Astronomy 294z:

Thursday, January 3

Professor Barbara Ryden

The History of the Universe

Before History: The Science of Origins

Astronomy 294z: The History of the Universe Geological Sciences 110: History of Life on Earth Anthropology 200: Intro to Physical Anthropology

Where do we come from? What are we? Where are we going?

Barbara Ryden

Office: 4035 McPherson

Office hours: Mon Wed, or by appointment [292-4562]

The Professor:

The Textbook:

On the Shores of the Unknown, by Joe Silk

The Website:

Contains: Lecture PowerPoint, syllabus, problem sets, & useful links.

www.astronomy.ohio-state.edu/~ryden/ast294z/

The science that studies the history (& future) of the universe

is called “cosmologycosmology”.

“kosmos” = order, harmony

“logos” = word, law

What is Science?

Systematic study of the universe, using

the scientific method.

Scientific MethodGather facts

Guess an explanation (guess = “hypothesis”)

Test hypothesis

Modify hypothesis

A well-tested hypothesis = “theory”

Looking south, 3 pm today

Earth (opaque)

Sky (blue)

Sun ↑

horizon

Cosmology version 1.0: Domed sky over flat earth.

And God said, “Let there be a vault in the midst of the waters,

and let it divide water from water.” And God made the vault and it divided the water beneath the vault from the water above the

vault, and so it was. – Genesis 1:6

[Robert Alter translation]

Aristotle (4th century BC): First to give reasons why the Earth is spherical.

Aristotle’s 1st reason:

Gravity pulls matter to center of Earth, compressing the Earth into as compact a shape as possible.

…a sphere!

2nd reason:

You see different stars from the

south than from the north.

Big Dipper

Southern Cross

During a lunar eclipse, Earth’s shadow is alwaysalways circular.

Only object whose shadow is always circular is a spheresphere.

3rd reason:The shape of the Earth’s shadow.

Morocco

India

Morocco and India are close together on the far side of the round Earth!!

4th reason: Elephants

How large is the Earth?

Question answered by EratosthenesEratosthenes (ca. 200 BC).

Alexandria •

• Syene

What Eratosthenes readread: At noon on June 21, Sun is at zenith seen from Syene.

What he sawsaw: At noon on June 21, Sun is 7.2° south of zenith seen from Alexandria.

What he assumedassumed: Earth is spherical. Sun is very far away.

Distance D = 1/50 of circumference.

Angle α = angle β = 7.2° = 1/50 of circle.

Circumference of Earth = 50 × distance from Alexandria to Syene.

Distance from Alexandria to Syene = 5000 stades

Circumference of Earth = 50 × 5000 stades = 250,000 stades.

(about 46,000 kilometers – true value is 40,000 kilometers)

Looking north, 7 pm tonight.

horizon

Earth (opaque)

Sky (black)

stars

Constellations

Stars in a constellation are not all at the same distance from us.

Constellations are arbitrary (but useful).

7 pm

9 pm

11 pm

Constellations appear to travel in counterclockwise circles around Polaris

(the North Star).

Strong visual illusion: stars are attached to a celestial spherecelestial sphere,

rotating around the Earth.

Cosmology version 2.0: Celestial sphere

surrounding spherical Earth.

Earth stationary at center.

Stars attached to celestial sphere, which rotates.

This geocentricgeocentric (Earth-centered) model was generally accepted from

ancient times until 16th cent. AD.

Dante

Divine Comedy

AD 1300

← spherical Earth

Q: Why did astronomers believe all the stars were at the same distance from

Earth, glued to a celestial sphere?

A: It’s impossible to get a sense of depth looking up at the sky with

your naked eyes.

Andromeda Galaxy

Comet Hale-Bopp

Q: Why did astronomers believe that the Earth is not moving?

A: Well, do you feel any motion?

Speed of rotation at Earth’s equator = circumference / rotation period =

40,000 kilometers / 24 hours = 1667 kilometers/hour

Tuesday’s Lecture:

From an Earth-centered to a Sun-centered universe

Reading:

Prologue