The Astronomy Book

21

See also: The geocentric model 20 ■ Consolidating knowledge 24–25 ■

The Copernican model 32–39 ■ Stellar parallax 102

A

n astronomer and

mathematician from the

Greek island of Samos,

Aristarchus is the first person

known to have proposed that the

sun, not Earth, is at the center

of the universe, and that Earth

revolves around the sun.

Aristarchus’s thoughts on this

matter are mentioned in a book

by another Greek mathematician,

Archimedes, who states in The

Sand Reckoner that Aristarchus

had formulated a hypothesis that

“the fixed stars and sun remain

unmoved” and “Earth revolves

about the sun.”

Unfashionable idea

Aristarchus persuaded at least

one later astronomer—Seleucus of

Seleucia, who lived in the second

century bce—of the truth of his

heliocentric (sun-centered) view

of the universe, but otherwise it

seems his ideas did not gain wide

acceptance. By the time of Ptolemy,

in about 150 ce, the prevailing view

was still a geocentric (Earth-

centered) one, and this remained

the case until the 15th century, when

the heliocentric viewpoint was

revived by Nicolaus Copernicus.

Aristarchus also believed that

the stars were much farther away

than had previously been imagined.

He made estimates of the distances

to the sun and moon, and their

sizes relative to Earth. His estimates

regarding the moon were reasonably

accurate, but he underestimated

the distance to the sun, mainly

because of an inaccuracy in one

of his measurements. ■

FROM MYTH TO SCIENCE

EARTH REVOLVES

AROUND THE SUN ON

THE CIRCUMFERENCE

OF A CIRCLE

EARLY HELIOCENTRIC MODEL

IN CONTEXT

KEY ASTRONOMER
Aristarchus (310 –230 bce)

BEFORE
430 bce Philolalus of Craton
proposes that there is a
huge fire at the center of the
universe, around which the
sun, moon, Earth, five planets,
and stars revolve.

350 bce Aristotle states that
Earth is at the center of the
universe and everything else
moves around it.

AFTER
150 ce Ptolemy publishes
his Almagest, describing an
Earth-centered (geocentric)
model of the universe.

1453 Nicolaus Copernicus
proposes a heliocentric
(sun-centered) universe.

1838 German astronomer
Friedrich Bessel is the
first to obtain an accurate
measurement of the
distance to a star, using a
method known as parallax.

Aristarchus was the

real originator of the

Copernican hypothesis.

Sir Thomas Heath

Mathematician and classical scholar