G E O C E N T R I S M

A R I S T O T L E , P T O L E M Y, A N D T H E

M E D I E VA L W O R L D V I E W

• Question: What causes the motion of the stars, sun, moon, and planets?

• Answer: they are carried along by celestial spheres rotating around a stationary Earth (diagram by Peter Apian, Cosmographia, 1524)

• This opinion was taken from the ancient greeks, particularly Aristotle (384-322 b.c.) and earlier Greek astronomers.

• The ancient Greeks astronomers were not unanimous (e.g. Pythagoreans)

• Geocentrism was “perfected” by Ptolemy in the 2nd century A.D.

• Lived (100-165 A.D.) in Alexandria, Egypt.

• Wrote on geography, mathematics, music, philosophy and astronomy

• Most famous work was The Almagest, which consisted of 13 books discussing the size, shape, location and motion of the Earth, sun, moon, planets and stars.

• The Almagest served as an astronomy textbook for over 1500 years

• The name (lit. “The Greatest”) derived from title given it by Arabic scholars who translated it from Ptolemy’s original Greek.

• It was translated much later from Greek into Latin by Peuerbach (1423-1461) and his student Regiomontanus (1436-1476).

C L A U D I U S P T O L E M Y

• Theoretical philosophy is divided into three areas: theology, physics and mathematics

• Astronomy serves a practical purpose in society by developing a student’s character/morals/soul

• Book I: summary of Aristotle’s (384-322 B.C.) world-view

• The heavens spin like a sphere

• The earth is a sphere in the middle of the heavens

• The earth is very small compared to the heavens

• The earth is stationary (contrary to Pythagoras)

• There are two primary motions of the heavens

• Books II - XIII: describe Ptolemy’s specific observations and contributions

P T O L E M Y ’ S W O R L D - V I E W

celestial equator

north star

celestial sphere

Earth

axisT H E C E L E S T I A L S P H E R E

• Centered on Earth

• Rotates around Earth once each day

• All stars are fixed to it

• Polaris lies at north pole

horizon

zenith

celestial equator

north star

celestial sphere

Earth

axis

• Half of the celestial sphere

lies above (and half below)

the horizon at any time

• Zenith (overhead point) is 90

degrees from horizon

T H E H O R I Z O N F O R A N O B S E R V E R I N M I LW A U K E E

horizon

horizon

zenith

celestial equator

north star

celestial sphere

Earth

axis

• Notice: all stars rise and set

at an angle with respect to

horizon

T H E H O R I Z O N F O R A N O B S E R V E R I N H A W A I I

horizon

horizon

zenithcelestial equator

north star

celestial sphere

Earth

axis

• Notice: Polaris is on (or just

below) the horizon

• All stars rise and set straight

up from horizon

T H E H O R I Z O N F O R A N O B S E R V E R O N T H E E Q U AT O R

horizon

zenith

celestial equator

north star

celestial sphere

Earth

axisB A C K I N M I LW A U K E E ( 4 4 D E G R E E S N L AT. )

zodiac

horizon

zenith

celestial equator

north star

celestial sphere

Earth

axis

• A set of 12 constellations

that form a narrow tilted belt

around the celestial sphere

• The sun always lies in front of

one of these constellations

• The sun travels (west to east)

through the zodiac once per

year

T H E Z O D I A C

Sun

horizon

zenith

celestial equator

north star

celestial sphere

Earth

axis

• The sun is fixed to a tilted

inner sphere

• The axis of this sphere is

fixed to the outer sphere.

• The inner sphere rotates with

the outer sphere daily.

• The inner sphere also rotates

once per year—in the

opposite direction.

H O W D O E S T H E S U N M O V E T H R O U G H T H E Z O D I A C ?

zodiac Sun

horizon

zenith

celestial equator

north star

celestial sphere

Earth

axis

• The sun is fixed to a tilted

inner sphere

• The axis of this sphere is

fixed to the outer sphere.

• The inner sphere rotates with

the outer sphere daily.

• The inner sphere also rotates

once per year—in the

opposite direction.

H O W D O E S T H E S U N M O V E T H R O U G H T H E Z O D I A C ?

zodiac

horizon

zenith

celestial equator

north star

celestial sphere

Earth

axis

T H E S U N ’ S O R B I T O N T H E D AY O F T H E V E R N A L E Q U I N O X

• The sun appears fixed

between Pisces and Aries

• The sun rises directly to the

east, and sets directly to the

west.

• The sun does not pass

through your Zenith

zodiac

horizon

zenith

celestial equator

north star

celestial sphere

Earth

axisT H E S U N ’ S O R B I T O N T H E D AY O F T H E S U M M E R S O L S T I C E

• The sun appears fixed

between Gemini and Cancer

• The sun rises to the north-

east, and sets to the north-

west.

• The sun does not pass

through your Zenith

zodiac

horizon

zenith

celestial equator

north star

celestial sphere

Earth

axisT H E S U N ’ S O R B I T O N T H E D AY O F T H E W I N T E R S O L S T I C E

• The sun appears fixed

between Sagittarius and

Capricorn.

• The sun rises to the south-

east, and sets to the south-

west.

• The sun hugs the southern

horizon

• Based largely on Aristotelian and Ptolemaic geocentrism

• Ptolemy’s cosmology worked: it allowed for extremely accurate predictions of the rising and setting times of the sun, moon and planets

• Problems with Ptolemy’s system eventually led to the acceptance of Copernicus’s heliocentric system (published 1543).

• The Copernican system was often worse than the Ptolemaic system in predicting celestial events.

T H E M E D I E VA L W O R L D - V I E W