General Astronomy: Lecture 2 - The Ancient Views of the Heavens
Summary
TLDRThis astronomy lecture explores the evolution of human understanding of the cosmos, from ancient civilizations' practical uses of astronomy for timekeeping and navigation to the development of the geocentric model by ancient Greeks. It highlights how observations led to the realization that Earth is a planet in a vast universe and discusses the contributions of figures like Pythagoras, Plato, Aristotle, and Ptolemy in shaping early astronomical theories.
Takeaways
- 🌌 Astronomy is one of the oldest sciences, with roots stretching back to antiquity.
- 🌍 Ancient civilizations had practical reasons for studying astronomy, such as timekeeping, seasonal changes, and navigation.
- 🌜 People in central Africa could predict rainfall patterns by observing the orientation of the waxing crescent moon.
- 📅 Our modern measurements of time, like the day, month, and year, are based on astronomical observations.
- 🌞 The seven days of the week were named after the seven planets of ancient times, including the Sun and the Moon.
- 🕰️ Ancient cultures used simple tools like Sun dials and obelisks to tell time by observing the Sun's path.
- 🗿 Structures like Stonehenge and the Templo Mayor were built to mark astronomical events and align with celestial bodies.
- 🌐 The geocentric model, which placed Earth at the center of the universe, was developed by Greek philosophers over several centuries.
- 📉 The Ptolemaic model, a geocentric system, was able to predict planetary positions with reasonable accuracy and was used for over 1500 years.
- 🚀 The transition from the geocentric to the heliocentric model marks a significant shift in our understanding of the universe, moving towards modern science.
Q & A
What is the significance of ancient astronomy in understanding the cosmos?
-Ancient astronomy is significant because it represents the earliest systematic observations of the sky and the development of the first scientific models to explain celestial phenomena. It laid the foundation for modern astronomy and our understanding of the universe.
How did ancient cultures use astronomy for practical benefits?
-Ancient cultures used astronomy for practical benefits such as timekeeping, tracking seasonal changes, and navigation. For example, people in central Africa predicted rainfall patterns by observing the moon's orientation.
What is the origin of the seven-day week in relation to astronomy?
-The seven-day week originates from the seven planets of ancient times, which included the Sun, the Moon, and the five planets visible to the naked eye: Mercury, Venus, Mars, Jupiter, and Saturn. Each day was named after one of these celestial bodies.
How did the ancient Egyptians use the Sun to tell time?
-The ancient Egyptians used the Sun's path across the sky to tell time. They likely used simple Sun dials based on the shadows cast by sticks and built obelisks that may have served as large, accurate timekeeping devices.
What is the significance of Stonehenge in the context of ancient astronomy?
-Stonehenge is significant because it was used as an astronomical device to mark the seasons and as a social and religious gathering place. It has alignments that correspond to the Sun's position on the summer solstice.
How did the Aztecs use astronomy in their Templo Mayor?
-The Aztecs used the Templo Mayor, with its twin temples on a pyramid, to align with the Sun's position on the equinoxes. From a royal observer's vantage point, the Sun would rise through the notch between the temples on the fall and spring equinoxes.
What is the major lunar standstill and how was it observed at the Sun Dagger site?
-The major lunar standstill is an 18.6-year cycle where the full moon reaches its most southerly point along the eastern horizon. At the Sun Dagger site, this event was marked by the shadow of the full moon passing through slabs of rock to light the edge of a spiral.
What was the geocentric model of the universe, and why was it significant?
-The geocentric model placed the Earth at the center of the universe, with all celestial bodies moving around it. It was significant because it was the dominant model in Western thought for almost two thousand years and was the culmination of many Greek philosophers' and astronomers' observations and theories.
Who was Claudius Ptolemy and what was his contribution to the geocentric model?
-Claudius Ptolemy was a Greek astronomer whose geocentric model, known as the Ptolemaic system, adapted and synthesized earlier ideas into a single system that agreed well with astronomical observations at the time. His model could predict planetary positions with reasonable accuracy and was used for over 1500 years.
