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 [ ] The Professor:

The Textbook: On the Shores of the Unknown, by Joe Silk

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

cosmology The science that studies the history (& future) of the universe is called “cosmology”. “kosmos” = order, harmony “logos” = word, law

What is Science? Systematic study of the universe, using the scientific method.

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

Aristotle’s 1 st reason: Gravity pulls matter to center of Earth, compressing the Earth into as compact a shape as possible. …a sphere!

2 nd reason: You see different stars from the south than from the north. Big Dipper Southern Cross

always During a lunar eclipse, Earth’s shadow is always circular. sphere Only object whose shadow is always circular is a sphere. 3 rd reason: The shape of the Earth’s shadow.

MoroccoIndia Morocco and India are close together on the far side of the round Earth!! 4 th reason:Elephants

How large is the Earth? Eratosthenes Question answered by Eratosthenes (ca. 200 BC). Alexandria Syene

read What Eratosthenes read: At noon on June 21, Sun is at zenith seen from Syene. saw What he saw: At noon on June 21, Sun is 7.2° south of zenith seen from Alexandria. assumed What he assumed: 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).

celestial sphere Strong visual illusion: stars are attached to a celestial 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.

geocentric This geocentric (Earth-centered) model was generally accepted from ancient times until 16 th 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