What’s in a Cal­en­dar?

The Washington Post Sunday - - SC4 -

What is a cal­en­dar?

A cal­en­dar is a sys­tem for di­vid­ing time and ar­rang­ing the di­vi­sions.

Kids and adults use cal­en­dars al­most ev­ery day. Whether they hang on the wall or are opened with a tap on a cell phone, cal­en­dars help us keep track of ap­point­ments, school days, sport­ing events and other im­por­tant dates.

Who in­vented the first cal­en­dar? Why do we have 24 hours in a day and 12 months in a year? The Mini Page found out more about how cal­en­dars were de­vel­oped.

Where do we start?

In an­cient times, peo­ple noted the pas­sage of time by sun­rises and sun­sets, sea­sons and moon phases. For ex­am­ple, it takes about 24 hours for the Earth to make one ro­ta­tion on its axis; this is the ba­sis for a day.

Our cal­en­dar bases months roughly on the time be­tween new moons. Can you re­mem­ber how many days each month has?

29. has it year, leap a in days; 28 has Fe­bru­ary year, reg­u­lar a In Fe­bru­ary. for ex­cept days, 31 have months other The Novem­ber.” and June April, Septem­ber, has days 30“ rhyme: this with has month each days many how re­mem­ber can You An­swer:

Read­ing the num­bers

In the year 585 in Rome, a scholar named Diony­sius in­vented a cal­en­dar that num­bered the years. It used B.C. and A.D. to tell the dif­fer­ence be­tween the years be­fore and af­ter the birth of Je­sus Christ.*

B.C. stands for be­fore Christ. If we say 225 B.C., we mean 225 years be­fore Christ was born.

A.D. stands for anno do­mini, a Latin phrase mean­ing “the year of our Lord.” When we say A.D. 435, we mean 435 years af­ter Christ’s birth.

Mod­ern cal­en­dars

In 1582, Pope Gre­gory XIII changed the cal­en­dar, and most peo­ple still use the Gre­go­rian cal­en­dar to­day.

This cal­en­dar is based on how long the Earth takes to make one or­bit around the sun. To­day this is cal­cu­lated as about 3651/ 4 days — which is why ev­ery four years, we have an ex­tra (leap) day in Fe­bru­ary. * To­day some peo­ple use B.C.E., or be­fore com­mon era, and C.E., or com­mon era, to iden­tify the years.

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