Ptolemy and the Geocentric Model

Early Astronomy

Part: 1 of 2

Ptolemy’s Universe

Many ancient cultures

Many ancient cultures have taken a keen interest…

…in the changing

…in the changing night sky.

Many artifacts have survived

Many artifacts have survived until the present time…

…which make that fact

…which make that fact more than abundantly clear.

Artifacts

Artifacts from Antiquity

The QuadrantThe Quadrant

One of the earliest tools

One of the earliest tools for astronomical observation…

…was an instrument

…was an instrument known as the quadrant.

The quadrant would divide the sky

The quadrant would divide the sky into (4) segments of 6-hour arcs

It could then calculate a star’s latitude

It could then calculate a star’s latitude above the Earth’s surface.

The AstrolabeThe Astrolabe

The ancient Persian Astrolabe

The ancient Persian Astrolabe was another useful tool in Astronomy.

The astrolabe was similar

The astrolabe was similar to an early version of a star-chart,...

…and was useful in predicting the positions

…and was useful in predicting the positions of the Sun, Moon, planets, and stars.

StonehengeStonehenge

One of the best examples

One of the best examples of an early interest in

Astronomy…

…is found in the ancient stone monolith

…is found in the ancient stone monolith known as “Stone Henge”.

Built between 2,000-3,000 B.C.

Built between 2,000-3,000 B.C. in southern England,…

…Stone Henge still stands today

…Stone Henge still stands today as a popular tourist attraction.

The positions of the great rocks

The positions of the great rocks of Stone Henge…

…were used to mark the important events

…were used to mark the important events during the celestial calendar.

The risings of the Moon and the Sun,

The risings of the Moon and the Sun, for example,…

…can be seen in the photos above

…can be seen in the photos above during the

summer solstice.

ArtworkArtwork

Throughout the history

ArtworkArtwork

Throughout the history of mankind,…

ArtworkArtwork

…Astronomy has played a prominent role

ArtworkArtwork

…Astronomy has played a prominent role in various forms of artwork.

ArtworkArtwork

Shown above are several examples

ArtworkArtwork

Shown above are several examples of paintings featuring Astronomy

themes.

ArtworkArtwork

Van Gogh’s famous “Starry Night”

ArtworkArtwork

Van Gogh’s famous “Starry Night” is the painting shown on

the left.

ArtworkArtwork

the left.

Starry Night

ArtworkArtwork

the left.

Starry Night

ArtworkArtwork

The PyramidsThe Pyramids

The pyramids of the Sun and the Moon

The pyramids of the Sun and the Moon can be found near Mexico City, Mexico.

Built by the ancient Aztec civilization

Built by the ancient Aztec civilization around 100 A.D.,…

…they are still a popular tourist attraction

…they are still a popular tourist attraction even 2,000 years later.

…and there are many, more examples

…and there are many, more examples of the ancients’ interest in Astronomy

from the periods of antiquity.

But unlike today,

But unlike today, the need to understand Astronomy

was neither scientific nor religious.

The ancients instead

The ancients instead had more practical reasons

to study the Heavens.

NavigationNavigation

The ancient seafarers,

The ancient seafarers, for example,…

…needed to understand the stars

…needed to understand the stars in order to navigate their vessels by

night.

FarmingFarming

Additionally, farmers needed to know

FarmingFarming

Additionally, farmers needed to know how to predict the seasons…

FarmingFarming

…in order to know on which dates

FarmingFarming

…in order to know on which dates to plant and to harvest their crops.

FarmingFarming

Cosmetology

Cosmetology VS

Cosmology

CosmetologyCosmetology

If you wanted to study

If you wanted to studythe field of cosmetology,…

…then one day

…then one day you might want to work here.

CosmologyCosmology

However, if you would rather study

CosmologyCosmology

However, if you would rather study the field of cosmology,…

CosmologyCosmology

…then you might prefer to work

CosmologyCosmology

…then you might prefer to work in an observatory like this instead.

CosmologyCosmology

Cosmology

Cosmology –

Cosmology – The scientific study

Cosmology – The scientific studyof the structure and evolution

of the Universe.

The Sun

The Planets

The Sun

The Planets

The Sun

The PlanetsThe Earth

The Sun

The Moon

The Sun

The Moon

The Sun

The Universe,

The Moon

The Sun

The Universe, to the ancient Greeks,…

The Moon

The Sun

The Moon

The Sun

…was basically

The Moon

The Sun

…was basically just the Solar System.

Perseus Galaxy ClusterPerseus Galaxy Cluster

Today’s view of the Universe, however,

Today’s view of the Universe, however, is quite a bit different.

