Intellectual Revolutions that Defined Society

4. Around 150 A.D. the
astronomer Ptolemy resolved
this problem by using a
system of circles to describe
the motion of planets. In
Ptolemy’s system, a planet
moves in a small circle,
called an epicycle. This
circle moves around
Earth in a larger circle,
called a deferent.
Ptolemy’s version of the
geocentric model
worked so well that it
remained the accepted
model of the universe for
more than a thousand years.

5. According to Ptolemy,
a planet moves on a
small circle (epicycle)
that in turn moves on
a larger circle
(deferent) around
Earth.
Ptolemy’s geocentric
model worked but it
was not only
complicated, it
occasionally made
errors in predicting
the movement of
planets.

6. • At the beginning of the 16th century A.D., Nicolaus Copernicus
proposed that Earth and all the other planets orbit the Sun. With the
Sun at the center, this model is called the heliocentric model or “sun-
centered” model of the universe. Copernicus’ model explained the
motion of the planets as well as Ptolemy’s model did, but it did not
require complicated additions like epicycles and deferents.
• Unlike the geocentric model, the heliocentric model had the Sun at
the center and did not require epicycles.
• Although Copernicus’ model worked more simply than Ptolemy’s, it
still did not perfectly describe the motion of the planets because, like
Ptolemy, Copernicus thought planets moved in perfect circles. Not
long after Copernicus, Johannes Kepler refined the heliocentric
model so that the planets moved around the Sun in ellipses (ovals),
not circles (Kepler’s model matched observations perfectly.

8. • Kepler’s model showed the planets
moving around the sun in ellipses. The
elliptical orbits are exaggerated in this
image.
• Because people were so used to thinking
of Earth at the center of the universe, the
heliocentric model was not widely
accepted at first. However, when Galileo
Galilei first turned a telescope to the
heavens in 1610, he made several striking
discoveries. Galileo discovered that the
planet Jupiter has moons orbiting around
it. This provided the first evidence that
objects could orbit something besides
Earth.

9. Galileo also
discovered that
Venus has
phases like the
Moon, which
provides direct
evidence that
Venus orbits
the Sun.

10. • Galileo’s discoveries caused
many more people to accept
the heliocentric model of
the universe, although
Galileo himself was found
guilty of heresy for his ideas.
The shift from an Earth-
centered view to a Sun-
centered view of the
universe is referred to as the
Copernican Revolution.

18. • From the late 1970s, innovations in information
technology and telecommunications started
spreading on a large scale, including the personal
computer and mobile phone. If the invention of the
computer back to the Second World War, it is truly
diffuse until 1977 with the release of personal
computers (launch on the Apple II).

19. Similarly, if the first mobile phones are
designed in the 1970s (first handheld mobile
phone developed by Dr. Martin Cooper), it is
only at the end of the 1990s that their use is
spreading widely.

20. These innovations profoundly modify both the
way of life of households and
businesses operate and are the source of
productivity gains that have significant
potential for growth.