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 to a international speace station.
They have orbited the Earth once every 90 minutes
I’ve had personal experience with people that 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 slightlymore complicated.
One of the things you can see yourself with a pair of binoculars,
Is if you actually go out to a lake
Thear are boats on it
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 where sometimes the sun is overhead in the sky.
And 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 Earthwas 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
能看到太阳角度是不同的 ——也就是在天空中 太阳的位置是不同的
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.
还有其他一些证据 虽然很模糊 但它们都是真实的 确实很可爱
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 lunareclipse.
Now,a lunar eclipse happens when the Earth casts a shadow on the moon.
月球实际上变暗了 事实上 如果你看过月食
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
月球看起来会有点暗 甚至有点红 这种场景真的很美
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.
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 Earthto 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,
有时只是从月亮边上扫过 只会有一点点阴影 在月球的顶部或底部
sometimes it just kindof 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 whosay,”I don’t really believe the Earth is flat,
but I’m not really sure what to thinkabout 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 atall.
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—
你用望远镜看到的那些行星 如土星 木星 火星
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… everythingelse we take a picture of is a sphere!”
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