So, Oscar, you asked the question, "What are
some of the easiest ways that you can prove
that the Earth is round?"
Because apparently, this is something that
we're debating—I have no idea why.
That's a hard thing for me to even start talking
about because there are so many proofs that
the Earth is round, it's difficult to know
where to start.
And it's not okay to think that the Earth
is flat.
This is not a viable argument.
I have friends who have been on the International
Space Station, they have orbited the Earth
once every 90 minutes; I've had personal experience
with people who have been up in space and
can see with their own eyes that the Earth
is round.
And of course, we've taken all of these amazing
pictures from space; they're so beautiful,
all those pictures of the Earth.
So I don't really know what's going on right
now with this 'Earth is flat' thing, but I
will tell you that this is one of the things
I really enjoyed teaching my own astronomy
class about because there are proofs all around
you.
It is not difficult to know that the Earth
is round.
In fact, people have known of this for way
more than 2,000 years.
The ancient Greeks actually had a number of
really elegant, wonderful proofs that the
earth was a sphere.
So let's start from the simple to the slightly
more complicated.
One of the things you can see yourself, with
a pair of binoculars, is if you actually go
out to a lake and there are boats on that
lake, the farther away a boat is the more
the bottom of the boat will disappear, and
you'll basically just see the mast of the
boat.
And as a boat goes farther and farther away
the last thing you will see is the very top
of the mast of that boat, and that's because
the boat is actually going over the horizon
that's curved—and that means that as it
goes farther and farther away you see less
and less of the bottom of it, and more of
the top of that.
You can see that with binoculars by an ocean,
by a lake, it's really easy.
That wouldn't happen if the Earth were flat—you
would simply see the boat getting smaller
and smaller and smaller as it went farther
away, but you'd be able to see the whole thing
with the same proportions.
Now, another way that you can tell that we're
on a sphere is to think about how there's
something called the tropics on the Earth,
and the tropics are places near the equator
of the earth were sometimes the sun is overhead
in the sky.
This was actually something that the Greeks
used, not only to prove that the Earth was
round about 2000 years ago, but they actually
measured the circumference of the Earth, accurate
to within just a couple percent.
2,000 years ago we've known that the Earth
was round.
There was a really brilliant Greek scientist
called Eratosthenes, and Eratosthenes noticed
that there was a town called Syene, and on
a certain date the sun would actually shine
straight down to the bottom of a well.
That meant the sun was directly overhead;
you could look down a well and see the sun
shining back at you.
And on the very same date, farther away in
the city of Alexandria, that didn't happen.
The sun was not directly overhead, it was
a slight angle, and all that Eratosthenes
did was he measured the difference in the
angle of the sun.
It was straight overhead in Syene; in Alexandria
it was a little bit less than overhead, and
he rationed that that change in angle from
one city to another was probably indicative
of us being on a curved surface, and you could
make all kinds of measurements even between
those two cities and see that the angles were
different—the sun was at a different place
in the sky.
Using this, he actually measured the circumference
of the Earth, and he got it right 2,000 years
ago.
So another really simple proof is that on
any given date, at different cities and different
places around the world, the sun is at different
angles in the sky.
That wouldn't happen if the Earth wasn't round.
Then there are some other proofs that are
a little more obscure, but they're actually
really lovely.
One is to observe what happens during a lunar
eclipse.
Now, a lunar eclipse happens when the Earth
casts a shadow on the moon.
The moon actually goes dark, in fact, if you've
seen one you can actually see the Earth's
shadow go across the moon, and when the moon
is entirely in the Earth's shadow the moon
looks kind of dark and even kind of red-colored;
it's really, really beautiful.
What's happening, in that case, is that the
sun is on one side of the Earth—the Earth
is in the middle—and the Earth is casting
a shadow on the moon, and as the shadow moves
across the moon you'll notice that the shadow
is curved, it's round.
And so something like the sun that's bigger
than the Earth and is able to cast a shadow
of the Earth on the moon can actually show
you the shape of the Earth.
"Ah-ha!" you might say, "but could the Earth
to be a disk?
Could it be flat but it's actually still shaped
like a disk, not like a sphere?"
There was a Greek scientist called Aristarchus
and what he noticed was that you can get a
lunar eclipse at many different angles where
the sun is; sometimes the shadow goes straight
across the moon, sometimes it just kind of
glances the moon—just a little bit is in
shadow just on the top or on the bottom.
From every different vantage point, every
different angle the sun is casting a shadow,
you always get a perfectly curved shadow.
The only shape that can cast a shadow that's
curved from any direction you put the light
is a sphere.
So people have known that the Earth is spherical
for thousands of years.
It's not okay to say that the Earth is flat.
This is some sort of strange denial, I don't
know where it comes from, and it's something
where I keep getting this question.
We really need to put this question to bed
because we've known the Earth is a sphere
for a long time.
There's even some well-meaning people who
say, "I don't really believe the Earth is
flat, but I'm not really sure what to think
about it."
And they've asked me some interesting questions,
like they've heard that space is a very hot,
that when you go up above the atmosphere the
temperature of space is millions of degrees,
which is true.
The problem is there's basically no air at
all.
So the gas right around the Earth is actually
millions of degrees hot.
That's actually true, but there's almost none
of it, there's almost nothing.
Like one single proton whizzes by you at a
temperature of a million degrees, it's not
the same as temperature in the air, it's not
the same thing at all.
So that's one that I get sometimes.
And the other one is—I actually said this
to somebody, and I couldn't believe they had
never thought of it—that with binoculars
you can see planets, you can see Saturn and
Jupiter, you can see Mars with a telescope,
the sun and the moon, everything else you
see in the solar system is a sphere.
So we're the one thing that is different?
And that actually made somebody who was more
interested in actually hearing information,
that actually got them to think.
They were like, "You're right… everything
else we take a picture of is a sphere!"
