The Astronomy Book

37

Copernicus thought it was just
too unlikely that thousands of
stars were spinning rapidly around
Earth every 24 hours. Instead, he
considered them to be fixed and
immovable in their distant, outer
sphere, and that their apparent
movement was actually an illusion
caused by Earth’s spin. To refute
the idea that a spinning Earth
would create huge winds, and that
objects on its surface would fly off,
Copernicus pointed out that Earth’s
oceans and atmosphere were part
of the planet and were naturally
part of this spinning motion. In his
own words: “We would only say
that not merely the Earth and the
watery element joined with it have
this motion, but also no small part
of the air and whatever is linked in
the same way to the Earth.”
Second, Copernicus proposed
that it is the sun that is at the
center of the universe, not Earth,
which is simply one of the planets,
all of which circle the sun at
differing speeds.

Elegant solution
These two central tenets of
Copernicus’s theory were of utmost
importance because they explained
the movements and variation in
brightness of the planets without
recourse to Ptolemy’s complicated
adjustments. If Earth and another
planet, such as Mars, both circle
the sun and do so at different
speeds, taking a different amount
of time to complete each revolution,
they will sometimes be close to
each other on the same side as the
sun and sometimes far from each
other, on opposite sides to the sun.
This, at a stroke, explained the
observed variations in brightness
of Mars and the other planets. The
heliocentric system also elegantly
explained apparent retrograde
motion. In place of Ptolemy’s ❯❯

FROM MYTH TO SCIENCE

In the Ptolemaic model

(top), Earth is at the center

and other celestial bodies

go around Earth. In the

Copernican system

(bottom), Earth together

with the moon have

swapped position

with the sun; the

sphere of the fixed

stars is much

farther out.

Sun

Mercury Venus

Moon

Earth

Jupiter

Saturn

Mars

Embedded

“fixed” stars

Sun

Venus

Mercury

Earth Moon

Jupiter

Saturn

Mars

Outer sphere

with embedded

“fixed” stars