General Astronomy: Lecture 2 - The Ancient Views of the Heavens

Summary

This 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

00:00

🌌 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.

05:00

🕰️ 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.

10:03

🔭 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.

15:05

🌐 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.

20:06

💫 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.

25:11

🌞 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.

Keywords

💡Astronomy

Astronomy is the scientific study of celestial objects, space, and the physical universe as a whole. In the context of the video, astronomy is portrayed as the oldest of the sciences, with roots stretching back to antiquity. It is highlighted by the careful observations of the sky that have been ongoing for thousands of years, indicating its significance in understanding the cosmos and our place within it.

💡Celestial Sphere

The celestial sphere is an ancient concept that refers to an imaginary sphere with the Earth at its center, upon which all celestial bodies are thought to be attached. Anaximander is credited with inventing this idea. In the video, the celestial sphere is mentioned as a key development in early astronomical models, showing how ancient civilizations began to understand the relative positions of celestial bodies.

💡Geocentric Model

The geocentric model is a now-disproven description of the universe with Earth at the center and all celestial bodies, including the Sun, Moon, and stars, orbiting around it. The video discusses how this model developed gradually over several centuries and was a dominant view until the Copernican heliocentric model was proposed. It illustrates the progression of human understanding of our place in the universe.

💡Retrograde Motion

Retrograde motion refers to the apparent backward movement of a planet against the backdrop of other stars, as viewed from Earth. This phenomenon was a challenge to the early geocentric model and is explained in the video as a reason for the development of more complex models, such as the Ptolemaic system, which used epicycles to account for these observations.

💡Epicycles

Epicycles are small circles in which celestial bodies were thought to move while those circles themselves revolved around a larger circle (deferent). This concept was developed by Apollonius and is discussed in the video as a method to explain the retrograde motion of planets within the geocentric model.

💡Copernicus

Copernicus is mentioned in the video as a pivotal figure in the shift from the geocentric to the heliocentric model of the solar system. His work is a stepping stone towards modern astronomy and represents a significant shift in understanding the structure of the solar system.

💡Ptolemy

Ptolemy was a Greek astronomer whose geocentric model, known as the Ptolemaic system, was widely accepted for over a thousand years. The video describes his model as a significant accomplishment in synthesizing earlier ideas into a system that could predict planetary positions with reasonable accuracy.

💡Stonehenge

Stonehenge is a prehistoric monument known for its alignment with the movements of the sun. In the video, it is used as an example of how ancient cultures used astronomical observations for practical purposes, such as marking the seasons and serving as a gathering place.

💡Aztec

The Aztec civilization is mentioned in relation to the Templo Mayor, a structure aligned with celestial events. This example from the video highlights how ancient cultures used astronomy for religious and ceremonial purposes, indicating the widespread significance of astronomy across different civilizations.

💡Precession

Precession, specifically the precession of the moon's orbit, is a concept touched upon in the video. It refers to the slow westward shift of the moon's nodes, which takes approximately 18.6 years to complete a cycle. The Sun Dagger, a petroglyph created by the ancestral Pueblo people, is mentioned as a structure that may have marked this lunar cycle.

💡Observational Science

Observational science is emphasized in the video as the foundation of astronomy. It involves making careful, repeatable observations of the sky to gather data and understand celestial phenomena. This approach is highlighted as a precursor to modern scientific methods, showing the video's theme of the evolution of scientific thought.

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]