Transcript Objection (Parallax)

Classical Greek
Astronomy
Mr. Catt
Astronomy
Quick History


The first traces of settlement date to around
3,000 B.C,
Several major civilizations rose and fell over
the centuries





Minoans: 2700-1450 B.C., mainly on Crete
Mycenaeans: 1550-1050 B.C, mainland Greece
Classical Greece: 776-323 B.C.
Macedonians: 323-88 B.C.
Romans/Byzantines: 88 B.C.-1453 A.D.
Problems




The biggest problem of studying Greek
astronomy is the lack of the original works
This is due to both the destruction of libraries,
such as at Alexandria, and the early Christian
Church's attitudes toward Greco-Roman culture
Most writings have been completely lost and
are known only through writings of later authors
Since the original works no longer exist, only
the general ideas, not the specific details are
known
The Beginning



The earliest astronomical references in Greece
were found in the works of Hesiod and Homer,
from nearly 3,000 years ago.
These pieces give practical advice for
navigation and farming
“When Orion and Sirius are come to the middle
of the sky and the rosy fingered dawn confronts
Arcturus, cut off all your grapes and bring them
home with you”-Hesiod, 700 B.C.
The First Scientists



The Greeks were notable in that they were the
first people that attempted to explain the world
in terms of natural phenomenon, leaving out
gods and myths
Obviously, many of the following theories are
wrong and may seem funny today, but they
were closer to explaining the world than the
stories of gods used by all previous cultures
The key innovation is the Greeks' attempt to
explain the world rationally
The Astronomers of Miletus
Thales





Lived 624-547 B.C.
One of the legendary Seven Sages, the first
known Greek to explain the world naturally
Originally a merchant who gathered knowledge
on his business travels, especially to Egypt
Learned to calculate distances/heights by using
geometry, which would later factor into
astronomy
Was said to have predicted a solar eclipse
Anaxamander

Lived 611-547 B.C.

Said to have been a student of Thales

Often called “The Father of Cosmology”



Gave the first physical explanations of the
celestial realm
The earth was a cylinder floating freely in space
The sun, moon, and stars were fire filed wheels
with holes that allowed their light to escape
Anaximenes




Lived 585-526 B.C.
Also gave mechanical explanations of the
universe
Believed that the stars were like nails, fixed to
the interior of a crystalline vault surrounding the
earth.
This idea would eventually grow into the
celestial sphere, which would dominate
astronomical thought for almost 2000 years
Pythagoras
Pythagoras



Lived 582-500 B.C.
Best known for his
theorem on right
triangles and his 5
perfect solids
Left no writings of his
own, so it's hard to
determine what ideas
were his and what
ideas were those of
his followers,
Astronomical Innovations




Believed that the Celestial bodies revolved in
circles around a hidden “central fire,” which was
rendered invisible by a “counter Earth”
Believed that the Earth and all other celestial
bodies were spheres
These two ideas were quickly accepted
Also believed that all of the celestial motion
makes a lot of noise, but that we can't recognize
it because we have no silence for comparison
The Pythagoras Model
Eudoxus
Universe Model Updated




Lived 408-355 B.C.
Expanded greatly on the idea of the crystalline
sphere by using many spheres to explain the
motions of celestial objects
The stars' motion was explained by the rotation
of a single sphere
The sun needed two spheres, one rotates
Westward once a day to account for
rising/setting and the other rotates Eastward to
account for motion through the Zodiac
Universe Model Updated





Planets were even more complicated in this
model, each needing 4 spheres
Spheres 3 and 4 rotate in the planet's synodic
period in opposite directions and slightly tilted to
account for the cyclical figure eight motion
Sphere 2 rotates Eastward to explain motion
through the Zodiac. This, combined with
spheres 3/4, accounts for retrograde motion
Sphere 1 rotates Westward once a day
What this model fails to account for is the
changing brightness of planets
The Eudoxus Model
Aristotle
Proofs of Earlier Ideas



Lived 384-322 B.C.
Offered proofs of earlier ideas
Earth is a sphere:




All falling objects move to the center of the Earth,
which is straight down. If the Earth were a cube, this
wouldn't always be true.
The Earth casts a circular shadow on the moon
during an eclipse
Some stars are visible in Greece that aren't in Egypt
Other celestial objects are spheres:

The shapes of the shadow on the moon in an
eclipse can only be true with a spherical moon
Long Lasting Ideas