What was the role of Aristotle in the development of the geocentric model?
-Aristotle contributed to the geocentric model by arguing that gravity pulled heavy things toward the center of the universe, forming a spherical Earth. He also believed in celestial spheres responsible for the motion of celestial bodies, reinforcing the idea of an Earth-centered universe.
Outlines
🌌 Introduction to General Astronomy
The paragraph introduces the viewer to general astronomy, highlighting the ancient views of the cosmos and how our understanding has evolved. It emphasizes the Earth's status as a planet orbiting an ordinary star within a vast universe. The lecture aims to explore how we've learned about the universe through careful observation, which has been ongoing for thousands of years. Ancient civilizations had practical uses for astronomy, such as timekeeping, seasonal tracking, and navigation. An example is given of how people in central Africa predicted rainfall patterns by observing the moon's orientation. The paragraph also discusses how the length of our day, month, and year are derived from astronomical observations, and how the seven days of the week are named after the seven planets known to ancient civilizations.
🕰️ Ancient Timekeeping and Astronomy
This section delves into how ancient cultures used astronomy for timekeeping, with the origins of our modern clock traced back to ancient Egypt. The Egyptians divided the day and night into twelve equal parts, leading to our AM and PM system. The paragraph discusses various ancient structures like Stonehenge and the Aztec Templo Mayor, which were used to mark the seasons and perform astronomical observations. It also mentions how many cultures aligned their buildings with cardinal directions to track the Sun's movement. The Sun Dagger, a feature created by ancestral Pueblo people in New Mexico, is highlighted as an example of a structure used to mark the Sun's positions on special dates like the solstices.
🔭 The Accuracy of Ancient Observations
Paragraph 3 discusses the importance of accurate, repeatable observations in ancient astronomy and how they laid the groundwork for modern science. It explains how structures like Stonehenge or the Templo Mayor required meticulous observation to be built with such precision. The paragraph also touches on the rise of ancient Greek civilization and its contributions to science, including the development of the geocentric model of the universe. A timeline of important figures in the development of the geocentric model is provided, emphasizing the gradual refinement of astronomical understanding over centuries.
🌐 The Development of the Geocentric Model
This paragraph traces the development of the geocentric model of the universe, which placed Earth at the center. It starts with the philosopher Thales, who first proposed a non-supernatural explanation for the universe's composition, and continues through the contributions of Anaximander, Pythagoras, Plato, Eudoxus, Aristotle, and finally Ptolemy. Each figure built upon the ideas of the previous, refining the model to better match observations. The paragraph explains how the geocentric model, despite being incorrect, was a significant step forward in scientific understanding because it was based on observations and could predict planetary positions with reasonable accuracy.
💫 Retrograde Motion and the Ptolemaic System
The paragraph discusses the challenge of retrograde motion, where planets appear to move backward in the sky, and how it was explained within the geocentric model. It introduces the concept of 'epicycles' and 'deferents' used by Apollonius to explain these motions. The paragraph also details how Ptolemy synthesized earlier ideas into the Ptolemaic model, which could predict planetary positions with remarkable accuracy for its time. This model, despite being based on the incorrect assumption that Earth was the center of the universe, remained in use for over a millennium.
🌞 Transition to the Heliocentric Model
The final paragraph sets the stage for the next lecture, which will discuss the Copernican revolution that shifted the model of the universe from the geocentric to the heliocentric system, with the Sun at the center. It reflects on the durability of the geocentric model and the challenge of scientific paradigm shifts, emphasizing the importance of questioning established beliefs and continually seeking to align our understanding with observations.
Mindmap
Keywords
💡Astronomy
💡Celestial Sphere
💡Geocentric Model
💡Retrograde Motion
💡Epicycles
💡Copernicus
💡Ptolemy
💡Stonehenge
💡Aztec
💡Precession
💡Observational Science
Highlights
Astronomy is considered the oldest of the sciences with roots stretching deep into antiquity.
Ancient civilizations practiced astronomy for practical benefits such as timekeeping, seasonal changes, and navigation.
