Greek Astronomy

Ancient View of the Cosmos

Universe is 2-D All celestial objects attached to a sphere.

Celestial Sphere is close Climb a high mountain and touch the sky

Celestial objects are self-luminous Earth is the center of the universe Objects move on perfect circles

Pre-disposition for success

Not Fatalists like Babylonians Greeks had a curiosity about nature

Model builders

Lunar Phases and Eclipses

Two important changes in thought needed

Universe is three dimensional

Some celestial objects are dark

Lunar Phases

Phases

New MoonFull Moon

1st Quarter

Earth (12,756 km)

Moon (3476 km)

Lunar Eclipses

Sun

EarthMoon

umbra

penumbra

Lunar Eclipses

b Eclipse shadow is always a section of a circle

b Earth must be a sphere

b Color of the eclipsed moon

Solar Eclipses

Total Solar Eclipse

Equator

North Pole

Eratosthenes

Sun’s RaysSyene

Alexandria 7°

Video

Eratosthenes

a 7° is about 1/50th of a circle

a Alexandria and Syene are separated by 800 km That 800 km must be 1/50th the

circumference of the Earth Earth Circumference = 800*50 = 40,000 km

Accepted value is 40,074 km

Relative sizes and distances

By 350 BC Greek Natural Philosophers knew the relative diameters and distances of the Sun, Moon, and Earth. The Sun was very large and very far away

and the Earth was bigger than the Moon.

Cosmologies

Heliocentric Model Aristarchus

of Samos

Cosmologies

Geocentric Model Aristotle

350 BC

Stationary Earth

Earth is heavy Easier to imagine the sky can move

It looks like the sky is moving We have no sensation of our motion

Rotating Earth would make objects fly off of the surface

Stellar Parallax

Stellar Parallax

Hipparchus 150 BC

Sun

January

June

Why did Hipparchus fail to observe Stellar Parallax?

0%

0%

0%

0%

10

1. He only looked at bright stars.

2. He was a poor observer.

3. The stars are too far away.

4. The Earth does not orbit the Sun.

Aristotelian UniverseTerrestrial Realm

Earth and Water tend to sink

Composition predicts motionnatural tendencies

Overall tendency to seek rest Objects following tendencies require no force Objects are corruptible (changing)

Fire and Air tend to rise

Aristotelian UniverseCelestial Realm

Celestial Objects composed of AetherSelf luminous but does not consume

Motion is constant, circular Objects are incorruptible (not changing)

Meteors and comets were phenomena of the Earth’s atmosphere

Link

Claudius Ptolemy (150 AD)

Accounted for retrograde motion within the confines of the Geocentric Model

The Sun and Moon

Earth

Moon

The Planets

Earth

Planet

EpicycleDeferent

Link

Retrograde Loop inPtolemy’s System

Ptolemaic Universe

John Milton, Paradise LostWith Centric and Eccentric

scribl'd o're, Cycle and Epicycle, Orb in Orb

Astronomy in the Dark Ages

Fall of Rome Greek knowledge went to Islam Alexandrian library destroyed

Universal Illiteracy No mass communications

Villages were isolated

Thomas Aquinas Giordano Bruno

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