The Ptolemaic and Copernican Models
Summary
TLDRThis video explores the Ptolemaic and Copernican models of the universe. The Ptolemaic model, refined by Claudius Ptolemy, was geocentric and used epicycles and deferents to explain planetary motion, including retrograde motion. It was widely accepted for over a thousand years. The Copernican model, proposed by Nicolaus Copernicus, was heliocentric, placing the Sun at the center and simplifying the explanation for retrograde motion. Despite being more intuitive, it was not significantly more accurate than the Ptolemaic model without additional modifications.
Takeaways
- 📚 The Ptolemaic model was a geocentric system that placed Earth at the center of the universe and was accepted for about 1400 years.
- 🌌 The Ptolemaic model used deferents and epicycles to explain the motion of planets, including the complex retrograde motion.
- 🔄 Retrograde motion is when a planet appears to move backward in the sky, which the Ptolemaic model attempted to explain.
- 🌞 The Copernican model, proposed by Nicolaus Copernicus, was a heliocentric system that placed the Sun at the center of the universe.
- 📈 Copernicus's model offered a simpler explanation for retrograde motion as a result of Earth overtaking other planets in its orbit.
- 📘 Claudius Ptolemy, the author of the Almagest, refined the geocentric model for better planetary predictions.
- 🌐 The Ptolemaic model was almost geocentric, with Earth being slightly off-center, and all celestial bodies orbiting around points near the Earth.
- 📊 The equant was a concept in the Ptolemaic model where the center of a planet's epicycle appeared to move with constant angular speed from a certain point.
- 📚 The Copernican model was not significantly more accurate than the Ptolemaic model initially, and Copernicus added epicycles to improve its accuracy.
- 📖 The transition from the Ptolemaic to the Copernican model was influenced by scholars like Galileo, who supported and expanded upon Copernicus's ideas.
- 📚 The script also suggests readings for those interested in the history of these models, including books by Davis Obel, Arthur Koestler, Owen Gingerich, and Timothy Ferris.
Q & A
What are the two main models of planetary motion discussed in the video?
-The two main models of planetary motion discussed are the Ptolemaic model and the Copernican model.
What is the Ptolemaic model and when was it published?
-The Ptolemaic model is a geocentric model of the universe that was a refinement of early Greek models. It was published around 150 AD.
How long was the Ptolemaic model accepted before being succeeded?
-The Ptolemaic model was accepted for approximately 1300 to 1400 years before being succeeded by the Copernican model.
What is retrograde motion and which planet's motion does the video use to explain it?
-Retrograde motion is when a planet appears to move backward in its orbit against the background stars. The video uses Mars' motion to explain this phenomenon.
What is the term for the circular orbit of the sun around a point near the earth in the Ptolemaic model?
-In the Ptolemaic model, the circular orbit of the sun around a point near the earth is called the deferent.
What is the term for the small circles that planets move along in the Ptolemaic model?
-In the Ptolemaic model, the small circles that planets move along are called epicycles.
Who was Claudius Ptolemy and what did he contribute to the Ptolemaic model?
-Claudius Ptolemy was an ancient astronomer who authored the Almagest, which refined the geocentric model for better planetary predictions.
What is the equant in the context of the Ptolemaic model?
-The equant is a point from which the center of a planet's epicycle appears to move with constant angular speed in the Ptolemaic model.
What is the Copernican model and how does it differ from the Ptolemaic model?
-The Copernican model is a heliocentric model where the sun is at the center of the universe. It differs from the Ptolemaic model by placing the sun, not the earth, at the center.
What was the profession of Nicolaus Copernicus and when did he live?
-Nicolaus Copernicus was a canon of the Catholic Church and lived from 1473 to 1543.
How did the Copernican model explain retrograde motion?
-The Copernican model explained retrograde motion by the relative positions and speeds of the planets as they orbit the sun, causing the apparent backward motion from Earth's perspective.
Outlines
🌌 Introduction to Ptolemaic and Copernican Models
The video begins by introducing two major models of the universe: the Ptolemaic and Copernican models. The Ptolemaic model, refined from early Greek models and published around 150 AD, was accepted for approximately 1400 years. It's noted for its ability to predict the paths of the planets against the background stars, including the phenomenon of retrograde motion, where planets appear to move backward. The Copernican model, which succeeded the Ptolemaic model, is also mentioned, with a focus on its acceptance by scholars. The video uses the example of Mars' path from July 2020 to January 2021 to illustrate retrograde motion, which occurs approximately every 26 months. The script also discusses the motivations behind developing these models, including understanding the universe and casting more accurate horoscopes, as many early astronomers were also astrologers.
