The Astronomy Book

20

See also: Consolidating knowledge 24–25 ■ The Copernican model 32–39 ■

The Tychonic model 44–47 ■ Gravitational theory 66–73

O

ne of the most influential

of all Western philosophers,

Aristotle, from Macedonia

in northern Greece, believed that

the universe was governed by

physical laws. He attempted to

explain these through deduction,

philosophy, and logic.

Aristotle observed that the

positions of the stars appeared to

be fixed in relation to each other,

and that their brightness never

changed. The constellations always

stayed the same, and spun daily

around Earth. The moon, sun, and

planets, too, appeared to move in

unchanging orbits around Earth.

Their motion, he believed, was

circular and their speed constant.

His observations of the shadow

cast by Earth on the moon’s

surface during a lunar eclipse

convinced him that Earth was

a sphere. His conclusion was

that a spherical Earth remained

stationary in space, never spinning

or changing its position, while the

cosmos spun eternally around it.

Earth was an unmoving object

at the center of the universe.

Aristotle believed that Earth’s

atmosphere, too, was stationary.

At the top of the atmosphere,

friction occurred between the

atmospheric gases and the rotating

sky above. Episodic emanations

of gases from volcanoes rose to

the top of the atmosphere. Ignited

by friction, these gases produced

comets, and, if ignited quickly,

they produced shooting stars.

His reasoning remained widely

accepted until the 16th century. ■

IT IS CLEAR

THAT EARTH

DOES NOT MOVE

THE GEOCENTRIC MODEL

IN CONTEXT

KEY ASTRONOMER

Aristotle (384–322 bce)

BEFORE

465 bce Greek philosopher

Empedocles thinks that there

are four elements: earth, water,

air, and fire. Aristotle contends

that the stars and planets are

made of a fifth element, aether.

387 bce Plato’s student

Eudoxus suggests that the

planets are set in transparent

rotating spheres.

AFTER

355 bce Greek thinker

Heraclides claims that the sky

is stationary and Earth spins.

12th century Italian Catholic

priest Thomas Aquinas begins

teaching Aristotle’s theories.

1577 Tycho Brahe shows that

the Great Comet is farther

from Earth than the moon.

1687 Isaac Newton explains

force in his Philosophiae

Naturalis Principia Mathematica.

Earth casts a circular shadow

on the moon during a lunar eclipse.

This convinced Aristotle that

Earth was a sphere.

moon

Earth’s shadow

Earth

sun’s

rays