Ptolemy toPtolemy to Wednesday,

CopernicusCopernicus, January 28

Astronomy o

Many ancient cultures tood l ti l h Tand celestial phenomena. T

in the heavens and were abThe Chinese, Egyptians, Brhave left us evidence of thhave left us evidence of th

f the ancients

k note of celestial objectsTh t d t i ttThey noted certain patternsble to construct calendars.ritons, Mayans, and others

heir interest in astronomyheir interest in astronomy.

Stonehenge canStonehenge canbe used as anastronomical

calculator.

Greek scientist Aristotle showe

Aristotle supported his statement thobservations.

The Earth’s shadow on the Moon dcircular.

The only object that always throws

ed that the Earth is spherical.

hat the Earth is round with

during a lunar eclipse is always

s a circular shadow is a sphere

The only object whosThe only object whoscircular is a sphere.p

Aristotle (384 BC –

se shadow is alwaysse shadow is always

322 BC)

Eratosthenes (ca 200 BEratosthenes (ca. 200 Bmeasured the Earth’s ci

Eratosthenes: 42 000 kmEratosthenes: 42,000 kmActual: 40,000 km

BC Alexandria Egypt)BC, Alexandria, Egypt) rcumference:

Greek astronomers deGreek astronomers de(Earth-centered) mo

Basic assumptions of Gree

● Spherical Earth is statiouniverseuniverse.

● Earth is corrupt, heaven

● Heavenly bodies move wtimotion.

Bad assumptions bBad assumptions b

eveloped a geocentriceveloped a geocentric odel for the universe.

ek astronomers:

nary, at the center of the

ns are perfect.

with uniform circular

ad conclusions.ad conclusions.

Hipparchus of Rho

Important early astronomer:● catalogue of 1000 starscatalogue of 1000 stars● classified stars by brightness● discovered precession of the equ● determined: obliquity of the ecli● synodic periods of planets● inclination of Moon's orbit● place of Sun's “apogee”● t i it f th “S ' bit”● eccentricity of the “Sun's orbit”● estimate of the Moon's distance,

a baselinea baseline● He put astronomy on a geometri

odes (190-120 BC)

uinoxesiptic

, using the diameter of the Earth as

ical basis.

Ptolemy used epicyPtolemy used epicyretrograde moti

Ptolemy worked in AlexandAD 140AD 140.

Used results of Hipparchusmeasurements to create asystem worked

Wrote an astronomy text, la(= “the best”). ( )

Predicted positions of planewere adequately accuratewere adequately accurate

ycles to explain theycles to explain the ions of planets.

dria, was active around

’ research and d l f h h la model of how the solar

ater called the “Almagest”

ets far into the future that ee.

Basic structure of

B li f i P l ’Belief in Ptolemy’s geocentric model l d il hlasted until the 16th century.

Cosmographia first published 1524Cosmographia, first published 1524

geocentric model:

Geocentric models havretrograde mot

Planets usually move west toretrograde motion thretrograde motion, th

ve problems explaining p p gtion of planets.

o east relative to stars; during hey move east to westhey move east to west.

Ptolemy’s explanationn of retrograde motion:

The planet (P) moves in a small circle called the epicycle.

The center of theThe center of the epicycle (A) moves in a large gcircle called the deferent.

ThThhe combination of small andhe combination of small and large circles produces “l th l ” ti“loop-the-loop” motion.

Ptolemy’s model

During the Middle Ages, Ptolemy’s model had toPtolemy s model had to be fiddled with – more epicycles were addedepicycles were added.

The model was needlessly complicated because it pwas based on erroneous assumptions.p

l: did not fit data

OCCAM’S RAZOR

entia non sunt multiplicanda praeter necessitatemp

entities should not bemultiplied beyond necessity

William of Occam (c. 1285–1347 ?)

Copernicus proposed aCopernicus proposed athe uni

Mikolaj Kopernik(1473 1543)(1473-1543)

Poland

Stated that Sun notStated that Sun, not Earth, was at the

center of thecenter of the universe.

a heliocentric model fora heliocentric model for iverse.

Basic structure of h

Sun is at center.Earth revolves around SunEarth revolves around Sun.Earth rotates around axis.

heliocentric model:

In the heliocentric model oIn the heliocentric model omotion of planets is

Retrograde motions occurfurther from the Sun mofurther from the Sun mo

Example: Earth and Map

Earth’s orbital radius = 1 A

Earth’s orbital speed = 30

Mars’ orbital radius = 1 5Mars orbital radius = 1.5

Mars’ orbital speed = 24 k

of Copernicus retrogradeof Copernicus, retrograde naturally explained.

r naturally if planets ove more slowlyove more slowly.

ars

A.U.

km/sec

A UA.U.

km/sec

As Earth “laps” Marspbackward as seen by

● Earth catches up with M

– a-b

Passes it● Passes it

– b-f

– Apparent westward moti

S i● Sees it move to east aga

– gg

s, Mars appears to go , pp gy observer on Earth.

Mars

ion

iain

Heliocentric model ofHeliocentric model ofconsiderable (scie

Why? It implies that distah h dimuch greater than distan

● Stars do not vary much● Stars do not vary much course of a year.

● Stars do not show a largcourse of a yearcourse of a year.

f Copernicus met withf Copernicus met with entific) resistance.

ance from Sun to stars is f S E hnce from Sun to Earth:

in brightness over thein brightness over the

ge parallax over the

r

The parallax to the nearesst stars is about 1 arcsec (˝)

R di l fRadical aspects of

● Earth is not at center.

E th i i● Earth is moving.

● Earth is just another planEarth is just another plan

● Space is big – REALLY

Conservative aspects o

● Uniform Circular Motio

E i l till i d● Epicycles still required.

C i d lCopernican model:

net.net.

Y big.

of Copernican model:

on assumed.

Few closing

1) Do the inner planets h dshow retrograde

motions?

2) See picture on the i h I i l?right. Is it real?

3) In that picture, could you ) p , yhave Venus instead of Saturn? (tricky)Saturn? (tricky)

g questions:

Few closing ques

4) See picture on the right Is it real?right. Is it real?

5) In that picture, could ) pyou have Saturn instead of Venus?instead of Venus?

6) Could you have i d fMercury instead of

Venus?

stions continued: