The Astronomy Book

38

complicated epicycles, Copernicus

explained that such motion could

be attributed to changes in

perspective caused by Earth

and the other planets moving

at different speeds.

Distant stars

Another of Copernicus’s tenets

was that the stars are much farther

away from Earth and the sun than

had previously been believed. He

said: “The distance between Earth

and the sun is an insignificant

fraction of the distance from

Earth and sun to the stars.” Earlier

astronomers knew that the stars

were distant, but few suspected

just how far away they were, and

those who did, such as Aristarchus,

had not managed to convince

anyone. Even Copernicus probably

never realized quite how far

away the stars are—it is now

known that the very closest are

about 260,000 times more distant

than the sun. But his assertion

was extremely important because

THE COPERNICAN MODEL

of its implications for stellar

parallax. For centuries, supporters

of geocentrism had argued that

the absence of parallax could

only be due to Earth not moving.

Now, there was an alternative

explanation: the parallax was not

absent, but because of the great

distance to the stars, it was simply

too tiny to be measured with the

instruments of the time.

Copernicus additionally

proposed that Earth is at the

center of the lunar sphere.

Copernicus maintained that

the moon circled Earth, as it did

in the geocentric model. In his

heliocentric model, the moon

moved with Earth as it circled the

sun. In this system, the moon was

the only celestial object that did

not primarily move around the sun.

In the Ptolemaic model (left), the occasional retrograde

(backward-moving) motion of Mars was regarded as due to loops that

the planet makes in space. In the Copernican model (right), retrograde

motion was caused simply by changes in perspective because Earth

and Mars orbit the sun at different speeds. Earth would from time to

time “overtake Mars on the inside” as shown here, causing Mars to

reverse its apparent direction of movement for several weeks.

Those things which I am

saying now may be obscure,

yet they will be made clearer

in their proper place.

Nicolaus Copernicus

Earth

Mars’s deferent

Epicycle

Mars

Motion

of Mars

Mars’s orbit

Earth’s orbit

Earth

Mars

Sun

View as

seen from

Earth