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(Music)

One of the funny things about owning a brain

is that you have no control over the things that it gathers and holds onto,

the facts and the stories. And as you get older, it only gets worse.

Things stick around for years sometimes

before you understand why you're interested in them,

before you understand their import to you.

Here's three of mine.

When Richard Feynman was a young boy in Queens,

he went for a walk with his dad and his wagon

and a ball. And he noticed that when he pulled the wagon,

the ball went to the back of the wagon.

And he asked his dad, "Why does the ball go to the back of the wagon?"

And his dad said, "That's inertia."

He said, "What's inertia?" And his dad said, "Ah.

Inertia is the name that scientists give

to the phenomenon of the ball going to the back of the wagon.

But in truth, nobody really knows."

Feynman went on to earn degrees

at MIT, Princeton, he solved the Challenger disaster,

he ended up winning the Nobel Prize in Physics

for his Feynman diagrams describing the movement of subatomic particles.

And he credits that conversation with his father

as giving him a sense

that the simplest questions could carry you out to the edge of human knowledge,

and that that's where he wanted to play.

And play he did.

Now Eratosthenes was the third librarian at the great Library at Alexandria,

and he made many contributions to science.

But the one he is most remembered for

began in a letter that he received as the librarian,

from the town of Swenet, which was south of Alexandria.

The letter included this fact that stuck in Eratosthenes' mind,

and the fact was that the writer said at noon

on the solstice, when he looked down this deep well,

he could see his reflection at the bottom, and he could also see that his head

was blocking the sun.

Now, I should tell you — the idea that Christopher Columbus discovered that the world is spherical

is total bull. It's not true at all.

In fact, everyone who was educated understood that the world was spherical

since Aristotle's time, and Aristotle had proved it

with a simple observation.

He noticed that every time you saw the Earth's shadow on the Moon

it was circular,

and the only shape that constantly creates a circular shadow

is a sphere, Q.E.D. the Earth is round.

But nobody knew how big it was

until Eratosthenes got this letter with this fact.

So he understood that the sun was directly above the city of Swenet,

because looking down a well, it was a straight line

all the way down the well, right past the guy's head up to the sun.

Eratosthenes knew another fact.

He knew that a stick stuck in the ground in Alexandria

at the same time and the same day, at noon,

the sun's zenith, on the solstice,

the sun cast a shadow that showed that it was 7.2 degrees off-axis.

Now, if you know the circumference of a circle,

and you have two points on it,

all you need to know is the distance between those two points,

and you can extrapolate the circumference.

Three hundred and sixty degrees divided by 7.2 equals 50.

I know it's a little bit of a round number, and it makes me suspicious of this story too,

but it's a good story, so we'll continue with it.

He needed to know the distance between Swenet and Alexandria,

which is good because Eratosthenes was good at geography.

In fact, he invented the word geography.

The road between Swenet and Alexandria

was a road of commerce,

and commerce needed to know how long it took to get there.

One of the funny things about owning a brain

is that you have no control over the things that it gathers and holds onto,

the facts and the stories. And as you get older, it only gets worse.

Things stick around for years sometimes

before you understand why you're interested in them,

before you understand their import to you.

Here's three of mine.

When Richard Feynman was a young boy in Queens,

he went for a walk with his dad and his wagon

and a ball. And he noticed that when he pulled the wagon,

the ball went to the back of the wagon.

And he asked his dad, "Why does the ball go to the back of the wagon?"

And his dad said, "That's inertia."

He said, "What's inertia?" And his dad said, "Ah.

Inertia is the name that scientists give

to the phenomenon of the ball going to the back of the wagon.

But in truth, nobody really knows."

Feynman went on to earn degrees

at MIT, Princeton, he solved the Challenger disaster,

he ended up winning the Nobel Prize in Physics

for his Feynman diagrams describing the movement of subatomic particles.

And he credits that conversation with his father

as giving him a sense

that the simplest questions could carry you out to the edge of human knowledge,

and that that's where he wanted to play.

And play he did.

Now Eratosthenes was the third librarian at the great Library at Alexandria,

and he made many contributions to science.

But the one he is most remembered for

began in a letter that he received as the librarian,

from the town of Swenet, which was south of Alexandria.

The letter included this fact that stuck in Eratosthenes' mind,

and the fact was that the writer said at noon

on the solstice, when he looked down this deep well,

he could see his reflection at the bottom, and he could also see that his head

was blocking the sun.

Now, I should tell you — the idea that Christopher Columbus discovered that the world is spherical

is total bull. It's not true at all.

In fact, everyone who was educated understood that the world was spherical

since Aristotle's time, and Aristotle had proved it

with a simple observation.

He noticed that every time you saw the Earth's shadow on the Moon

it was circular,

and the only shape that constantly creates a circular shadow

is a sphere, Q.E.D. the Earth is round.

But nobody knew how big it was

until Eratosthenes got this letter with this fact.

So he understood that the sun was directly above the city of Swenet,

because looking down a well, it was a straight line

all the way down the well, right past the guy's head up to the sun.

Eratosthenes knew another fact.

He knew that a stick stuck in the ground in Alexandria

at the same time and the same day, at noon,

the sun's zenith, on the solstice,

the sun cast a shadow that showed that it was 7.2 degrees off-axis.

Now, if you know the circumference of a circle,

and you have two points on it,

all you need to know is the distance between those two points,

and you can extrapolate the circumference.

Three hundred and sixty degrees divided by 7.2 equals 50.

I know it's a little bit of a round number, and it makes me suspicious of this story too,

but it's a good story, so we'll continue with it.

He needed to know the distance between Swenet and Alexandria,

which is good because Eratosthenes was good at geography.

In fact, he invented the word geography.

The road between Swenet and Alexandria

was a road of commerce,

and commerce needed to know how long it took to get there.

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