Earth: Prove that it is Round
Can you give one undisputable answer to:
"Is the earth round, and how do we prove it."
Because we are having a debate in my class, and the teacher kept saying
'the earth is flat' And I have to give him one undisputable answer.
I may be wrong about this, but I think your teacher is trying to
demonstrate to you that anything and everything is open to dispute. In
fact, a rather famous theorem in the philosophy of science, the Duhem-Quine
theorem, states that it is impossible to conclusively disprove any
scientific hypothesis. That includes such hypotheses as the earth is flat,
photography steals the soul of the object being photographed, and apples
are highly toxic to humans. To hold on to such hypotheses, it may be
necessary to put forth some rather farfetched explanations of what we can
observe, but it can always be done.
In fact, science does not work by conclusively and indisputably refuting
bad hypotheses. Instead, hypotheses stand or fail on their ability to
explain known observations, and to predict the outcome of observations that
haven't been performed yet. Let's take the flat earth versus round earth
hypotheses as an example. A round earth is the most stable shape according
to gravitational theory; a flat earth would tend to collapse toward the
center, and people standing near the edges would feel a gravitational pull
toward the center of the flat surface instead of perpendicular to the
surface. Your teacher could counter that gravitational theory is wrong;
magic holds us to the earth. This may be true, but it doesn't explain why
gravitational theory works so well at explaining how things fall, and the
observed trajectories of the planets and their moons. You could tell your
teacher that photographs of the earth from space show that it is round,
just like the globe model in your classroom. He could counter that those
photographs are government fakes. Perhaps so, but we can see that other
planets and moons are round; wouldn't it make more sense to say that the
earth is round, too? Your teacher could say that the earth is special and
different. You could ask how numerous people have sailed or flown around
the earth if the earth is flat. He would have to say that every one of
these people, separated as they are in time and space, belongs to the same
great conspiracy aimed at brainwashing us into supposing that the earth is
round. You could ask him about the communications networks that are linked
by satellite - how can you have satellites orbiting a flat earth? He might
then tell you that there really aren't any satellite links - everything is
connected by wires, or a close-packed array of relay antennas.
We could go on and on, and I bet that in your classroom you HAVE gone on
and on. As you see, there is no single fact you could tell your teacher
that he couldn't dispute in some way. If you step back and look at it,
though,what kind of world would it have to be if your teacher were right?
Gravity doesn't function here like it does everywhere else in the solar
system, there's a huge conspiracy reaching across hundreds of years to
convince us (for no apparent reason) that the earth is round instead of
flat, the progression of the seasons, and even of day and night, are
controlled by the gods. Eventually, a reasonable person would have to
concede that it would make a lot more sense to agree that the earth is
There are some other theories to explain things that do not meet this test
of scientific reasonableness, yet are still put forth and defended by many
people in society. This example of the flat earth hypothesis lets you see
how difficult it is to change someone's mind when they're set on some
strange or unrealistic theory. It also lets you see that just if a theory
"can't be disproven" doesn't necessarily mean that it is correct, or that
it should be taken seriously for very long.
No matter what the Nebraska School Board says.
Richard Barrans Jr., Ph.D.
Three arguments come to mind. All of these date back to Greek times or
1. During lunar eclipses, the projected shadow of the earth on the moon is
always round. If the earth were flat, then this projection will not always
be circular (it could degenerate to a line!). But this is never observed,
regardless of the time of the lunar eclipse. Aristotle (384-322 BCE) wrote
about this. The third point, below, is also credited to Aristotle.
2. Eratosthanes (276-195(?) BCE) did a famous experiment on measuring the
angle of the sun at noon in Alexandria, Egypt, and in Syrene. Finding the
difference in the angles and knowing the distance between the two points,
the circumference can be calculated. He calculated the circumference in
stadia, which we do not know the conversion to our present day measure.
3. If you travel north or south a signigicant distance, you will see a
different set of stars at night. This will not happen on a flat surface.
Introductory astronomy texts deal with this topic. A good reference is
George Abell _Exploration of the Universe_
The next question is: "Prove that the earth is moving."
Nathan A. Unterman
I presume your teacher won't accept hearsay evidence (evidence
collected by someone else and told to you), such as testimony from
astronauts or folks who have traveled around the world by boat,
airplane or balloon.
So here are the standard facts observable by anyone which your
teacher would find difficult to explain with a flat Earth hypothesis:
(1) When ships put out to see, they sink below the horizon gradually.
First the hull goes down, then the masts, like so:
. / \ .
. . /
. here you can still see the mast . |\ /
but the hull is below the horizon \ \/
This suggests that the Earth's surface curves away and downward from
wherever you stand. It's hard to observe this effect on land, because
mountains and trees and so forth get in the way when something moves
away from you before it dips below the horizon.
(2) If you climb higher, you can see further. On top of a mountain or
lighthouse, or in an airplane, you can see things that are invisible
-- below the horizon -- when you are on the ground. For example, if
you watch the Sun set, and at the very moment when the Sun is just
below the horizon you climb quickly up a hundred feet, you will see
the Sun again. It is hard to explain why you can see further when you
climb higher unless the Earth's surface curves downward away from you
wherever you stand.
(3) The Earth casts a shadow on the Moon during a lunar eclipse. The
shadow is round.
(4) When the Sun is directly overhead in any place, it is NOT directly
overhead at the same time in any place a few hundred miles away:
Sun's rays are parallel because the Sun is very far away.
| | | | | | | | | | | | | | |
| | | | | | | | | | | | | | |
| | | | | | | | | | | | | | |
V V V V V V V V V V V V V V V
this stick ||
casts no ||
shadow || this stick
|| casts a shadow
. . //
. . //.
. . //...
. EARTH ....
If you were to put a stick in the ground sticking straight up at noon
in New York City, then telephone a friend in Chicago to ask him to
also put a stick in the ground sticking straight up, he would see a
shadow, and you would not. This is hard to explain unless ``straight
up'' (away from the Earth's surface) points in different directions
when you are at different places on the Earth's surface. That is, the
Earth is not flat.
(5) If you travel North, you can see stars at night which you never
saw before. Since there are so many stars, you only need to travel a
hundred miles or so. The new stars are stars that were hidden below
the horizon before you walked North. How could this be true if the
Earth were flat?
(6) There are tides in the oceans, which you can see at the shore,
that repeat every 24 hours or so. If the Earth were flat, then there
would be no tides, because the tides depend on their being a
substantial distance between the near and far sides of the Earth, from
the point of view of the Moon. This is kind of complicated, however.
That is easy.
Observation of the Earth from outside the planet shows it to be round,
I am sure that your teacher is trying to motivate you to think.
By the way, if you look at the stars, the sun, and the moon, you do not see
any of them to be non-round. Since you observe them at different times and
from different angles, it seems plausible to say that they are round. Why
should the Earth be an exception?
Dr. Ali Khounsary
Advanced Photon Source
Argonne National Laboratory
Argonne, IL 60439
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Update: June 2012