📚 Detailed Explanation of the Ptolemaic Model
The second paragraph delves into the specifics of the Ptolemaic model, which was nearly geocentric, placing Earth almost at the center of the universe. It describes the model's structure, which included deferents (large circles) and epicycles (smaller circles on the deferents) for each planet. The model accounted for retrograde motion through the use of epicycles. The script explains the concept of the equant, a point from which the center of a planet's epicycle appears to move at a constant angular speed. Despite its complexity, the Ptolemaic model was effective for many centuries, although it required each planet to have its own unique deferent, epicycle, and equant. The paragraph concludes with a mention of animations that simulate the Ptolemaic model, showing the motion of planets like Mars and the phases of Venus as predicted by the model.
🌉 Animation of Ptolemaic Model and Venus Phases
The third paragraph discusses an animation that demonstrates the Ptolemaic model, showing the motion of Mars against the background stars and the sun's apparent motion. It corrects a common misconception by clarifying that, in the Ptolemaic model, the sun would be inside the deferent for Mars. The animation illustrates how Mars appears to move backward (retrograde motion) and then forward again. Additionally, the paragraph explores the phases of Venus as predicted by the Ptolemaic model, which were not observable at the time of Ptolemy but later confirmed by Galileo's observations. The animation shows how Venus would appear as a thin crescent and how its phases change over time, according to the model.
🌏 The Resurgence of the Ptolemaic Model and the Copernican Revolution
This paragraph discusses the resurgence of the Ptolemaic model in Europe after the fall of the Roman Empire, which had been preserved by other cultures such as in India and Arabian countries. It then introduces Nicholas Copernicus, who proposed a revolutionary model with the sun at the center, known as the heliocentric model. Copernicus's work, 'De revolutionibus,' published in 1543, the year of his death, is highlighted. The paragraph contrasts the Ptolemaic and Copernican models, emphasizing the simplicity of the latter in explaining retrograde motion and the relative speed of planets. It also mentions that Copernicus's model was not significantly more accurate than the Ptolemaic model but laid the groundwork for future astronomical discoveries.
🔍 Copernican Model's Accuracy and Historical Context
The final paragraph addresses the accuracy of the Copernican model, noting that it was not much more accurate than the Ptolemaic model it replaced. Copernicus attempted to improve the model's accuracy by adding epicycles, but this only provided a marginal improvement. The paragraph also suggests readings for those interested in the history of these models, including books by Davis Obel, Arthur Kessler, Owen Gingerich, and Timothy Ferris. It concludes by setting the stage for a discussion of Galileo, who was a significant supporter of the Copernican model in the generations following Copernicus's death.
Mindmap
Keywords
💡Ptolemaic Model
💡Copernicus
💡Retrograde Motion
💡Deferent
💡Epicyclic
💡Equant
💡Heliocentric
💡Geographic Model
💡Astrology
💡Almagest
💡Revolutionibus
Highlights
The Ptolemaic model was a refinement of early Greek models and was accepted for about 1400 years.
The Copernican model succeeded the Ptolemaic model in many scholars' minds.
Planetary retrograde motion was a significant challenge for early models of the universe.
Claudius Ptolemy authored the Almagest, which refined the geocentric model for better planetary predictions.
The Ptolemaic model used deferents and epicycles to explain the motion of planets.
The equant was introduced in the Ptolemaic model to account for constant angular speed.
The Ptolemaic model predicted the phases of Venus, which were later confirmed by Galileo.
The Ptolemaic model was lost to European civilization after the Roman Empire's collapse but was preserved in other cultures.
Nicholas Copernicus proposed a heliocentric model of the universe with the sun at the center.
Copernicus's model provided a simple explanation for retrograde motion.
The Copernican model was not significantly more accurate than the Ptolemaic model initially.
Copernicus added epicycles to his model to improve its accuracy.
The Copernican model's circular orbits were not an accurate representation of actual planetary paths.
Galileo was a significant supporter of Copernicus's model in the generations after Copernicus's death.
The video also discusses the importance of understanding planetary motion for casting accurate horoscopes.
The Ptolemaic model's complexity included the Earth being slightly off-center in the universe.
Copernicus's work 'De revolutionibus' was published shortly before his death in 1543.
The video includes animations to illustrate the Ptolemaic and Copernican models' explanations of planetary motion.
