WEBVTT

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- WHO DEVELOPED UNITS OF TIME?

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HOW DID THEY DECIDE
HOW LONG A SECOND SHOULD BE

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OR A MINUTE OR AN HOUR

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OR EVEN A DAY?

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WE LOOK TO THE SKIES
TO TELL US THE TIME,

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AND THE ROTATION OF THE EARTH
GIVES US SOME ANSWERS.

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THINK OF EARTH AS A ROUNDED
BUT NOT PERFECTLY ROUND SPHERE.

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LOOKING DOWN ON THE SPHERE
FROM SPACE, WE SEE A CIRCLE.

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BUT THE ANCIENT GREEKS RELIED
ON OBSERVATION AND MATHEMATICS

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TO FIGURE OUT THE SHAPE
AND SIZE OF EARTH.

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THEY DIVIDED THE GLOBE INTO
360 SEGMENTS CALLED DEGREES.

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VERTICAL LINES CALLED MERIDIANS
OR LINES OF LONGITUDE

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WERE USED TO MARK EACH DEGREE.

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THE EARTH IS ROTATING,

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SO THE SUN APPEARS
TO MOVE ACROSS THE SKY.

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IN A 24-HOUR DAY--

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THE TIME FROM SUNRISE
TO SUNRISE EACH DAY--

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EARTH HAS MADE
ONE COMPLETE TURN,

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OR ROTATION,
ON ITS AXIS.

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SO HOW FAR HAS EARTH TURNED
IN ONE HOUR?

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QUICK, DO THE MATH.

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360 DIVIDED BY 24 IS...

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YOU GOT IT, 15.

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THE EARTH ROTATES
ABOUT 15 DEGREES EVERY HOUR.

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THE LINE OF LONGITUDE
EVERY 15 DEGREES

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ALSO MARKS A NEW TIME ZONE,

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BEGINNING WITH 0 DEGREES
LONGITUDE,

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A LINE WE CALL
THE PRIME MERIDIAN.

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AND YOU GUESSED IT:
TIME IN THIS ZONE

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IS CALLED GREENWICH MEAN TIME,
NAMED FOR GREENWICH, ENGLAND,

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A CITY THE PRIME MERIDIAN
RUNS THROUGH.

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THE ROYAL NAVAL OBSERVATORY
IN GREENWICH, ENGLAND,

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IS ABOUT 75 DEGREES FROM
NASA LANGLEY RESEARCH CENTER

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IN HAMPTON, VIRGINIA.

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TO FIND OUT
HOW MANY TIME ZONES THAT IS,

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WE JUST DO THE MATH AGAIN.

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WE KNOW THAT EACH TIME ZONE
IS 15 DEGREES,

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SO 75 DIVIDED BY 15 EQUALS 5,

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WHICH MEANS THE TIME IN ENGLAND
IS FIVE HOURS AHEAD

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OF THE TIME IN VIRGINIA.

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SO IF IT IS 11:00 P.M.
IN ENGLAND,

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WE WOULD SUBTRACT FIVE HOURS

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TO FIND OUT WHAT TIME IT IS
AT LANGLEY.

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THE TIME IN VIRGINIA
WOULD BE 6:00 P.M.

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IF THE ASTRONAUTS
WERE TRYING TO TALK TO PEOPLE

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AT NASA'S JOHNSON SPACE CENTER,

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WE WOULD HAVE TO SUBTRACT
ANOTHER HOUR,

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BECAUSE HOUSTON'S LONGITUDE
PUTS IT

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IN A DIFFERENT TIME ZONE
THAN VIRGINIA.

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AND THE THREE NASA CENTERS
IN CALIFORNIA

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ARE LOCATED
IN A DIFFERENT TIME ZONE YET.

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IN FACT, THE TEN NASA CENTERS
ACROSS THE UNITED STATES

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ARE LOCATED IN THREE
DIFFERENT TIME ZONES,

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AND THAT'S NOT EVEN COUNTING
THE TIME ZONES

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FOR ALL THE OTHER COUNTRIES
INVOLVED

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IN THE INTERNATIONAL
SPACE STATION.

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- AS WE ALL KNOW,

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WE RARELY THINK
ABOUT OUR 24-HOUR DAY,

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THAT IT'S ACTUALLY DEFINED
ABOUT THE ROTATION OF THE EARTH.

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IT MAKE SENSE TO SPEND OUR TIME
AWAKE AND WORKING

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WHILE THE SUN IS UP

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AND RESTING
WHILE THE SUN IS DOWN.

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- SO THE LENGTH OF A DAY
IS RELATED

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TO HOW FAST A PLANET ROTATES
ON ITS AXIS.

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AND HOW ABOUT A DAY ON MERCURY,
WHICH CAN LAST 88 EARTH DAYS?

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HEY, I'D HAVE PLENTY OF TIME

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TO STAY UP REAL LATE
EACH NIGHT

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AND SLEEP IN EVERY MORNING.

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- BUT HAVE YOU EVER THOUGHT
ABOUT WHAT LIFE WOULD BE LIKE

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IF THE EARTH ROTATED
AT A DIFFERENT SPEED?

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IF WE LIVED ON JUPITER,

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WHICH ROTATES
ONCE EVERY TEN HOURS,

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PERHAPS YOU WOULD ONLY
BE AT SCHOOL

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FOR THREE HOUR--
FOUR HOURS A DAY

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BEFORE THE SUN
WOULD BEGIN TO SET

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AND YOU WOULD HAVE
TO GO BACK HOME TO SLEEP.

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ON MERCURY, THE SUN IS UP
FOR 88 EARTH DAYS.

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IMAGINE WHAT LIFE
WOULD BE LIKE

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IF WE HAD THREE MONTHS
OF DAYTIME

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AND THEN THREE MONTHS
OF NIGHTTIME.

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DO YOU THINK WE WOULD ADJUST

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AND SLEEP
FOR THREE MONTHS STRAIGHT?

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OUR PERCEPTION OF TIME
IS TIED DIRECTLY

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TO LIVING ON THE SURFACE
OF EARTH.

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BUT WHILE WE'RE HERE IN ORBIT,

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WE DON'T SENSE THIS PASSAGE
OF DAYS

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LIKE WE DO ON EARTH.

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BUT OUR BODIES ARE USED
TO THE 24-HOUR DAYS ON EARTH,

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SO WE KIND OF GO TO WORK
AND TO SLEEP

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BACK AND FORTH
EVERY 45 MINUTES.

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WE CONTINUE TO OPERATE
ON A 24-HOUR DAY.

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SO THE NEXT TIME YOU LAY DOWN
TO GET READY FOR BED,

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REMEMBER THAT SOMEONE
ON THE OTHER SIDE OF THE WORLD

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IS JUST WAKING UP.

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ANOTHER ONE IS EATING THEIR
LUNCH IN THE AFTERNOON SUN.

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AND STILL OTHERS ARE FLYING
350 KILOMETERS, OR 200 MILES,

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ABOVE THE EARTH ON
THE INTERNATIONAL SPACE STATION,

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WHERE THE ENTIRE DAY
JUST SEEMS TO FLY BY.

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- TIME HAS REALLY
GOTTEN AWAY FROM US HERE.

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SO NO MATTER WHAT TIME ZONE
YOU'RE IN,

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BE SURE TO CATCH US NEXT TIME
ON "REAL WORLD."