Beginnings of Astronomy

Overview of Chap02a

!!Time KeepingTime Keeping

!Early Observatories

!Early Records

!Early Myths

!Earth centered Model

!Ancient Greek

Astronomers

! Heliocentric Theory

(Sun-Centered Model) " Seasons marked by equinoxes and solstices were important dates

for agriculture and hunting;

" Motion of the Sun, the Moon and stars tracked the time of day and

night for orientation and later navigation needs.

" The planets were known out to Saturn. Their wandering motion

made them distinct from the fixed stars.

Beginnings of Astronomy Time of Day ---Time of Day ---

Position of the SunPosition of the SunTime KeepingTime Keeping

Time of Year--

Position of Sun

relative to the horizon

and the Constellations

Local herders built a

stone circle on the

shoreline of a lake

that has long since

dried up. The stone

slabs three meters

high were dragged

over a kilometer to

create the site

marking the

solstices.

Nabta Early structures to

mark timeSouthern Egypt

! 6000 BC

Aubry holes ring of 56 holes,

Movement of a marking stone

would complete a circle in 18.67

years -- approximately the

period for the "nodes", needed

to predict the eclipses

Stonehenge ( 3000-2000 BC)English county

of Willtshire

Summer

Solstice

Sec 2.1

Ring of Sarsen

Stones

33 meters) in

diameter

Heel Stone on

midsummer's morning

the sun rose directly

over the Heel Stone

and the first rays

shone into the centre

of the monument.

-- a temple and/or astronomical calculator?.

SW United States: “Sun Dagger” marks summer solstice

"Spokes of the Big Horn Medicine Wheel built approximately 1050 AD by

the Lakota Native Americans

"aligned with rising and setting of Sun and other stars

"its 28 "spokes" may symbolize the days in a lunar month.

The "Caracol" at Chichén Itzá is

believed to have served as

an observatory( ! A. D. 600-850).

Certain doors and windows line up with

various planetary orbits.

Sec 2.1

Early Record: Babylonian

Tablet with a list of eclipses

between 518 and 465 B.C.

Around 4000 B.C: The oldest astronomical observations ever recorded

(Egypt and Central America)

Around 3000 B.C: The first written materials on astronomy (Egypt, China,

Mesopotamia and Central America)

2697 B.C.: The oldest preserved relation on the Sun eclipse (China)

Around 2000 B.C.: The first solar-lunar calendars in Egypt and Mesopotamia

Early Time Line

Tablet containing the text of Enûma Elish 12th

century BC

The Enuma Elish "When on High . . .”

The Mesopotamian/Babylonian Creation Myth

recounting the struggle between cosmic order

and chaos.

Early Myths of the Origin

In ancient Egypt the dome of

the sky was represented by

the goddess Nut, who

arched her back over the

earth so that only her hands

and feet touched the

ground. The sun god Ra

was born from her every

morning.

From Myths to Scientific Models

Thought that natural phenomena -

including the heavens - could be discussed

as processes governed by natural laws,

rather than relying on supernatural

explanations.

Scientific Method

Thales of Miletus 624- 547 BC

Thales of Miletus 624- 547 BC Model of the Universe

"In the beginning there was only water, that the world and all things were

composed of water. “Water” in this context was some unifying principle.

"The Earth was a large (flat) disk floating on an infinite ocean of water,

and that earthquakes resulted from disturbances in this ocean that shook

and cracked the Earth.

Aristotle

(384-322 BCE)

Ptolemy

(85 -165AD)

Thales of Miletus

624- 547

Ancient Greek Astronomers

Aristotle

(384-322 BC )

Claudius

Ptolmey

A.D. 85-165 Plato

427-347 BC

Thales of

Miletus

624- 547

BC

Pythagoras

560-500 BC

The father of Greek

mathematics

Entire universe can be

described in numbers

Systematized logic,

which forms the

basis of

western science

Jupiter

Venus (right)

The Greeks distinguished planets from stars because of their ‘wandering

paths’ through the sky (“planets” from ‘wanderer’ in Greek)--- Mercury,

Venus, Mars, Jupiter & Saturn plus the Sun and Moon

“Wandering Stars”

Saturn

(left),

Mars (top),

General Observations of the PlanetsTo naked eye, planets look like stars, but they change their position

relative to the other stars. Greeks called them "wandering stars" Motion

somewhat like sun and moon

#They were carried by the celestial

sphere from night to night with the

fixed stars.

#However, in addition they had

their own motion and they followed

their own circle path through stars.

#Each planet had its own period for

its circular path through stars.

#Planets sometimes reverse course

relative to stars, RETROGRADE

motion.

The Planets were objects of !perfect" shape andmoved on perfect paths, i.e., spheres and circles.

Greek Astronomy

Heavenly bodies were unchanging and perfect

wandering stars: Mercury, Venus, Mars,

Jupiter & Saturn plus the Sun and Moon

The 7 planets and the stars all moved in perfect

circular orbits, with perfectly uniform speeds.