The Copernican model's simplicity and placement of the sun at the center was a significant shift from the Ptolemaic model.
Transcripts
in this video we're going to look at the
ptolemaic and the copernican models
the ptolemaic model was a refinement of
the early greek models
and it was published in about 80 150 or
so
it was the model that worked or was
accepted for about the next
13 or 1400 years so for quite a long
time
the copernican model is the model that
succeeded the ptolemaic model at least
in
many scholars minds after this we'll
learn about more people's models but
we'll start with this one for reminders
you may have watched the video that i
made on
the paths of the planets against the
background stars
here's the path of mars back in july
2020 to about january 2021 so for about
a six month period
and it would
be the yellow line that starts over
there
to the right of the center
and then
most of the time mars will be moving
from left to right against the
background stars that's toward the east
but every once in a while it does this
weird little
backwards loop
and it's hard to see that as a loop if
we were viewing this from a different
angle you'd be able to tell but it does
something like that
and
the backwards part of it is called
retrograde motion
here's the path mars will take from
november 2026 to july 2026
the start of this in november 2026 would
be up at this end and
for most of the time it's moving from
west to east against the background
stars but then it reverses direction
retrograde motion here
then it goes back to pro-grade motion
heading from west to east again
and so
that is the motion
of mars against the background stars it
repeats its retrograde motion about
every two years and two months so about
every 26 months or so again and again
the other planets do this as well
sometimes in more complicated ways
especially for mercury and venus
understanding and predicting this stuff
was
a big challenge for early models of the
universe this motion looks complicated
and it is
they had a couple of incentives one was
just understanding
how the universe worked
and another one
another reason for doing so was so that
more accurate
horoscopes could be cast by the early
astronomers virtually all of them
up through galileo and kepler
were
let's see copernicus probably wasn't but
most of them were astrologers as well as
being astronomers
so first claudius ptolemy we have no
idea what he looked like but
this is a picture someone did hundreds
of years after his death
lived from about 85 to 165 we don't know
for sure when he was born or when he
died
he was the author of
something that became known as
almagest
and that's a combination of arabic and
greek the al is
arabic for the the majest is
greek for
the majestic or majestic
it was talking about the majestic things
in the heavens and their motions
but people
also thought of the book itself
as pretty majestic
because
it worked pretty well for a long time as
long as you didn't need to be closer
than about 10 degrees to the actual
position of something
10 degrees on the night sky is about the
width of your fist held at arm's length
in it ptolemy refined the
geocentric model for better planetary
predictions geocentric means
earth-centered you'll see as soon as we
look at ptolemy's actual model that it
was not actually earth-centered earth
was a little bit off-center
but things orbited the earth more or
less ptolemy was also known for a book
on geography mapping
and another on astrology i'm guessing
that is pronounced tetrabiblus or
something to that effect
so here's the ptolemaic model
and this is a fairly simplified version
of it but
it'll have the main features that we're
concerned with i'll show you a somewhat
more complicated model here in a minute
if you can contain this model in your
brain and describe it well
you'll have the hang of the ptolemaic
model first of all the ptolemaic model
was almost geocentric the earth was
almost at the center
but if you look at the
circles that are drawn for mercury's
orbit and venus and the sun and then
mars jupiter and saturn you may start to
notice that they're not exactly centered
on earth the sun had a circular orbit
around a point near the earth
and all of the planets had circular
orbits
around points
near the earth
and they all had epicycles epicycles are
the small
circles
that are on the larger circles the
larger circles are called deference and
the smaller circles are called epicycles
venus and mercury had interesting orbits
both of them orbited points on a line
between the earth and the sun
now the earth stayed fixed in the middle
of this thing
all of these things orbited around the
earth turned around the earth once a day
in fact
while continuing their motion along the
deference and the epicycles here's a
more complete picture of what a
planetary orbit would look like
here's the earth
right there
this is the center of the larger circle
here which is the deferent
there's an equine point
here and i'll talk about that in a
little bit
the planet itself is up here
and it moves around
the epicycle circle at the same time
that the epicycle circle
is moving around the deferent
and so this is pretty complicated
the point marked center
is the center of the deference circle
that puts the
deferent off center from the earth or
eccentric as a way of describing that
and the epicycle is a small circle
the planet moves along that at constant
speed
and that provides the appearance of
retrograde motion
if you look up there at the epicycle
circle the point marked x
when the planet is there
it'll be moving on the epicycle in the
same direction that the epicycle is
moving on the deferent
and so
the planet will appear to be moving in a
