History of Astronomy

Arny, 3rd Edition, Chapter 1

The Cosmic Landscape Lesson 4: Early Ideas of the Heavens: Classical Astronomy 2

Which Theory is Correct?

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Which Theory is Correct?• Classical theories (500 B.C. to 150

A.D.)• Some ideas very complicated and

wrong • Others ideas simple and correct• People just like you and me

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Lesson overview What is the Earth’s shape? How big are the Moon and Earth? How far is it to the Sun and Moon? What is the structure of the universe? How does Occam’s razor question geocentric

models of the universe?

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Introduction• Astronomers of ancient Greece and Egypt first to

explain the heavens• Some ancient Greeks are famous in astronomy:– Pythagoras – Aristotle– Aristarchus – Eratosthenes– Ptolemy

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Introduction (cont’d)

• Determined important things using naked eye and simple math

• Also got some things wrong• Made models to explain motions of Sun, Moon,

and planets

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What is the Earth’s shape?

• Earth is nearly round• Ancient astronomer’s beliefs about the

Earth’s shape not really science

Pythagoras

500 B.C.

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What is the Earth’sshape? (cont’d)

• Pythagoras said Earth was round: – Sphere a perfect shape– Gods made Earth

• Was Pythagoras right?– He never could prove it.

VOCABULARY

• ob·late• – a spheroid flattened at the poles

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What is the Earth’sshape? (cont’d)

• Aristotle—two things “prove” Earth’s spherical shape:– Earth’s shadow on the Moon – What a traveler moving south sees in the sky

300 B.C.

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How big is the Moon?

• Aristarchus estimated – Relative size of Earth

and Moon– Moon’s diameter

related to Earth’s

250 B.C.

(he guessed 1/3;

actually, 1/4th

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How big is the Earth?• Eratosthenes first measured Earth’s size

• Used simple math and measurements of shadows

• Accurately estimated Earth’s circumference 25

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Time Zones

How many time zones?About how wide is each, in miles?So, how fast does the Earth rotate?Is it possible to fly in an airplane fast enough to stay in daylight for an entire 24-hour flight?Which direction would you have to fly to do that?

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How far is it to the Sun and Moon?• Aristarchus estimated – Sun about 20 times farther away from the Earth than the

Moon (ACTUAL: 238,857 miles compared to 93 million miles = 389 times!)

– Sun to have seven times Earth’s diameter (ACTUAL: 2,713,406 divided by 24,901 = ~ 109 times!)

How could he be so accurateabout Earth’s size, and so wrong

on these other points?

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How far is it to the Sun and Moon? (cont’d)

Angle B had to be 90 degrees for the Moon to appear to be perfectly half-lit.

Knowing angle A, Aristarchus set the scale of the triangle to estimate the distance of the Sun from Earth.

He measured angle A—the angle between the Sun and Moon— when the Moon was exactly half-lit.

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What is the center of the universe?• Aristarchus recognized the Sun was larger

than Earth • Said it was the center of our solar system

AND the center of the universe

Was he right?Was he popular for that belief?

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Eudoxus believed Earth is the center of the universe

• Geocentric theory

• Objects in the sky generally move east to west

ge·o·cen·tric   – representing the earth as a center: a geocentric theory of the universe.

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How could our ancestors believe Earth is the center?

• Each celestial body mounted on its own revolving transparent sphere

• Bodies moving fastest across the sky were closest to Earth

• Right or wrong?

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Ptolemy’s geocentric theory• What Ptolemy of Alexandria

assumed about planetary movement

• Theory included epicycles   – a small circle the center of

which moves around in the circumference of a larger circle: attempts to account for observed periodic irregularities in planetary motions. (Spirograph)

• 80 of these needed to explain motions of Sun, Earth, Moon and the 5 planets known at the time

• How well did his model predict motion?

150 A.D.

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Problems with Ptolemy’s geocentric theory

• Ptolemy’s first model couldn’t cover: • Ptolemaic models became overly complex• Belief in his theory declined – though they

were in vogue until mid-1500s 1,400+ years!

Why could these “epicycles” never explain celestial motion

perfectly?

STARS!

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Geocentric theory fails thetest of “Occam’s razor”

• Occam’s razor: a rule in science created by William of Ockham

• Occam’s razor says:“If you can choose between a very

complicated theory and simple theory, the simple one is probably better and is more

likely to be correct.”

OCCAM’S RAZOR ACTIVITY

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Lesson review• What is the Earth’s shape?– What we know and what Pythagoras believed – What Aristotle showed and how

• How big is the Earth?– What we know and how ancients guessed– What Eratosthenes estimated the Earth’s

circumference to be and how

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Lesson review (cont’d)• How far is it to the Sun and Moon?– Aristarchus estimated distances from the Earth to the

Moon and Sun—right or wrong?– He estimated the Sun’s and Earth’s relative sizes—right

or wrong?

• How does Occam’s razor question geocentric models of the universe?– What Ptolemy proposed – What Occam’s razor says about complicated theories

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Activity—Questions for review• Who is Pythagoras and how did he help our understanding of

astronomy? • Who is Aristotle and how did he help our understanding of

astronomy? • Who is Eratosthenes and how did he help our understanding of

astronomy? • Who is Aristarchus and how did he help our understanding of

astronomy? • Who is Ptolemy and how did he help our understanding of

astronomy? • How does Occam’s razor help us figure out which scientific

theories may be right and which are likely to be wrong?

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Activity—Test yourself1. Name an ancient Greek astronomer and tell his contribution to astronomy.

2. What are two observations the ancient Greek astronomers used to make their calculations?

3. Why was Ptolemy’s theory of the universe finally abandoned?

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Summary

What is the Earth’s shape? How big is the Moon and the Earth? How far is it to the Sun and Moon? What is the structure of the universe? How does Occam’s razor question geocentric

models of the universe?

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NextDone — Key thinkers and classical theories Next — Four great Renaissance astronomers