50 Chapter 1
From the Geocentric Theory to the Heliocentric
The mysterious behavior of planets puzzled people for a long time, as evi-
denced by the fact that etymologically, the word planet comes from the Greek
word for wandering star. People thought that the planets wandered around
and changed their locations, while the other stars—that is, the so-called fixed
stars*—revolved while maintaining the same positional relationships on the
celestial sphere.
Oh yeah, I heard
that explanation
at the planetar-
ium once.
Yeah, I bet the name planetarium
comes from the word planet.
You sure know
a lot of things,
don’t you?
That’s because I went to
school in America.
Well, aren’t you
Enough! I don’t want to hear your
In the first geocentric model that Professor Sanuki drew, this kind of planetary
behavior could not be explained. It failed to account for the fact that the Sun
and the Moon don’t move in the same way.
So although the geocentric theory
should have been abandoned at
this stage...
* The stars do “move—just not in relation to other stars in the sky.
It didn’t go away. Ptolemy arrived
on the scene at that point.
From the Geocentric Theory to the Heliocentric Theory 51
Who? When did he live?
Claudius Ptolemy
(about 90–168 AD)
A Greek astronomer and
geographer; the world maps he
drew in his book called Geography
used latitude and longitude for
the first time and established the
convention that “North is up” that
we still use today.
We’re not exactly sure when he lived,
but we know he was an astronomer and
geographer in Roman Egypt during the
second century. The world maps he left
behind were used until the Middle Ages.
Only a very talented individual
like Ptolemy could have come up with
a method to explain the motion of the
planets using the geocentric theory.
Model of the geocentric theory, proposed by Ptolemy
That’s right, that’s right.
The model shown here
is his.
52 Chapter 1 Is Earth the Center ofthe Universe?
That’s a clever
idea, isn’t it?
Why are Mercury, Venus, Earth, and
the Sun all lined up in a straight line
in Ptolemy’s model??
This is how Ptolemy explained why
Mercury and Venus always appear
closer to the Sun.
What’s the matter?
The retrograde motion of Mars in the night sky, as observed from Earth
Of course. Since they revolve around
Earth together with the Sun, they
will always appear closer to the Sun.
The ancient Greeks believed that the Moon, the Sun, and the other planets
revolved around Earth, and Ptolemy’s geocentric theory didn’t challenge this.
However, the earlier geocentric theories from ancient Greece were unable to
explain the retrograde motion of the planets.
What do we mean by retrograde? Planets appear to move in an eastward
direction most of the time. However, planets sometimes appear to reverse directions
in the night sky, which we call retrograde motion. All planets periodically exhibit
this behavior. Ptolomy's geocentric model explained this motion as a revolution
around a point in a planet's orbit. (In reality, this apparent backward motion is
caused by Earth "lapping" the planet in a revolution around the sun.)
From the Geocentric Theory to the Heliocentric Theory 53
Planetary motion, according to Ptolemy's geocentric theory
He certainly didn’t make it believable,
did he? If his model were true, the
planets would all revolve like swirl-
ing eddies.
It feels like he had to keep fudging the geocentric model—to try to keep it in
line with new scientific discoveries.
It looks totally fake! It
wouldn’t fool anybody.
I wonder...I think this model
is well constructed, but...
Why don’t we try to compare our
impressions of this model with
our impressions of Copernicuss
model of the heliocentric theory?
Good idea! Ptolemy’s model was accepted as true for nearly 1,400 years.
But then Copernicus challenged it in 1543 with his heliocentric theory,
which he presented in his book On the Revolutions of the Celestial Spheres.
For details, read “Just How Progressive Was Copernicus?” on page 71.
Doesn’t it seem like he went way
overboard in creating this diagram?
54 Chapter 1 Is Earth the Center ofthe Universe?
Galileo Galilei (1564–1642)
Italian physicist, astronomer, and philosopher
Very straight-
It’s so simple,
isn’t it?
Personally, I think that
the way Ptolemy tried
to explain planetary
motion within the
geocentric theory is
brilliant, but...
Kanta...why are you supporting the geocentric theory so much? Looking at this,
it’s evident that the heliocentric theory is correct, isn’t it?
What? Isn’t that the opposite of
what you said before?
Well, we must be open to new
thinkers and their new ideas!
If everyone were like you, perhaps
Galileo would have been spared
from the Roman Inquisition.

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