People in central Africa predicted rainfall patterns by observing the orientation of the waxing crescent moon.
The length of our day, month, and year, as well as the seven days of the week, have astronomical roots.
Ancient cultures used the Sun's path and shadows to tell time, like the ancient Egyptians with obelisks.
Stonehenge was used as an astronomical device and a social and religious gathering place.
The Aztec city featured the Templo Mayor, where the Sun rose through the notch between the temples on the equinoxes.
Many cultures aligned their buildings and streets with the cardinal directions to track the Sun's positions.
The Sun Dagger in New Mexico marked the sun's positions on special dates like the winter or summer solstice.
Ancient observations led to the development of modern science, with elements present in many early human cultures.
Greek philosophers developed a tradition of understanding nature without relying on supernatural explanations.
The Greeks used mathematics to give precision to their ideas about the universe.
Aristotle's geocentric model dominated Western thought for almost two thousand years.
Ptolemy's model could predict planetary positions within a few degrees of arc, keeping it in use for the next 1500 years.
The geocentric model, although incorrect, was a way of explaining observations and a part of scientific progress.
The next lecture will discuss the Copernican revolution, moving from the geocentric to the heliocentric system.
Transcripts
hello and welcome to general astronomy
lecture to the ancient views of the
heavens today we know that the earth is
a planet orbiting a rather ordinary star
and a galaxy more than 100 billion stars
that is in an incredibly vast universe
we know that the earth along with the
entire cosmos is in constant motion we
also know that on the scale of cosmic
time human civilization has existed for
only the briefest moment how do we come
to learn these things so that's what
we'll look at today careful observations
of the sky have been going on for many
thousands of years astronomy has been
called the oldest of the sciences
because its roots stretch deepest into
antiquity ancient civilizations did not
always practice astronomy in the same
way or for the same reasons that we
study it today but nonetheless had some
amazing achievements understanding this
ancient astronomy can give us a greater
appreciation of how and why science
developed through time interest in
astronomy likely stems due to parts of
our fundamental curiosity but ancient
cultures also had discovered that
astronomy had practical benefits for
timekeeping keeping track of seasonal
changes and navigation one amazing
example comes from the people of central
Africa although we do not know exactly
when they developed this skill people in
some regions learned to predict rainfall
patterns by making careful observations
of the moon the orientation of the horns
of a waxing crescent moon which is the
moon that you see in this chart relative
to the horizon varies over the course of
the year primarily because the angle at
which the sun's path through the sky
intersects the horizon that changes
throughout the year this orientation
also depends on latitude in tropical
regions in which there are distinct rain
and dry seasons rather than the four
familiar seasons at temperate latitudes
where we live the orientation of the
crescent moon can be used to predict how
much rainfall should be expected over
coming days and weeks so you can see on
this chart here how
dry season is typically from October to
February and rainy season is from March
to September and based on that relative
to the orientation of the moon so how
much it's angled to the luster to the
right they were able to use that to
predict when the rain and dry seasons
would be so a pretty remarkable
achievement way back then the impact of
ancient astronomical observations is
still with us in our modern measurements
of time the length of our day is the
time it takes for the Sun to complete
one full circuit of the sky because
there are throat eighths in 24 hours the
length of a month comes from the moon's
cycle of phases so the moon cycle of
phases takes about 29 and a half days so
that's where a month comes from the year
is based on the cycle of the seasons in
other words how long it takes us to
revolve not rotate around the Sun the
seven days of the week were named after
the seven planets of ancient times we
didn't know of all the planets we know
of now background so the seven planets
of Asian times which were the Sun the
moon and the five planets that are
easily visible to the naked eye Mercury
Venus Mars Jupiter and Saturn the
ancient definition of planets which
comes from a Greek word meaning wanderer
applied to any object that appeared to
wander among the fixed stars that is why
the Sun in the moon were on the list
while the earth was not because we do
not see our own planet moving in the sky
so back then they just considered any
wandering object a planet so that
wouldn't explain why the Sun and the
moon are on there because they appear to
be moving through the sky several of the