counterclockwise circle around the earth
but when the
planet is down here at point y or near
why
it'll be going on the epicycle in a
direction opposite the direction of the
deferent that motion can
produce an appearance that the planet is
moving backwards in space now a
complication tossed in here from earth's
viewpoint the epicycle does not move
along the deferent at constant speed
this would be something you wouldn't
actually try to produce an animation
with this thing or at least the ancient
greeks would not have done so
what they would do is have an equation
that described the speed of the center
of the epicycle
moving along the deferent they could
calculate all this stuff
the equant
equint the equint is a point
from which the center of the planet's
epicycle appears to move with constant
angular speed so if you were standing
there on the equine looking at the
epicycle and you were able to see the
center of the epicycle
that epicycle
would move along the deferent in such a
way that it looked to you like it was
always moving at the same speed to make
things worse each planet has to have its
own deferent
its own epicycle
and its own equine
so that was the ptolemaic model pretty
complicated
we can look at a couple of animations
that simulate things in the ptolemaic
model
here's the simulator
there are
links to this in your textbook if you
read the chapter section on this this is
the ptolemaic model earth is the blue
dot here
the gray dot is the center
of the deferent which is this circle
the green cross here
is the equine
and then
this circle is the epicycle
and then this red dot here represents
mars and that's just the one that we'll
happen to look at
and i'll start the animation
oh we need to back out here a little bit
we want to be able to see this zodiac
strip down here
and
the zodiac strip is where you'll be able
to see the apparent motion of mars
against the background stars
and in this case it actually is caused
by mars moving around this fixed earth
in the center
you can also see the apparent motion of
the sun
against the background stars now one
thing about this animation that isn't
correct is that it has the sun outside
of mars
the sun in the ptolemaic model would
actually be inside
the deferent for mars so it would have
been inside this circle
but appearance wise this will produce
exactly what the ptolemaic model would
show for a view so let's start the
animation
at times i'll slow this thing down
and so you can
see something
so here's just what the model looks like
you can see mars moving along that
epicycle while the epicycle is moving
along the deferent
and
you can watch that for a couple turns
here
and then down here on the zodiac strip
you can watch the sun moving against the
background stars with pretty constant
speed
you can have mars moving against the
background stars but every once in a
while it does something kind of weird so
i'm going to slow it way down
the next time
it starts getting close to
the sun here
mars is falling behind the sun by quite
a bit but it's reversing direction right
now
it's doing its weird little thing on the
epicycle
now the sun's going to catch up to and
past mars
now mars is going to be starting on that
part of its epicycle where it's moving
in toward the earth and it's when it's
along this part that it looks like it's
going backwards
against the background stars
there down here on the zodiac strip you
will soon see mars stopping
reversing direction
and then going forward again
that is the
retrograde motion
that you could see for mars
a simulation of that now there's one
more thing that i want to show you
with the ptolemantic model
and that is the phases of venus now at
the time ptolemy developed the model
the telescope was still let's see about
1
1450 years in the future they didn't
have any idea that there might be phases
of venus when galileo came along
he could talk about the phases of venus
that would be predicted by the ptolemaic
model and compare them to what he
actually saw in the sky
this time we're just looking at venus
and here we do have the correct geometry
we have earth here
venus is
deferent here and venus's epicycle here
and then the sun is outside of that
and
if things aren't lined up just perfectly
with earth venus and the sun
you can see a
small crescent extremely thin crescent
along the right side of venus there
so let's start the animation
and if you look at the bottom picture
here you can see
it's weird venus is a growing crescent
and it always pauses before it gets to a
half venus and then
disappears and goes back to
a crescent on the opposite side
this is what the ptolemaic model would
predict if you could watch venus for a
long enough time
now you'd have to be watching it for
long periods of time to be able to see
this
i'll slow it down a bit
but it's interesting the path of venus
or the phases of venus that would be
predicted by this okay let's move on to
the next model the ptolemaic model was
lost to european civilization after the
collapse of the roman empire
but other cultures in the world kind of
kept it alive in india and in
the arabian countries they hung on to it
translated it into their own languages
sometimes modified things a little bit
in the early years of the renaissance or
shortly after 1200 a.d it worked its way
back into european countries
people started copying it spreading it
around
it made its way back there
eventually someone comes along nicholas
copernicus who proposes a different
model of the universe
he lived from 1473 to 1543
he was a cannon of the catholic church
that's a
like an administrative position
a clerical position kind of thing not
clerical he actually had
