[MUSIC] A long time ago on a far-away island, the people kept time by sunrise, sunset, and the cannon.

Every day at noon, on the hill above town, soldiers would fire the cannon and set life's pace in the village below.

One day, a boy wanted to know how the soldiers knew when to fire.

The artilleryman told the boy he gets the order from the captain.

When he asked the captain, he said he keeps his watch synchronized to the great clock in the village.

And when the boy asked the watchmaker how he sets the great clock, the watchmaker said, "I set it to the noonday cannon!"

The way we tell time isn't so different today.

The cannon and soldiers have been replaced by tiny machines, but somehow we've all agreed to agree when, exactly, is now.

So who decided what time it is, everywhere?

[MUSIC] Time, in the physics sense, is very real.

It's what makes "now" different from "then" across the universe.

But "time", in the sense of hours, minutes, seconds and you and I agreeing that you're a half hour late for lunch?

We made that up.

The rhythms of time are built into our bodies, and most living things instinctively obey daily or yearly cycles by following the big bright daytime sky thing.

For most of human history, though, sunrise, sunset, and some seasons were all the timekeeping we needed.

And maybe knowing halfway in between for lunch.

And maybe the tides if you're the Saxons.

The first calendars were made to please the gods, and cows and crops didn't need clocks.

But with civilization came schedules.

People had meetings to make and employees to manage.

Egyptians made the first clocks using shadows and split the day in 12, but that doesn't work in the dark, so Egyptians marked nighttime hours by watching 12 of their favorite constellations rise.

Why 12?

Because why not!

Egyptians preferred duodecimal numbers to match the lunar cycles in a year or maybe the finger joints on each hand, but either way, that's how we got a 24 hour day.

Or they might have stolen it from the Babylonians.

Ancients eventually switched from watching stars to watching water drip, but hours weren't a fixed length until Greek astronomers decided to make it that way.

Later, Greeks decided to divide up circles into a very random 360 "degrees", and each degree into 60 smaller "minutes", and each of those into 60 smaller "seconds".

Why?

Greek astronomers "borrowed" a lot of their math from guess who!

the Babylonians, who used a base-60 number system they "borrowed" from the Sumerians.

In the end, hey, I guess clocks are circles, so why not use minutes and seconds for time too?

What's clear is the 24 60 60 system doesn't make much sense, it's just how it happened.

Water-clocks spread all over the world and became pretty sophisticated, adding bobs and gears to show the water level and time on a face.

But Europeans were the first to get rid of the water entirely, and just use gears and gadgets, mostly because European astronomers needed extremely accurate time measurements to do experiments.

Science molded clocks into intricate counting machines: Just figure out some regular wiggler, like Galileo's pendulum or Christian Huygens' spring, then count the wiggles.

Congrats!

You're a Time Lord, Harry.

This new precision time created a new kind of human day.

People knew when to go to work and when to meet.

More and more people disconnected their day from nature, and turned it over to machines.

Which sounds kind of familiar.

By the Industrial Revolution, schedules could be regulated to the minute, workers became slaves to the clock and productivity was measured by what you made in how much time, because after all: Time is money!

This was a huge shift in human history.

Clocks even revolutionized exploration and fueled the colonial era.

Precise longitude measurements require precise clocks.

Sailors synced fancy new spring-watches to a master clock at Royal Greenwich Observatory so they'd always know where, and when, they were.

But remember that noonday cannon?

Accurate timepieces don't work if you don't agree on whose clock has the right time.

Cities and companies would often follow their own clocks, choosing 12 noon to be whenever the sun was overhead in their city.

Trains and telegraphs made this a big problem, so in 1884, President Chester A. Arthur held a conference where the world decided once and for all what time it is.

England was like "Hey, we've got that awesome clock in Greenwich" so everyone agreed Greenwich Mean Time would be The Time... adjusted for time zones, which are a whole other thing.

In the 1900s, scientists started using electronics to keep time.

When quartz crystals are hooked up to special electric circuits, they vibrate 32,768 times every second.

Tinier wiggles than a spring or pendulum, but same principle.

Most of the gadgets we tell time with today use these little crystally wigglers.

But clocks got so good, the definition of a second was now less precise than the things measuring seconds.

The lengths of days and years aren't always exactly the same; the tug of the moon, earthquakes, gravity from other planets all tweak Earth's rotation ever so slightly, and shift the year by a few seconds every few thousand years.

Doesn't sound like much, but super-precise science machines like GPS and particle colliders need to accurately measure down to billionths of a second.

The solution was to change the definition of a second.

Unlike springs and pendulums, atoms wiggle pretty much the same everywhere in the universe.

Today, atomic clocks count the wiggling of electrons in atoms, a little over 9 billion electron oscillations of a cesium atom per second to be exact, and they're so precise they may drift by just one second in tens of millions of years.

The Babylonians would be very proud.

And probably think we were wizards.

Today, our clocks work so well that we completely take them for granted, yet it took more than 3,000 years, with Egyptian priests and Galileo and even Chester A. Arthur all working together to synchronize the world.

But they did, and now we can finally rest, knowing that some magic machine knows exactly what time it is.

And as always, thanks for watching.

Watching!

Stay curious.