English names come for dramatic come
from with Germanic gods which you can
see here in this table for example our
day Tuesday comes from the Germanic God
- Wednesday comes from voting Thursday
from Thor and Friday from free on so you
can kind of see where some of these
things are coming from though
in the daytime ancient peoples could
tell time by observing sun's path
throughout the sky many cultures likely
use sticks and the shadows they cast as
simple Sun dials the ancient Egyptians
in fact Duty built huge obelisks often
decorated in homage to the Sun which
were likely served as simple clocks as
well this ancient Egyptian obelisk that
you see in the image which stands 83
feet tall and weighs 331 tons resides in
st. Peter's Square at the Vatican in
Rome it is one of twenty one surviving
obelisks from ancient Egypt much of what
most of which are now scattered
throughout the world shadows cast by the
obelisk may have been used to tell time
[Music]
we can trace the origins of our modern
clock to ancient Egypt some four
thousand years ago the Egyptians debated
divided daytime and nighttime into
twelve equal parts which is how we got
our twelve hours of each AM and PM the
abbreviations for a.m. and p.m. stand
for the Latin terms and meridiem and
post meridiem respectively which means
before the middle of the day and effort
the middle of the day so you're starting
to see that everything has these
astronomical roots many ancient cultures
built structures to help mark the
seasons Stonehenge is a well-known
example that served both as an
astronomical device and as a social and
religious gathering place the sketch on
the right shows the archaeologists or
how archaeologists believe that
Stonehenge looked upon its completion at
about 1550 BC several astronomical
alignments are shown as they appear from
the center for example the Sun rises
directly over the heel stone on the
summer solstice so the first day of
summer so Stonehenge is actually much
larger than what we typically see in
photos most of it has been deteriorated
over time but you can see for example
that on the first day of summer the Sun
will aligned directly over this heel
stone and there's lots of other
alignments as well
so it's amazing that even way back then
they are able to determine these things
in the Americas one of the most
spectacular structures was the templo
mayor and the Aztec city of I always say
this wrong tenor I can't say it I'm
sorry this is going to happen a few
times but it's modern-day Mexico City so
pardon me for not being able to
pronounce on these words it features
twin temples on a flat-topped pyramid
from the vantage point of a royal
observer with watching from the opposite
side of the plaza the Sun rose through
the knotch between the temples on the
equinoxes so that would be the fall and
the spring equinox before conquistadors
destroyed it Spanish visitors reported
elaborate rituals at the templo mayor
sometimes including human sacrifices
that were held at times determined by
astronomical observations so happy
thoughts this actually happens quite a
lot in the class we'll have a lot of
these happy thought moments but we'll
get through many cultures aligned their
buildings and streets with the cardinal
directions north south east and west
which made it easier to keep track of
the changing rise and set positions of
the Sun over the course of the year this
type of alignment is found at such
diverse sites as the Egyptian pyramids
the Forbidden City in China on the right
image and among the ceremonial Kiva's
built by the ancient ancestral Pueblo
people of the American Southwest
believes that ones in New Mexico they're
in the center this large structure more
than 20 meters in diameter is a Kiva in
Chaco Canyon New Mexico got ahead of
myself it was built by the ancestral
Pueblo people approximately 1,000 years
ago and its main axis which you see kind
of pointing from the top left to the
bottom right is aligned almost precisely
north-south
[Music]
other structures were used to mark the
sun's positions on special dates such as
the winter or summer solstice many sub
structures can be found around the world
but one of the most amazing is the Sun
dagger made by the ancestral Pueblo
people in New Mexico these large slabs
of rock lie in front of a carved spiral
which you see there on the left and the
right in such a way that they produce
special patterns of light and shadow at
different times throughout the year for
example a single dagger of sunlight
pierces the center of the spiral only at
noon on the summer solstice while two
daggers of light will bracket the spiral
at the winter solstice so here you see
that summer solstice where the lights at
noon will pierce the center of the
circle only on the summer solstice the
first day of summer and then here on the
right on the winter solstice you see the
two lines of light bracketing that
spiral only on that day the Sun Dagger
may have also been used to mark a
special cycle of the moon that had
ritual significance to the end ancestral
Pueblo people the rise in set positions
of the full moon vary in an eighteen
