eastward across the night sky, but sometimes
will stop and go backward before going
eastward again.
Ptolemaic System
Recent History
Nicolaus Copernicus (1473-1543)
Proposed Earth as a planet
Supported heliocentric model
Explained most motions by a rotating Earth.
Religious/Social implications = goes against the
Roman Catholic Church
Think Inquisition of 1600’s! Acceptance means
to be fined or ostracized.
Tycho Brahe (1546-1601)
Famous for detailed and precise
measurements of the heavens
Extensive amounts of very precise data
Conclusions? None published, the Catholic
Church reigns supreme, also a believer of
Ptolemaic System. (Trying to prove it with
his research.)
Galileo Galilei (1564-1642)
1609 constructed a telescope(first to use
for astronomy, not the inventor.)
Discovered 4 satellites, moons, of Jupiter
Data that supported Copernicus
Planets are round
Venus has phases
Moon’s surface has mountain, craters, etc.
Sun has sunspots, and rotates.
Johannes Kepler (1571-1630)
Assistant to Brahe
Kept/Stole/Inherited data?
Using Brahe’s data, discovered the three
laws of planetary motion.
Kepler’s Laws of Planetary Motion
Law Number 1
The orbit of a planet around the sun is an ellipse
with the sun's center of mass at one focus.
Law #1-Summary
All orbits are ovals!
We draw circles with one focus. Ellipses
have two foci instead of one.
A.k.a. a mathematically perfect oval.
Circles are a special kind of ellipse.
So, all circles are ellipses,
But all ellipses are not circles.
In orbits, the sun is at one focus.
Sun (focus)
Path of planet
(ellipse)
QuickTime™ and a
DV - NTSC decompressor
are needed to see this picture.
Kepler’s Laws of Planetary Motion
Law Number 2
A line joining a planet/comet and the Sun
sweeps out equal areas in equal intervals of
time.
Another way of saying this is that the planet
moves FASTER in its orbit the closer it is to
the sun.
Perihelion = Point in orbit where planet is closest
to the sun, therefore, traveling faster
Aphelion = Point in orbit where planet is farthest
from the sun, therefore, traveling slower
QuickTime™ and a
DV - NTSC decompressor
are needed to see this picture.
Great! So?
Why would the planet move faster as it got
closer?
Many factors to consider. . .
Can partially explain it by the Universal
Law of Gravitation.
Sir Isaac Newton(1642-1727)
Universal Law of Gravitation
 All things have a force of gravity.
 Dependant upon mass and distance.
 The greater the mass the more gravity.
 The closer two objects, the more gravity.
 INVERSE RELATIONSHIP!
 Difference between mass and weight.
 Mass is the same everywhere.
 Weight is a measure of the force of gravity.
 Why don’t the Astronauts drift away from the moon?
 2nd law Tie in: Force of gravity is greatest when the planet is
closest to the sun.
Gravity in a nutshell
Mass matters!
We fall to the earth, not our neighbor, because Earth
is so much more massive.
Astronauts feel lighter on the moon because it is less
massive and has less gravity.
Distance matters!
We fall to the earth, not the sun, because Earth is
much closer.
Sun is still massive enough to keep Earth, and other
planets in orbit.
How does it relate to Kepler’s Laws?
When the planet is nearing perihelion, it will feel a
greater force of gravity from the sun. It will go
faster!
Gravity according to
Einstein (1879-1955)
Einstein has given us a frame of reference
for understanding Newton’s law of gravity.
Bent space model - Anything with mass
“bends” space.
More massive objects will stay put and bend the
space around them.
Less massive objects will “sink” in toward the
more massive ones.
Will investigate this in lab later this week.
Einstein’s Gravity
Kepler’s Laws of Planetary
Motion
Law Number 3
The period of a planet increases rapidly with the
radius of its orbit. (T2=d3)
Period=Time it takes for the planet to make one
revolution around the sun.
Law #3-Summary
Distance
 Don’t memorize the
Planet
Period
from sun
formula! (We’ll save
that for your physics
Mercury 0.38 AU
88
class.)
days
 Logically-it takes longer
Neptune 30 AU
60,266
to travel a larger
days
distance.
(165
 Think about the outside
lane on a race track vs.
the inside lane.
years)