0:00:14.000,0:00:17.000
We check the time every day, all day long.

0:00:17.000,0:00:20.000
But did you ever wonder - where did telling time come from?

0:00:20.000,0:00:22.000
Why does it matter what time it is?

0:00:22.000,0:00:24.000
Who determined the clock

0:00:24.000,0:00:28.000
and why in the world are there so many different time zones?

0:00:28.000,0:00:30.000
The first form of telling time was the sundial

0:00:30.000,0:00:33.000
and the earliest sundials known from the archaeological record

0:00:33.000,0:00:37.000
are obelisks from nearly 5,000 years ago.

0:00:37.000,0:00:40.000
Sundials indicate the time by casting a shadow

0:00:40.000,0:00:42.000
onto a surface.

0:00:42.000,0:00:47.000
The object that casts the shadow is a stick in the center known as a nomen.

0:00:47.000,0:00:50.000
A well-constructed sundial can measure time with remarkable accuracy,

0:00:50.000,0:00:56.000
and sundials were used to monitor the performance of clocks until the modern era.

0:00:56.000,0:00:58.000
But sundials have their limitations too.

0:00:58.000,0:01:01.000
Obviously they require the sun to shine,

0:01:01.000,0:01:04.000
so they don't work at all during the night when it's dark.

0:01:04.000,0:01:08.000
Many different devices have been used over the years to estimate the passage of time:

0:01:08.000,0:01:13.000
candles and sticks of incense that burn down at fairly predictable speeds

0:01:13.000,0:01:15.000
have been used, along with the hourglass.

0:01:15.000,0:01:18.000
Hourglasses are devices in which fine sand

0:01:18.000,0:01:21.000
pours through a tiny hole at a constant rate

0:01:21.000,0:01:26.000
and indicates a predetermined passage of an arbitrary period of time.

0:01:26.000,0:01:28.000
The origin of the hourglass is uncertain,

0:01:28.000,0:01:32.000
although beginning in the 14th century, the hourglass was used commonly,

0:01:32.000,0:01:34.000
especially on board ships.

0:01:34.000,0:01:37.000
The motion of the boat on the water did not affect the hourglass,

0:01:37.000,0:01:41.000
unlike other time-measuring devices.

0:01:41.000,0:01:44.000
The mechanical clock was invented in the 13th century

0:01:44.000,0:01:48.000
which sparked a big change in traditional timekeeping methods.

0:01:48.000,0:01:52.000
This modern clock relied on the swing of a pendulum

0:01:52.000,0:01:55.000
or the vibration of a quartz crystal, which was far more accurate than sand

0:01:55.000,0:01:57.000
or candles.

0:01:57.000,0:02:00.000
Today, the basis for scientific time

0:02:00.000,0:02:02.000
is a continuous count of seconds

0:02:02.000,0:02:08.000
based on atomic clocks all around the world, known as the international atomic time.

0:02:08.000,0:02:10.000
Why does it matter that we keep track of time?

0:02:10.000,0:02:13.000
Well, time regulates our daily lives

0:02:13.000,0:02:16.000
and makes it possible to accurately communicate with people

0:02:16.000,0:02:18.000
all over the world.

0:02:18.000,0:02:22.000
Without a time system, we would have many challenges in farming,

0:02:22.000,0:02:25.000
social structures, communication, and business.

0:02:25.000,0:02:28.000
Take the American railroad system, for example.

0:02:28.000,0:02:32.000
In the mid-19th century, each railroad used its own standard time

0:02:32.000,0:02:35.000
generally based on the local time of its headquarters,

0:02:35.000,0:02:40.000
and the railroad's train schedules were published using its own time.

0:02:40.000,0:02:42.000
Some major railroad junctions

0:02:42.000,0:02:44.000
served by several different railroads

0:02:44.000,0:02:48.000
had a separate clock for each railroad, each showing a different time.

0:02:48.000,0:02:52.000
The distance between New York and Boston is about 2 degrees, or 8 minutes,

0:02:52.000,0:02:56.000
which can be the difference between making or missing your train connection.

0:02:56.000,0:02:59.000
If the difference between New York and Boston

0:02:59.000,0:03:02.000
is 8 minutes, imagine the difference

0:03:02.000,0:03:04.000
between Boston and Australia.

0:03:04.000,0:03:07.000
The use of time zones irons out these differences

0:03:07.000,0:03:10.000
and makes communication significantly smoother.

0:03:10.000,0:03:12.000
A time zone is a region on earth

0:03:12.000,0:03:14.000
that has a uniform standard time.

0:03:14.000,0:03:17.000
There are 40 time zones on land

0:03:17.000,0:03:19.000
because the earliest and latest time zones

0:03:19.000,0:03:21.000
are 26 hours apart.

0:03:21.000,0:03:24.000
Any given calendar date exists at some point

0:03:24.000,0:03:27.000
on the globe for 50 hours.

0:03:27.000,0:03:29.000
So the next time someone asks you

0:03:29.000,0:03:31.000
"What time is it?"

0:03:31.000,9:59:59.000
Your answer may be a whole lot more complicated than it used to be.