Cosmology entails looking at the Universe

Cosmology entails looking at the Universe on scales so large today…

…that even entire galaxies only appear

…that even entire galaxies only appear as mere pinpoints of light in

space.

Shown above is the Perseus Galaxy Cluster

Shown above is the Perseus Galaxy Cluster located 300 million LYs away from Earth.

And each pinpoint of light

And each pinpoint of light is a galaxy,…

…representing 100 billion

…representing 100 billion or more stars.

Imagine the total number of planets

Imagine the total number of planets contained in all of these galaxies.

Ptolemy and the

Ptolemy and the Geocentric Model

Claudius PtolemyClaudius Ptolemy

83 A.D. – 165 A.D.

The ancient Greek astronomer,

The ancient Greek astronomer, Claudius Ptolemy,…

…devised the Geocentric Model

…devised the Geocentric Model of the Universe.

The Geocentric ModelThe Geocentric Model

In the geocentric model

In the geocentric model of the Universe,…

…the Earth lay at the center

…the Earth lay at the center of all creation.

Everything else in the Heavens

Everything else in the Heavens then orbited the stationary Earth.

But unlike

But unlike the Sun, Moon, and stars,…

…the ancients noticed that the planets

…the ancients noticed that the planets would wander across the night sky.

Prograde Motion

If you watch the motions of the planets

If you watch the motions of the planets over many months,…

…you’ll notice that they drift eastward

…you’ll notice that they drift eastward across the night sky.

AprilMay

eastward

AprilMay

eastward

AprilMay

eastward

This forward and eastward drift

AprilMay

This forward and eastward drift is called “prograde

motion”,…

eastward

AprilMay

eastward

motion”,…

AprilMay

motion”,…

AprilMay

Septprograde motion

motion”,…

AprilMay

Septprograde motion

AprilMay

…and prograde motion is caused

prograde motion

AprilMay

…and prograde motion is caused by the planets’ orbits around the

prograde motion

Retrograde Motion

AprilMay

If you watch a planet’s motion

AprilMay

If you watch a planet’s motion more closely, however,…

AprilMay

…you’ll notice that the planet

AprilMay

…you’ll notice that the planet does something interesting over time.

AprilMay

After the planet moves forward

AprilMay

After the planet moves forward in the eastward direction for a while,…

AprilMay

eastward

AprilMay

eastward

AprilMay

eastward

…it then stops,

AprilMay

eastward

…it then stops, backs up,

AprilMay

eastward

…it then stops, backs up, and goes westward for a while.

AprilMay

eastward

AprilMay

westwardeastward

AprilMay

westwardeastward

AprilMay

westwardeastward

But then it stops,

AprilMay

westwardeastward

But then it stops, turns around,

AprilMay

westwardeastward

But then it stops, turns around, and moves in the eastward direction again.

AprilMay

westwardeastward

and moves in the eastward direction again.But then it stops, turns around,

AprilMay

westward

eastwardeastward

and moves in the eastward direction again.But then it stops, turns around,

AprilMay

westward

eastwardeastward

AprilMay

westward

While the forward and eastward motion

eastwardeastward

AprilMay

westward

While the forward and eastward motion was called “prograde

motion”,…

eastwardeastward

AprilMay

westward

motion”,…

eastwardeastward

AprilMay

westward

motion”,…

AprilMay

prograde motion

westward

motion”,…

AprilMay

prograde motion

westward

AprilMay

prograde motion

…this backward and westward motion

westward

AprilMay

prograde motion

…this backward and westward motion is called “retrograde

motion”.

westward

AprilMay

prograde motion

motion”.

westward

AprilMay

prograde motion

motion”.

AprilMay

prograde motion

retrograde motion

prograde motion

motion”.

Now watch the planet Mars

Now watch the planet Mars as it demonstrates this motion.

As the planet moves eastward,

As the planet moves eastward, it demonstrates the prograde

motion.

prograde motion

motion.

prograde motion

motion.

prograde motion

motion.

prograde motion

motion.

prograde motion

motion.

prograde motion

motion.

prograde motion

motion.

prograde motion

motion.

prograde motion

But then it stops,

prograde motion

But then it stops, turns around,

prograde motion

But then it stops, turns around, and shows westward retrograde motion.

prograde motion

retrograde motion

prograde motion

retrograde motion

prograde motion

retrograde motion

prograde motion

retrograde motion

prograde motion

retrograde motion

prograde motion

retrograde motion

prograde motion

retrograde motion

prograde motion

retrograde motion

prograde motion

retrograde motion

And finally it stops,

prograde motion

retrograde motion

And finally it stops, turns around again,

and shows the prograde motion again.