The earth was spherical and placed it at the

center of the Universe.

Aristotle

(384-322 BC )

Celestial motion

Heavenly bodies are

perfect spheres and

move on perfect

circles

The orbit of the

moon separated the

imperfect terrestrial

realm from the

perfect heavens

AST121,

Robert Zimmerman

University of Oregon

• All planets generally moveeastward with respect tothe background of stars.This is called progrademotion

Retrograde motion is

westward

primary motion is

eastward

When planets reverse

direction and move

westward w.r.t. stars: this

is retrograde motion

Problem with circular motion:

Retrograde

Motion

Claudius Ptolemy A.D. 85-165

Used EPICYCLES to account for

retrograde motion

! In his great 13-booktreatise, the Almagest,Ptolemy presents hiswork on the path of theSun, the Moon, and thefixed stars.

! Ptolemy’s geocentricuniverse accuratelypredicted the motion ofthe planets with ageocentric model withcircular orbits.

! This was the bestmodel of the cosmosuntil the late 15th

century.

Claudius Ptolemy A.D.

85-165 the Almagest,

Ranked by

how long

they took to

make a

complete

cycle

Moving Earth

(Heraclites, BC 387-312) Earth rotateson its axis

(Aristarchos BC 310-230) Sun-centred solar system

1. Earth “feels” stationary. Wouldn’t

clouds, air, birds, etc. be left behind if

the Earth were moving?!

Observation against a moving

Earth

No stellar parallax could be measured ---

Hipparchus tried very hard! (150 BC)

The first stellar parallax was measured in

1838 by Friedrich Bessel

2. No Stellar Parallax

Eratosthenes (267-194 BC)

noticed that when Sun was

directly overhead in one city, it

was at an angle in another

Measuring the angle and the distance

between the cities gives the radius.

7.2/360= dist/circum

Measuring Earth’s radius

7.2

Measurement: R! 6366 km

Today’s Value

R! 6378 km---(! 2-20 %)

! Arabs conquered many of

these countries starting in the

7th century, and preserved a

lot of the work done by the

Greeks

! Religious motives required

accurate timekeeping and

astronomical observations

! Translated Ptolemy’s

Almagest into arabic. It

contained a catalogue of over

1,000 stars that were given

arabic names

! The Almagest became the

foundation and ideas for the

15th and 16th centuries.

Islamic Period: 700-1300 AD

Galileo Galilei

(1564–1642)

Argued for Copernicus’

Model

Isaac Newton

(1642 – 1727)

Law of Gravity

Tycho Brahe 1546 -1601

Pre-Telescopic Measurements

Nicolaus Copernicus

1473-1543

Sun-Centered Model

Johannes Kepler

1571 - 1630

Three Laws

Heliocentric Theory:

Planets move around

the Sun rather than the

Earth

In the book, Copernicus stated that

1. the Sun was the center of the universe and

that the Earth had a double motion around

this center.

2. Copernicus still retained the circular motion

for the orbits

3. He argued that his system was more elegant

than the traditional geocentric system.

4. All the Planets revolve around the Sun!

5. Stars are very much farther away than the

Sun-----the parallax motion is too small

to observe

Nicolaus

Copernicus

1473-1543

A stationary Sun and moving Earth clashed with many biblical passages.

Protestants and Catholic alike often dismissed heliocentrism on these

grounds. The book’s preface stated that the theory put forward in this book

was only a mathematical hypothesis.

Heliocentric Universe

Spent his life as a physician, lawyer, and

church administrator. He wrote

De Revolutionibus Orbium Coelestium,

("On the Revolutions of the Celestial Orbs"),

which was published in Nuremberg in 1543, the year of his

death.

Retrograde motion had a natural explanation

CopernicusCopernicus

Tycho Brahe (1546-1601)# made precise observations before the

invention of the telescope.

Sec 3

#His observations of planetary motion,

particularly that of Mars, provided the

data for Kepler to construct his three

laws. Carefully measured the orbit of

Mars for 20 years

#He made observations of a supernova

in 1572 and a comet in 1577.

#Hired Kepler as an assistance in 1600.

oThe"star" appeared suddenly where none had

been seen before and was visible for about 18

months before fading from view.

oBrahe's observations showed that the

supernova did not change positions with

respect to the other stars (no parallax).

Therefore, it was a real star, not a local object.

Observed a new star (Supernova) in 1572

!This contradicted the teachings of

Aristotle, who had held that comets

were atmospheric phenomena ("gases

burning in the atmosphere" was the

common explanation among

Aristotelians).

! Supernovae and comets

represented an obvious change on the

celestial sphere that was supposed to

be unchanging

Observed a comet in 1577

By measuring the parallax for the

comet, he was able to show that the

comet was further away than the Moon.

" Born in Germany in 1584

" in 1589 he began his university education at

Tübingen where he studied theology

" In 1597 Kepler published, The Cosmographic

Mystery, in which he explain the relative

distances of the planets from the Sun in the

Copernican System.

