The Astronomy Book

18

T

he traditions on which

modern astronomy is

built began in ancient

Greece and its colonies. In nearby

Mesopotamia, although the

Babylonians had become highly

proficient at celestial forecasting

using complicated arithmetic,

their astronomy was rooted in

mythology, and their preoccupation

was with divining the future. To

them, the heavens were the realm

of the gods, outside the scope of

rational investigation by humans.

By contrast, the Greeks tried

to explain what they observed

happening in the sky. Thales

of Miletus (c.624–c.546 bce) is

regarded as the first in a line of

philosophers who thought that

immutable principles in nature

could be revealed by logical

reasoning. The theoretical ideas

put forward two centuries later

by Aristotle (384–322 bce) were to

underpin the whole of astronomy

until the 16th century.

Aristotle’s beliefs

Aristotle was a pupil of Plato,

and both were influenced by the

thinking of Pythagoras and his

followers, who believed that the

natural world was a “cosmos” as

opposed to “chaos.” This meant

that it is ordered in a rational way

rather than incomprehensible.

Aristotle stated that the

heavenly realms are unchanging

and perfect, unlike the world of

human experience, but he promoted

ideas that were consistent with

“common sense.” Among other

things, this meant Earth was

stationary and at the center of the

universe. Although it contained

inconsistencies, his philosophy

was adopted as the most acceptable

overall framework of ideas for science

and was later incorporated into

Christian theology.

Geometrical order

Mathematically, much of Greek

astronomy was based on geometry,

particularly motion in circles,

which were considered to be the

most perfect shapes. Elaborate

geometrical schemes were created

for predicting the positions of

the planets, in which circular

motions were combined. In 150 ce,

the Graeco–Egyptian astronomer

Ptolemy, working in Alexandria, put

together the ultimate compendium

of Greek astronomy. However, by

500 ce, the Greek approach to

astronomy had lost momentum.

In effect, after Ptolemy, there were

INTRODUCTION

C.550 BCE

C.530 BCE

C.200 BCE

C.15 0 CE

350 BCE

C.220 BCE

In Alexandria, Eratosthenes

measures the circumference

of Earth and estimates the

distance to the sun.

Pythagoras establishes a

school in Croton, where he

promotes the idea of a

cosmos in which bodies

move in perfect circles.

In his On the Heavens, Aristotle

outlines an Earth-centered

model of the universe. Many

of his ideas will dominate

thinking for 2,000 years.

Aristarchus of

Samos proposes a

sun-centered model

of the universe, but his

idea does not gain

wide acceptance.

Anaximander of
Miletus produces one
of the earliest attempts
at a scientific
explanation of
the universe.

Ptolemy writes the

Almagest, which sets

out an Earth-centered

model of the universe

that becomes

widely accepted.