administrative powers he would be
responsible for
seeing that taxes were collected in some
area
in those
years the catholic church was the
government in many places in europe and
so
they would have to collect taxes
uh
provide for the defensive areas from
marauding nights or something like that
have some way of delivering charity to
the poor all those sorts of things
his great
work called the revolutionibus it had a
longer title in that but the
revolutionibus is
good enough
it was published in 1543 you may notice
that there's a coincident date with the
top line and
that bottom line there
he may not have been aware of his book's
publication i bought a a new well new to
me book on copernicus it was actually
written in about 1970 i think the author
of this book i'll mention a bit later
you'll see his name here copernicus may
not have been aware of the publication
when the last parts of it were
delivered to where he was living and
working at the time he had had a
cerebral hemorrhage a few months before
it arrived
and one of his co-workers in a letter to
their local bishop said his memory and
mental powers had abandoned him but
here's the copernican model
in the simplified version he made it
more complicated later and i'll mention
those things but
it has the sun at the center it'll look
familiar it's got mercury venus earth
the moon mars jupiter and saturn
it had the approximate correct spacing
for these planets this is a simplified
version of it where
the planets seem to increase in orbital
diameter in a regular way but
copernicus's model wasn't that simple
still
they're all circles
but it's got the sun at the center we
call that
heliocentric helios is an ancient word
for the sun it had circular orbit it had
a simple explanation for retrograde
motion
let's look at another computer animation
for that here we have the sun at the
center we have an observer planet which
will be earth
we'll have a target planet which for
this one will be mars
it's got uh we haven't learned what
these are yet but uh
an au is an astronomical unit
it happens to be the diameter or the
radius of earth's orbit pretty close to
it
and then mars orbit is has an orbital
radius of about 2.4 astronomical units
let me slow down the animation a little
bit for starters
and
again you can watch the motion up here
or you can watch the apparent motion of
the planet against the background stars
as
they're traveling around the center of
this thing
now i maybe slowed it down way too much
but you can see that the sun
has a pretty constant motion
against the background stars
now
earth is going to
be moving into such a position that
mars will be behind the sun at this time
and the sun catches up to and passes
mars
i'll speed this up a bit so you don't
fall asleep
we're soon going to have i paused it
here we're soon going to have
earth catching up to and passing mars in
its orbit
and watch the bottom as that's occurring
mars stops
moves backward for a time
and then moves forward
and that is the simple explanation for
retrograde motion
copernicus said
the narrower a planet is to the sun the
faster it travels
and just by saying that
it takes care of explaining the
retrograde
the retrograde motion of mars jupiter
and saturn it also explains the weird
behavior of mercury and venus that we
can see
not perfectly however
so that's something that we'll have to
concern ourselves with let's have it
catch up earth catch up again and then
i'll
slow things way down
okay mars appears to be frozen in space
but as earth
passes it it looks like it moves
backwards for a time
against the background stars
at some point it freezes in space again
and then starts going forward
or
eastward against the background stars
again
so that's the copernican explanation for
retrograde motion
simple explanation
accuracy well
for starters the copernican model was
hardly any better than the old ptolemaic
model which over the years had gotten
less and less accurate
as far as predicting planetary positions
people had tried to update it from time
to time
but still it suffered from that and
copernicus's model did as well
and so to improve the accuracy of his
model
he made it more complicated he added
epicycles for the planets
and
it only slightly improved things if he'd
spent enough time on it he might have
narrowed it down made it a bit more
accurate but
the problem was you're trying to say
that things are traveling in circular
orbits and they're not doing that so
that's just the way it goes by the way
if you like reading about history
here's some books that i've read dealing
with this era galileo's daughter by
davis obel
interestingly
galileo and one of his daughters maria
celeste will learn about
her or him later
but
she appeared to have been his
intellectual equal
and they wrote letters back and forth
for many years galileo's copies of those
letters were saved but they'd never been
translated to english until
just the first part of the 2000s when
davis obell did that
letters to my father is the letters
themselves galileo's daughter is
in large part a biography of galileo
the sleepwalkers by arthur kessler
is kind of a complete history it's
tough going it's not a really exciting
book
the book i really enjoyed was the great
copernicus chase by owen gingrich
and the book no red nobody read by owen
gingrich
and another coming of age in the milky
way by timothy ferris i really enjoyed
that last book i read it close to 30
years ago and i've re-read it two times
since then he has a a way with words in
writing about history
okay next up will be galileo
galileo was the big supporter of
copernicus in the generations after
copernicus died
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