point six year cycle that's the cycle of
precession of the moon's orbit that
we'll get into later so the full moon
rises at its most southerly point along
the eastern horizon only once every
eighteen point six years at this time
known as a major luminal lunar
standstill the shadow of the full moon
passes through the slabs of rock to
light tangent to the edge of the spiral
in the Sun Dagger then nine point three
years later half of that time the lunar
shadow cuts through the center of the
spiral the major lunar stance though can
also be observed with structures nearby
at chimney rock and in cliff dwellings
at the Colorado Mesa Verde National Park
so it's incredible already that they can
use this to mark the first thing as
somewhere in winter but we believe that
it could also be used
could have also been used to predict the
moon's motions over a 20-year span
almost it's incredible
unfortunately however within just twelve
years of the site's discovery in 1977
the rocks had shifted so that the effect
no longer occurs and we believe that
these shifts were due to erosion of the
trails below the rocks because of a
large number of visitors so we we pretty
much destroyed it but still at least we
know what it is and we have visual
evidence of what it looks like so it's a
really remarkable feature before a
structure such as Stonehenge or the
temple mayor could be built careful
observations had to be made and repeated
over and over to ensure their accuracy
otherwise how would they know to build
them in that perfect way careful
repeatable observations also underline
the modern or underlie modern science
elements of modern science were
therefore present in many early human
cultures the past that led to modern
science emerged from the ancient
civilizations of the Mediterranean and
the Middle East especially from ancient
Greece Greece gradually rose to power in
the Middle East beginning around 800 BC
and was well established by about 500 BC
Greek philosophers developed at least
three major innovations that helped pave
the way for modern science first they
developed a traditional I'm sorry they
developed a tradition of trying to
understand nature without relying on
supernatural explanations and of working
communally to the baits and challenge
each other's idea so this is a really
big turning point up until those points
I mean almost everything was believed to
be supernatural but now they take away
that supernatural influence and they
debate and they talk to one another so
this was really the the birth of modern
scientific community second the Greeks
used mathematics to give precision to
their ideas which allowed them to
explore the implications of new ideas in
much greater depth than whatever
otherwise been possible so now they're
adding math into the equation
third while much of their philosophy
actively consisted of subtle debates
grounded only in thought and was not
scientific in the modern sense the
Greeks also saw the power of reasoning
from observations they understood that
an explanation could not be right if it
disagreed with observational facts and
so this is very important all three of
these I mean the community the use of
math and then the reliance and
observation this is really the beginning
to of the birth of modern science so I'm
taking this now and taking a look at
what the Greeks came up with to describe
the universe we start here with our own
solar system they had a way to describe
our solar system that we call the
geocentric model the Greek geocentric
model of the cosmos so named because it
placed a spherical earth at the center
of the universe develops gradually over
a period of several centuries because
this model is so important in the
history of science let's briefly trace
its developments so we're going to spend
the rest of this lecture really taking
ourselves from nothing to this model how
do we get to this point is basically
what we're looking at so this geocentric
model places the earth at the center of
the universe
hence geocentric and it's comparable to
a Mary ground as you see on the left so
you could consider the earth to be the
center the axis of that rotating Mary
ground and then perhaps the children on
the rides as the planets are moving
around that Center so the first thing
I'm going to show you now is actually a
timeline it's two slides because it
couldn't fit it on one slide this is a
timeline of all the important people
that led us to that geocentric model now
I'm not going to sit here and just list
them all off we will go through most of
the important people in the coming
slides so I put this in here simply for
you to reference it's a great resource
so here's the other half of it it's a
great place to look if you just wanted
some quick information about who did
this or when did this person do that but
we will go into detail of some of these
people know so these slides do serve as
a reference
so you go back a slide so we're going to
begin at the top left we're going to
begin with salaries we generally trace
the origin of Greek signs to the
philosopher Follies valleys was the
first person known to have addressed the