Aristotle had the best reasoned argument for a
geocentric solar system
Taking the earlier idea of Empedocles that the
universe is made of earth, air, fire, and water,
Aristotle reasoned that since all of the heavy
materials are in the Earth, it is heavy and
therefore must be stationary
This reasoning helped cement the idea of an
Earth-centered solar system, which would
dominate for almost 2000 years
Argument for Spherical Earth
Spherical Earth
Flat Earth
Aristarchus
Astronomy and Geometry




Lived 310-230 B.C.
Used geometry to calculate the distance to the
moon and the ratio of the distances between
the earth and sun and earth and moon
Used sound geometry, but wrong data for the
distances, which resulted in estimates that
weren't even close
Once the size of the Earth was determined
(later) new calculations were made that were
remarkably accurate
Heliocentricism




Aristarchus was one of the few astronomers to
support the idea of a sun centered solar system
Aristarchus calculated the sun to be 7 times
bigger than the earth
To Aristarchus, it made no sense to place the
much smaller Earth at the center
To account for the motion of the sky,
Aristarchus reasoned that the motion was only
apparent, caused by the Earth spinning on its
axis once a day
Objections




The heliocentric model was largely rejected, the
only other astronomer known to support it was
Seleucus of Selucia
One objection was that, if the Earth moved, there
should be a wind caused by its motion
A second was that anything dropped should land to
the West of where it was dropped from as Earth
rushed by during the fall
A third was that if the Earth orbited the sun, two
stars should appear different distances relative to
each other in different seasons (Parallax)
Objections (Parallax)
Aristotle argued that the absence of parallax for the stars
in the sky implied that the Earth must be at the center of
the solar system.
–
This is a valid scientific argument.
Parallax
is the
apparent shifting of
nearby objects with
respect to distant
ones as the position
of the observer
changes.
Objection (Parallax)
Eratosthenes
Earth Measured




Lived 276-195 B.C.
Used geometry to calculate the circumference
of the Earth
The drive to do this came from reports that the
Sun cast no shadows at noon in Syene, located
in Southern Egypt, on the first day of summer
At the same time on the same day in
Alexandria, located in Northern Egypt, the Sun
cast shadows
The Method



Knowing the sun was directly 90 degrees
overhead in Syene, Eratosthenes measured the
sun's height in Alexandria, which was 82.8
degrees, meaning that the latitudes were 7.2
degrees, 1/50th of a circle in difference
He then had the distance between the two cities
measured, which was done by men specially
trained in pacing out the distance.
The end result came within a few percent of the
Earth's actual circumference
Measuring the Earth
Therefore the Earth’s circumference
was about 360/7  50 times the
distance between the two cities.
Knowing this distance he was
able to find the Earth’s diameter.
His calculation was very close to the
correct value.
Linear distance between Syene and
Alexandria: ~ 574 miles
Earth Radius ~ 4,597 miles (~ 14 % too
large) – better than any previous
radius estimate. (Actual radius is
3,963 miles)
Hipparchus
Antiquity's Greatest Astronomer





Lived 190-120 B.C.
Is responsible for recording the first variable
star, which drove him to make a catalog of the
sky, noting stars' position and brightness.
We can thank Hipparchus for magnitudes
When charting positions, he noticed that all
stars were about 2 degrees off from older
reference materials he was using
With this observation, Hipparchus discovered
the Precession of the Equinoxes
Precession Pictured
Claudius Ptolemy
Last Great Classical Astronomer





90-168 A.D.
Worked in Alexandria, Egypt
His greatest accomplishment was the perfection
of the geocentric system
The earlier model of Eudoxus failed to account
for the changing brightness of the planets, the
fact that different planets retrograde differently,
and was very complex.
Ptolemy's system was far simpler and worked
even better for explaining planets' motion
Ptolemy's Universe




Geocentric with the Earth at the center, planets
and sun orbit.
Each of the planets, in addition to orbiting the
Earth, moves in circles, called epicycles, on
their own orbits, which explains both retrograde
motion and planets' changing brightness
Since the system wasn't totally accurate for
predicting planetary motion, Ptolemy modified
the model, moving the Earth slightly out of
center
This system goes unchallenged for 1500 years.
Ptolemy's Model
An epicycle is
the circular
orbit of a
planet, the
center of which
revolves
around the
Earth in
another circle.
In Motion
The Marriage of Aristotle and
Christianity



In the 13th century St. Thomas Aquinas blended
the natural philosophy of Aristotle and Ptolemy’s
work with Christian beliefs.
A central, unmoving Earth fit perfectly with
Christian thinking and a literal interpretation of
the Bible.
People during the Middle Ages placed a great
reliance on authority, especially authorities of the
past.