Geocentric Theory vs. Heliocentric TheorytheOutcomeof a Bale Royale 69
Until the Apollo spacecraft placed a corner cube prism on the surface of the Moon,
modern scientists were still using the same method that ancient Greeks had used to cal-
culate the distance to the Moon. This method is easy to understand—just find one location
(A) from which the center of the Moon’s surface is visible at the zenith and another location
(B) from which the center of the Moon’s surface is visible at the horizon, at the same time of
day. If points A and B are at the same longitude, the difference in latitude between them will
be B0C. Then, you can use the radius of Earth (B0) to calculate the distance to the Moon
by figuring out BO × tan(difference in latitude), where O is the center of Earth.
Geocentric Theory vs. Heliocentric Theory—
theOutcomeof a Battle Royale
We know from many experiences that what we observe with our own eyes is not necessar-
ily the truth. The best example of this is a mirror—when you gaze into it, you see a person
who is the spitting image of yourself. However, no one would look into a mirror and get all
excited, saying, “There’s another me over there!”
You immediately understand when you turn over a mirror that there is not another
world on the other side of it. You can comprehend that it is only your own image reflected,
even if you don’t think about the physical phenomenon of the reflection of light. However,
when it’s the universe that is being observed, people have a hard time buying any explana-
tion that is different from what they can see.
The Sun, the Moon, and many stars that shine in the evening sky all certainly seem
to revolve around our Earth. Therefore, it’s natural that early models of the universe con-
formed to the geocentric theory.
It seems that if Earth were moving, we wouldn’t be able to stand on the ground, would
we? We’d be thrown off and fly somewhere into space! Before physics was developed, it cer-
tainly wasn’t easy to answer these questions.
Since no one even considered the possibility of a heliocentric theory except some
thinkers in ancient Greece, the geocentric theory had a monopoly until Nicolaus Copernicus
(1473–1543) appeared on the scene.
Moon
A
B
C
Earth
0
Difference
in latitude
One method of measuring the distance from Earth to the Moon

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