question what is the universe made of
without resorting to any supernatural
explanation his own guess that the
universe fundamentally consisted of
water and that the earth is a flat disc
floating in an infinite ocean was not
widely accepted even in his own time
nevertheless just by asking the question
he suggested that the world is
inherently understandable and thereby
inspired others to come up with better
models for the structure of the universe
so even though he was very incorrect as
we know today and not many people
believed him it was still a big turning
point in story and astronomy next is
Anaximander a more sophisticated idea
followed soon after proposed by a
student of the Hollies named Anaximander
he suggested that earth floats in empty
space so we're making progress
surrounded by a sphere of stars and two
separate rings along which the Sun and
Moon travel we therefore credit him with
inventing the idea of what we will call
the celestial sphere you hear me talk
about that a lot once we get through
this history lesson interestingly
Anaximander imagined earth itself to be
cylindrical rather than spherical in
shape now that sounds odd but think
about this he probably chose this shape
because he knew earth had to be curved
in a north-south direction to explain
changes in the constellations with
latitude because the visible
constellations do not change with
longitude he no he saw no need for
curvature in the east-west direction
right so as you travel north to south
along the globe what you see in the sky
in terms of constellations changes but
if you travel say from Durant Oklahoma
straight east or west you will see the
exact same constellations so it's kind
of reasonable that he thought curvature
was only along one direction
so that assumed the earth was
cylindrical even though seen as weird is
actually not too inaccurate to think
about back then all right next we have
Pythagoras we do not know precisely when
the Greeks first began to think that
earth was round but this idea was taught
as early as about 500 BC by the famous
mathematician Pythagoras he and his
followers envisioned earth as a sphere
floating at the center of the celestial
sphere much of their motivation for
adopting a spherical earth was full
philosophical the pythagoreans had a
mystical interest in mathematical
perfection and they considered a sphere
to be geometrically perfect so they
believed that we were the center of
everything we were perfect and all that
so they thought that the earth was
spherical not because it was an
observational fact at the time but
because that was perfection and that's
the way they viewed it and so it's
pretty amazing that it could be so
simple next is Plato the Pythagorean
interest and heavenly perfection became
deeply ingrained in most Greek
philosophers so that idea really really
held on for a long time as you'll see as
we go on it took on even more
significance after Plato asserted that
all heavenly objects move in perfect
circles and at constant speeds and
therefore must reside on huge fears
encircling the earth so not only now do
we think the earth is a perfect sphere
at the center of everything well we
believe everything else moves in perfect
spheres at perfect speeds around us the
Platonic belief and perfection
influenced astronomical models for the
next 2,000 years of course those Greeks
who made observations found plato's
models to be problematic the apparent
retrograde motion of the planets which
we'll talk about in a second the
backward motion of planets in the sky
already well known by that time clearly
show that the plans do not move at
constant speeds around earth so there
was already some issue with this idea
an ingenious solution came from plato's
colleague Eudoxus
who created a model in which the Sun the
moon and the planets each had their own
spheres nested within several other
spheres individually the nested spheres
turned into perfect circles by carefully
choosing the sizes rotational axes and
rotation speeds for the invisible
spheres Eudoxus was able to make them
work together in a way that reproduced
many of the observed motions of the Sun
the moon and the planets in our sky
other Greek models refined the model by
comparing its predictions to
observations and adding more spheres to
improve the agree agreement so here in
the top you see what we call that
retrograde motion so in the sky this is
for example the path of Mars at the
beginning of January starts over here
and moves from west to east throughout
the months but then for a little while
it moves
apparently backwards through the sky
that's that retrograde motion I
mentioned and then it continues its path
from west to east so that was a
challenge to this perfect circle idea so
you DOCSIS came along and started to
come up with some ideas of these nested
spheres which we'll get into more in
just a moment following Eudoxus is
Aristotle whether you DOCSIS and his
followers thought of the nested spheres
as real physical objects is not clear
but Aristotle certainly did an
Aristotle's model all the sphere is
responsible for celestial motion were
