- WHO DEVELOPED UNITS OF TIME?
HOW DID THEY DECIDE HOW LONG A SECOND SHOULD BE
OR A MINUTE OR AN HOUR
OR EVEN A DAY?
WE LOOK TO THE SKIES TO TELL US THE TIME,
AND THE ROTATION OF THE EARTH GIVES US SOME ANSWERS.
THINK OF EARTH AS A ROUNDED BUT NOT PERFECTLY ROUND SPHERE.
LOOKING DOWN ON THE SPHERE FROM SPACE, WE SEE A CIRCLE.
BUT THE ANCIENT GREEKS RELIED ON OBSERVATION AND MATHEMATICS
TO FIGURE OUT THE SHAPE AND SIZE OF EARTH.
THEY DIVIDED THE GLOBE INTO 360 SEGMENTS CALLED DEGREES.
VERTICAL LINES CALLED MERIDIANS OR LINES OF LONGITUDE
WERE USED TO MARK EACH DEGREE.
THE EARTH IS ROTATING,
SO THE SUN APPEARS TO MOVE ACROSS THE SKY.
IN A 24-HOUR DAY--
THE TIME FROM SUNRISE TO SUNRISE EACH DAY--
EARTH HAS MADE ONE COMPLETE TURN,
OR ROTATION, ON ITS AXIS.
SO HOW FAR HAS EARTH TURNED IN ONE HOUR?
QUICK, DO THE MATH.
360 DIVIDED BY 24 IS...
YOU GOT IT, 15.
THE EARTH ROTATES ABOUT 15 DEGREES EVERY HOUR.
THE LINE OF LONGITUDE EVERY 15 DEGREES
ALSO MARKS A NEW TIME ZONE,
BEGINNING WITH 0 DEGREES LONGITUDE,
A LINE WE CALL THE PRIME MERIDIAN.
AND YOU GUESSED IT: TIME IN THIS ZONE
IS CALLED GREENWICH MEAN TIME, NAMED FOR GREENWICH, ENGLAND,
A CITY THE PRIME MERIDIAN RUNS THROUGH.
THE ROYAL NAVAL OBSERVATORY IN GREENWICH, ENGLAND,
IS ABOUT 75 DEGREES FROM NASA LANGLEY RESEARCH CENTER
IN HAMPTON, VIRGINIA.
TO FIND OUT HOW MANY TIME ZONES THAT IS,
WE JUST DO THE MATH AGAIN.
WE KNOW THAT EACH TIME ZONE IS 15 DEGREES,
SO 75 DIVIDED BY 15 EQUALS 5,
WHICH MEANS THE TIME IN ENGLAND IS FIVE HOURS AHEAD
OF THE TIME IN VIRGINIA.
SO IF IT IS 11:00 P.M. IN ENGLAND,
WE WOULD SUBTRACT FIVE HOURS
TO FIND OUT WHAT TIME IT IS AT LANGLEY.
THE TIME IN VIRGINIA WOULD BE 6:00 P.M.
IF THE ASTRONAUTS WERE TRYING TO TALK TO PEOPLE
AT NASA'S JOHNSON SPACE CENTER,
WE WOULD HAVE TO SUBTRACT ANOTHER HOUR,
BECAUSE HOUSTON'S LONGITUDE PUTS IT
IN A DIFFERENT TIME ZONE THAN VIRGINIA.
AND THE THREE NASA CENTERS IN CALIFORNIA
ARE LOCATED IN A DIFFERENT TIME ZONE YET.
IN FACT, THE TEN NASA CENTERS ACROSS THE UNITED STATES
ARE LOCATED IN THREE DIFFERENT TIME ZONES,
AND THAT'S NOT EVEN COUNTING THE TIME ZONES
FOR ALL THE OTHER COUNTRIES INVOLVED
IN THE INTERNATIONAL SPACE STATION.
- AS WE ALL KNOW,
WE RARELY THINK ABOUT OUR 24-HOUR DAY,
THAT IT'S ACTUALLY DEFINED ABOUT THE ROTATION OF THE EARTH.
IT MAKE SENSE TO SPEND OUR TIME AWAKE AND WORKING
WHILE THE SUN IS UP
AND RESTING WHILE THE SUN IS DOWN.
- SO THE LENGTH OF A DAY IS RELATED
TO HOW FAST A PLANET ROTATES ON ITS AXIS.
AND HOW ABOUT A DAY ON MERCURY, WHICH CAN LAST 88 EARTH DAYS?
HEY, I'D HAVE PLENTY OF TIME
TO STAY UP REAL LATE EACH NIGHT
AND SLEEP IN EVERY MORNING.
- BUT HAVE YOU EVER THOUGHT ABOUT WHAT LIFE WOULD BE LIKE
IF THE EARTH ROTATED AT A DIFFERENT SPEED?
IF WE LIVED ON JUPITER,
WHICH ROTATES ONCE EVERY TEN HOURS,
PERHAPS YOU WOULD ONLY BE AT SCHOOL
FOR THREE HOUR-- FOUR HOURS A DAY
BEFORE THE SUN WOULD BEGIN TO SET
AND YOU WOULD HAVE TO GO BACK HOME TO SLEEP.
ON MERCURY, THE SUN IS UP FOR 88 EARTH DAYS.
IMAGINE WHAT LIFE WOULD BE LIKE
IF WE HAD THREE MONTHS OF DAYTIME
AND THEN THREE MONTHS OF NIGHTTIME.
DO YOU THINK WE WOULD ADJUST
AND SLEEP FOR THREE MONTHS STRAIGHT?
OUR PERCEPTION OF TIME IS TIED DIRECTLY
TO LIVING ON THE SURFACE OF EARTH.
BUT WHILE WE'RE HERE IN ORBIT,
WE DON'T SENSE THIS PASSAGE OF DAYS
LIKE WE DO ON EARTH.
BUT OUR BODIES ARE USED TO THE 24-HOUR DAYS ON EARTH,
SO WE KIND OF GO TO WORK AND TO SLEEP
BACK AND FORTH EVERY 45 MINUTES.
WE CONTINUE TO OPERATE ON A 24-HOUR DAY.
SO THE NEXT TIME YOU LAY DOWN TO GET READY FOR BED,
REMEMBER THAT SOMEONE ON THE OTHER SIDE OF THE WORLD
IS JUST WAKING UP.
ANOTHER ONE IS EATING THEIR LUNCH IN THE AFTERNOON SUN.
AND STILL OTHERS ARE FLYING 350 KILOMETERS, OR 200 MILES,
ABOVE THE EARTH ON THE INTERNATIONAL SPACE STATION,
WHERE THE ENTIRE DAY JUST SEEMS TO FLY BY.
- TIME HAS REALLY GOTTEN AWAY FROM US HERE.
SO NO MATTER WHAT TIME ZONE YOU'RE IN,
BE SURE TO CATCH US NEXT TIME ON "REAL WORLD."