The Science Book

36 NICOLAUS COPERNICUS

T

hroughout its early history,

Western thought was

shaped by an idea of

the universe that placed Earth

at the center of everything. This

“geocentric model” seemed at

first to be rooted in everyday

observations and common sense—

we do not feel any motion of the

ground on which we stand, and

superficially there seems to be no

observational evidence that our

planet is in motion either. Surely

the simplest explanation was

that the Sun, Moon, planets and

stars were all spinning around

Earth at different rates? This

system appears to have been

widely accepted in the ancient

world, and became entrenched in

classical philosophy through the

works of Plato and Aristotle in

the 4th century BCE.

However, when the ancient

Greeks measured the movements

of the planets, it became clear

that the geocentric system had

problems. The orbits of the known

planets—five wandering lights in

the sky—followed complex paths.

Mercury and Venus were always

seen in the morning and evening

skies, describing tight loops around

IN CONTEXT

BRANCH

Astronomy

BEFORE

3rd century BCE In a work

called The Sand Reckoner,

Archimedes reports the ideas

of Aristarchus of Samos, who

proposed that the universe

was much larger than

commonly believed, and that

the Sun was at its center.

150 CE Ptolemy of Alexandria

uses mathematics to describe

a geocentric (Earth-centered)

model of the universe.

AFTER

1609 Johannes Kepler resolves

the outstanding conflicts in the

heliocentric (Sun-centered)

model of the solar system by

proposing elliptical orbits.

1610 After observing the

moons of Jupiter, Galileo

becomes convinced that

Copernicus was right.

If the Lord Almighty

had consulted me before

embarking on creation thus,

I should have recommended

something simpler.

Alfonso X

King of Castile

the Sun. Mars, Jupiter, and Saturn,

meanwhile, took 780 days, 12 years,

and 30 years respectively to circle

against the background stars, their

motion complicated by “retrograde”

loops in which they slowed and

temporarily reversed the general

direction of their motion.

Ptolemaic system

To explain these complications,

Greek astronomers introduced

the idea of epicycles—“sub-orbits”

around which the planets circled

as the central “pivot” points of the

At the center of

everything is the Sun.

Earth appears to be

stationary, with the Sun, Moon,

planets, and stars orbiting it.

However, a model of the

universe with Earth at its center

cannot describe the movement of

the planets without using a very

complicated system.

Placing the Sun at the center

produces a far more elegant model,

with Earth and the planets orbiting the Sun,

and the stars a huge distance away.