transparent and interconnected like the
gears of a giant machine Earth's
position at the center was explained as
a natural consequence of gravity
Aristotle argued that gravity pulled
heavy things toward the center of the
universe and allowed the lighter things
to float toward the heavens thereby
causing all of the dirt rock and water
of the universe to collect at the center
and form the spherical earth I guess not
a terrible idea back then
we now know that Aristotle was wrong
about both gravity and Earth's location
however largely because of his
persuasive arguments for an
earth-centered universe this geocentric
view dominated Western thought for
almost two thousand years to explain the
apparent retrograde motion of the
planets
Apollonia suggested that each planet
moved around earth on a small circle
that turned upon a larger circle the
small circle sometimes referred to as an
epi cycle and the large circle is often
called a defferent or deferment so two
important terms a planet following this
circle upon circle motion that you can
see in the schematic on the right would
trace a loop as seen from Earth with the
backwards portion of the loop mimicking
the apparent retrograde motion so that
graph that I showed you a moment ago
with that retrograde motion this could
explain it so this idea of a circle upon
circles so the idea is that the planet
itself moves in a little circle but as
it's moving in that little circle that
circle as a whole is moving around a
larger circle so if you were to trace
this out it would form this little bit
of a backward motion for some time
before returning back to that
west-to-east motion so right there you
could explain the retrograde motion
because of this idea among his many
accomplishments a park is developed the
circle upon circle idea of Apollonius
into a model that could predict
planetary positions so this is the first
time now that we are taking something
that we see and using it to predict
future positions to do this
the park is had to add several features
to the basic idea for example he
included even smaller circles that moved
upon the original set of small circles
and he positioned the large circle
slightly off centered from Earth so now
not only are we starting to predict
things but we're also slightly starting
to move away from this idea of
perfection by placing them off centered
from the earth so we're getting there
but the culmination of it all if this
geocentric model comes from : a Greek
modeling of the cosmos culminated in the
work of Claudius Ptolemy Toula mais
model still placed earth at the center
of the universe but it differed in
significant ways from the nested sphere
ideas of Eudoxus and Aristotle we refer
to Ptolemies geocentric model as the
Ptolemaic model to distinguish it from
the earlier models so he has his own
name now for this model the Ptolemaic
model tolas great accomplishment was to
adapt and synthesize earlier ideas into
a single system that agree quite well
with the astronomical observations made
at the time so even though it was still
wrong what he put together agreed to
reasonable extent with the observations
that were being made at that time so he
basically took everything and put into
one nice package in the end he created
and published a model that could
correctly forecast future planetary
positions within a few degrees of Arc
which means an arc a degree of Arc is a
sixtieth of a degree which we'll get
into later as well oh I can explain it
too I mean it's also about the angular
size of your hand held at arm's length
against the sky so right now you can
hold your arm out all the way extend it
with your hand sticking up and he could
predict planetary positions within a few
degrees of Arc which is the width of
your hand so that gives you an idea this
was sufficiently accurate at the time to
keep the model in use for the next 1500
years so this geocentric model came
together pretty quickly relatively
speaking through all these different
people and until my Tola may put it into
one nice package where the earth is at
the center of the universe and
everything moved around us even though
it was incorrect it was a way of
explaining what they saw so it was still
great that they were using science to
agree with the observations they were
making and that's a part of what makes
science so tough sometimes I mean for
thousands of years
they believe this was correct and it did
agree with observation so I mean who
knows even today we could be sure about
something and then a thousand years are
going to find out that we were totally
wrong and so we know that the earth is
not the center the universe so in our
next lecture we're going to talk about
the Copernican revolution that takes us
away from ancient the ancient Greeks
model of the earth centered system and
it takes us to the heliocentric system
where the Sun is the center of our solar
system which is in fact correct so now
we're going to start getting closer and
closer to modern science so as always
thanks for watching and I'll see you in
the next lecture
[Music]
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