DAY AND NIGHT

WE KNOW THAT DAY AND NIGHT ARE CAUSED BY THE ROTATION OF THE EARTH ON ITS AXIS,SO THAT FOR PART OF THE TIME A PLACE ON THE GLOBE FACES TOWARDS THE SUN AND FOR THE REST OF THE TIME IT IS TURNED AWAY FROM IT .
TO EARLY PEOPLE A DAY MEANT THE SPAN OF TIME BETWEEN SUN RISE AND SUN SET . WE STILL USE THE WORD IN THIS WAY TO DISTINGUISH IT FROM THE PERIOD OF DARKNESS THAT WE CALL NIGHT . BUT AS CIVILIZATION DEVELOPED , IT BECAME NECESSARY TO IDENTIFY DAYS MORE ACCURATELY AND TO WORK OUT WHEN THEY BEGAN AND ENDED .

THE ANCIENT GREEKS MEASURED THEIR DAY FROM SUNSET TO SUNSET . THE ROMANS , UNLIKE THE GREEKS , MEASURED THEIR DAY FROM MIDNIGHT TO MIDNIGHT . MOST MODERN COUNTRIES USE THIS ROMAN METHOD .

BEFORE MECHANICAL CLOCKS WERE INVENTED , PEOPLE DIVIDED THE DAY AND NIGHT INTO 12 HOURS EACH , BUT THIS METHOD PROVED IMPRACTICAL BECAUSE THE LENGTH OF THE DAY DIFFERED WITH THE SEASONS . THE INVENTION OF THE CLOCK HELPED IN DEVELOPING A SYSTEM OF MEASURING TIME THAT IS NOW USED ALMOST EVERYWHERE . UNDER THIS SYSTEM THE DAY LAST FOR TWENTY FOUR HOURS FROM MIDNIGHT TO MIDNIGHT . ALTHOUGH WE NO LONGER DIVIDE THE DAY AND NIGHT INTO TWELVE HOURS EACH , WE STILL USE TWO PERIODS OF TWELVE HOURS   EACH . THESE EQUAL HALVES ARE DISTINGUISHED BY THE ABBREVIATIONS A.M [ANTI MERIDIAN  , A LATIN PHRASE MEANING BEFORE MID DAY ] . FOR THE PERIOD FROM TWELVE MIDNIGHT TO TWELVE NOON , AND P.M [ POST MERIDIAN , MEANING AFTER MID DAY ] FOR THE PERIOD FROM TWELVE NOON TO TWELVE MIDNIGHT . OTHER WAYS OF SPLITTING UP THE DAY INCLUDE THE DIVISION INTO MORNING [ FROM MID NIGHT TO NOON ] , AFTER NOON [FROM NOON TO DUSK ] , AND NIGHT [ FROM DUSK TO MIDNIGHT ] . A VARIABLE PERIOD , CALLED THE EVENING , FALLS BETWEEN ABOUT FIVE P.M AND THE END OF TWILIGHT . IT IS HE PERIOD AFTER SUNSET WHEN THE SUN IS ONLY JUST BELOW THE HORIZON . IN SUMMER THE EVENINGS ARE LONGER THAN IN WINTER .