prograde motion

retrograde motion

prograde motion

retrograde motion

prograde motion

retrograde motion

prograde motion

retrograde motion

prograde motion

retrograde motion

prograde motion

retrograde motion

prograde motion

retrograde motion

prograde motion

retrograde motion

prograde motion

retrograde motion

prograde motion

retrograde motion

prograde motion

retrograde motion

prograde motion

retrograde motion

prograde motion

While it requires many months of observation,

prograde motion

retrograde motion

prograde motion

While it requires many months of observation, to witness the retrograde motion of Mars,

prograde motion

retrograde motion

prograde motion

retrograde motion

prograde motion

…it was nevertheless a phenomenon

prograde motion

retrograde motion

prograde motion

…it was nevertheless a phenomenon the ancients did not fail to notice.

The entire retrograde motion sequence

The entire retrograde motion sequence can be seen in this photo collage.

Can you explain why

Can you explain why this phenomenon is happening?

This slide shows an animation

This slide shows an animation of the retrograde

motion of Mars.

This slide shows an animation of the retrograde

motion of Mars.

Ptolemy Explains

Ptolemy Explains Retrograde Motion

In the Geocentric Model

In the Geocentric Model of the Universe,…

…Ptolemy had to explain this phenomenon

…Ptolemy had to explain this phenomenon of the retrograde motion of the planets.

Ptolemy explained that when Mars

Ptolemy explained that when Mars orbited the Earth,…

…that it did so

…that it did so using (2) pivot arms.

The larger of the (2) arms

The larger of the (2) arms makes a loop called a “deferent”,…

deferent

…while the smaller of the (2) arms

deferent

…while the smaller of the (2) arms makes a loop called an “epicycle”.

deferent

epicycle

When both of the pivot arms

When both of the pivot arms are moving in circles,…

…then the planet exhibits prograde motion

…then the planet exhibits prograde motion throughout most of its orbit.

But sometimes, however,

But sometimes, however, Mars also appears to

backup,…

…and when this happens,

…and when this happens, the planet exhibits retorgrade

motion.

Like this…

While Ptolemy did not correctly explain

While Ptolemy did not correctly explain the retrograde motion of

Mars,…

…his explanation nevertheless sounded

…his explanation nevertheless sounded both credible and plausible.

But Ptolemy’s explanation

But Ptolemy’s explanation for the retrograde motion of Mars,…

…did not alone prove

…did not alone prove that the Geocentric Model was correct.

The ancients also offered

The ancients also offered (4) other arguments,…

…in defense of the Earth’s position

…in defense of the Earth’s position at the center of the Universe.

4 Arguments for the

4 Arguments for the Geocentric Model

1) We Can’t Detect Earth’s Motion1) We Can’t Detect Earth’s Motion

The first argument given,

The first argument given, to support the Geocentric Model,…

…was that if the Earth revolved the Sun,

…was that if the Earth revolved the Sun, then we would feel the Earth’s motion.

But since the Earth’s motion,

But since the Earth’s motion, could not be detected,…

…then the Earth, they thought,

…then the Earth, they thought, could not possibly be moving.

But there are actually (5) ways

But there are actually (5) ways in which the Earth does in fact move.

1) The Earth is rotating

on its imaginary axis.

2) The Earth is revolving

around the Sun.

3) The Earth is “precessing”,

or wobbling, on its axis.

4) The Sun is revolving

around the center of the Milky Way.

4) The Sun is revolvingaround the center of the Milky Way.

5) The galaxies are moving apart,

as a result of the Big Bang.

5) The galaxies are moving apart, as a result of the Big Bang.

separating

But none of these motions

separating

But none of these motions could be felt by the ancients.

separating

And since none of these motions

separating

And since none of these motions were detectable,…

separating

…then the ancients incorrectly concluded

separating

…then the ancients incorrectly concluded that the Earth could not

possibly be in motion.

separating

Of course, the real reason

separating

Of course, the real reason why the Earth’s motion was not

detected,…

separating

…is because motion is relative

separating

…is because motion is relative and cannot be detected.

separating

But until Albert Einstein came along,

separating

But until Albert Einstein came along, 19 centuries later,…

separating

…the dynamics of relative motion

separating

…the dynamics of relative motion would not be fully understood.

separating

…the dynamics of relative motion would not be fully understood.

2) We Don’t Observe Stellar Parallax2) We Don’t Observe Stellar Parallax

A second argument given

A second argument given in support of the Geocentric Model,

…had to do with the failure of the ancients

…had to do with the failure of the ancients to observe parallax among the stars.