" Because of his talent as a mathematician , Kepler

was invited by Tycho Brahe to Prague to become

his assistant. Kepler moved to Prague in 1600.

" Kepler served as Tycho Brahe's assistant until

the latter's death in 1601 and was then appointed

Tycho's successor as Imperial Mathematician, the

most prestigious appointment in mathematics in

Europe.

" In 1609 he published his first two laws and in

1619 he published the third law

Johannes

Kepler

1571 - 1630

Kepler’s First LawThe orbits of planets are ellipses, with the Sun at one

focus and nothing at the other focus.

Focus

• Major Axis

• Minor Axis

• Semi-Major Axis (a)

Ellipse perihelionaphelion

0.2 .007 .017 .09 .048 .056 .046 .01 .25

Mer Ven Earth Mars Jup Sat Ur Nep Pluto

a( 1-e)

-------------------------------------------------------a( 1+e)-----------------

a( 1+e)

x

------------------ a------------------------

----

2nd Law: Orbiting objects sweep out equal areas in equal times

speed of planetsspeed of planets

- fastest at perihelion, slowest at aphelion- fastest at perihelion, slowest at aphelion

Kepler’s Third Law

61,500.61,600.248.39.5Pluto

27,200.27,300.165.30.1Neptune

7,060.7,080.84.019.2Uranus

870.868.29.59.54Saturn

142.141.11.95.20Jupiter

3.533.511.881.52Mars

1.001.001.001.00Earth

0.3780.3780.6150.723Venus

0.0580.0580.2410.387Mercury

P2a3P (year)a (AU)Object

aAU3 = PYr

2Kepler’s Third Law outer planets move slower

Galileo Galilei (1564-1642)

• Dutch invent telescopes

in early 1600’s

•Galileo learns of the

invention and within a

month --- he creates his

own.

• Galileo was the first to

use the telescope for

science observations.

1. Milky Way

2. “Spots” on the Sun!

3. The Moon has mountains,

craters, rocky surface with

imperfections!

4. The “planet” Jupiter is not a

pinpoint star – but a disc in the

sky! WITH MOONS!

5. Saturn is not round!

6. Venus has “PHASES” like the

MOON!

Galileo Galilei (1564 Galileo Galilei (1564 –– 1642) 1642)

1610 “Siderius Nuncius” (The Starry

Messenger)

The results were astounding,

and completely at odds with

what most people thought

about the universe.

Milky Way

Saturn

Milky Way: Objects exist that

Aristotle knew nothing about - the

combined light of many faint stars

can produce an observable result.

Sunspots: “Spots” on the

Sun! Showed they were on

the Sun and not in the

atmosphere.

Sunspots

Saturn: He thought the rings were "handles" or largemoons on either side of the planet. He said

"I have observed the highest planet [Saturn] to betripled-bodied. This is to say that to my very greatamazement Saturn was seen to me to be not a singlestar, but three together, which almost touch eachother".

it was not a perfect

sphere as he had

been taught, but

rough and mountainous.

The Moon

It was clearly made

of solid stuff and

was another world.

Four objects that do not

orbit the Earth.

Sec 3

Phases of Venus cannot be explained by geocentric model

The full phase cannot

happen in the

Geocentric Model

Copernican

Model

Heliocentric Model--Phases of Venus

--Phases of VenusGeocentric Model

Dialogue

Concerning the

Two Chief World

Systems 1632

1632: wrote the Dialogue Concerning the Two Chief

World Systems which was written as a dialogue between

two spokesmen, one for Copernicus, and the other for

Aristotle. He completed the Dialogue in 1630, and tried to

get it published, but could not. It was eventually printed

in March 1632.

1616: Galileo was instructed

to give up holding or defending

the idea that 'the sun stands still

at the centre of the spheres

while the Earth is in motion'.

A few copies were circulated, and then the printer received an instruction

to halt all further printing and Galileo was to appear before the

Inquisition.

1642: He died in 1642 -

the year Isaac Newton

was born.

Galileo was forced to

confess his views

and to condemn

them. He was

sentenced to

permanent house-

arrest at his villa at

Arcetri near Florence

villa at Arcetri near Florence

Aberration of starlight:

Discovered by the English astronomer James

Bradley and explained by him in 1729. The

phenomenon is caused by the orbital motion of the

earth

Analogy---vertically falling

raindrops appear to fall

diagonally when viewed by a

moving person.

Telescopes must be tilted

on moving Earth

100 years later (1729)--Proof that the

Earth Orbits the Sun

In 1851, Jean-Bernard-Leon

suspended a Foucault pendulum

from the dome of the Pantheon in

Paris.

dome of the

Pantheon in Paris.

200 years later (1851)--

Proof that the Earth Rotates

The plane of its motion, with respect to the earth, rotated

slowly clockwise.

This motion is produced by a rotating earth.

End of Chapter 2a

The Copernican Revolution

Go to Chapter 2b