If the Earth were truly in motion,

If the Earth were truly in motion, the thinking went,…

…then a star would appear to shift positions

…then a star would appear to shift positions as the Earth orbited the Sun.

A star would appear in one position,

A star would appear in one position, in June for example,…

…and then appear in different position

…and then appear in different position(6) months later.

June December

And as the Earth

June December

And as the Earth revolved around the Sun,…

June December

…the star would then seem to demonstrate

June December

…the star would then seem to demonstrate a certain amount of parallactic shift.

June December

parallactic shift

June December

parallactic shift

June December

parallactic shift

But since stellar parallax

June December

parallactic shift

But since stellar parallax was not observed by the ancients,…

June December

parallactic shift

June December

parallactic shift

…then they thought that the Earth

June December

parallactic shift

…then they thought that the Earth could not possibly be revolving the Sun.

June December

parallactic shift

June December

parallactic shift

June December

parallactic shift

June December

parallactic shift

June December

parallactic shift

Of course, the real reason why the ancients never observed

parallax,…

June December

parallactic shift

June December

parallactic shift

…is because stars are too far away

June December

parallactic shift

…is because stars are too far away to observe parallax with the naked eye.

June December

parallactic shift

…is because stars are too far away to observe parallax with the naked eye.

Heliocentric Model

3) The Moon Could Not Keep Up 3) The Moon Could Not Keep Up

Heliocentric Model

The third argument given

Heliocentric Model

The third argument given in support of the Geocentric Model,

Heliocentric Model

…was that if the Earth orbited the Sun,

Heliocentric Model

…was that if the Earth orbited the Sun, that the Moon would lag behind.

Heliocentric Model

Since there is no tether

Heliocentric Model

Since there is no tether to pull the Moon along,…

Heliocentric Model

…then the Moon would be unable

Heliocentric Model

…then the Moon would be unable to keep up with the Earth’s motion.

Heliocentric Model

The flaw in the ancients’ thinking,

Heliocentric Model

The flaw in the ancients’ thinking, however,…

Heliocentric Model

…lay in the their failure to properly understand

Heliocentric Model

…lay in the their failure to properly understand Newton’s first law of motion.

Heliocentric Model

All objects move through space

Heliocentric Model

All objects move through space because they have inertia.

Heliocentric Model

And since the Moon and the Earth

Heliocentric Model

And since the Moon and the Earth were already in motion together,…

Heliocentric Model

…then the Moon has no problem at all

Heliocentric Model

…then the Moon has no problem at all keeping up with the moving Earth.

Heliocentric Model

But since the ancients

Heliocentric Model

But since the ancients knew nothing at all about inertia,…

Heliocentric Model

…then they incorrectly believed

Heliocentric Model

…then they incorrectly believed that the Earth could not possibly be in

motion.

Heliocentric Model

motion.

Heliocentric Model

motion.

Heliocentric Model

They therefore incorrectly concluded,

Heliocentric Model

They therefore incorrectly concluded, that the Earth must remain

stationary,…

Heliocentric Model

…and that the Geocentric Model

Heliocentric Model

…and that the Geocentric Model must be correct.

Heliocentric Model

…and that the Geocentric Model must be correct.

4) Objects Would Fly Off Earth 4) Objects Would Fly Off Earth

The fourth argument given

The fourth argument given in support of the Geocentric Model,

…had to do with the Earth’s rotation

…had to do with the Earth’s rotation on its axis.

If the Earth were to rotate on its axis,

If the Earth were to rotate on its axis, the ancients believed,…

…then anything not nailed down

…then anything not nailed down would fly off into space.

Since the ancients failed to observe

Since the ancients failed to observe objects flying off into space,…

…then they incorrectly concluded

…then they incorrectly concluded that the Earth must not be rotating.

Of course, the real reason why objects do not fly off into space,…

…is because the Earth

…is because the Earth is not spinning fast enough.

If the Earth were to spin

If the Earth were to spin 17x as fast as it currently spins,…

…then everything not nailed down

…then everything not nailed down would suddenly go airborne.

Since none of these questions

Since none of these questions could be answered,…

…then Ptolemy and the Geocentric Model

…then Ptolemy and the Geocentric Model remained unchallenged for 14 centuries.

End of Part 1

A Note from the Author~

If you liked my presentation, please email me at AstronomyTeacehr@aol.com and tell me why you liked it. I have made a powerpoint as thorough as this one for each Astronomy subject. I placed this one online just to see if it is getting any response and usage.

I am an Astronomy teacher in northern Virginia, age 50. I have taught Astronomy to the students of northern VA for 24 years.

March 24, 2012

Ptolemy and the Geocentric Model

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Transcript of Ptolemy and the Geocentric Model

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