At some distributor point in elementary school , your science teacher probably explained to you that there are 365 day in a yr because that ’s how long it takes for Earth to discharge one full rotation around the Lord’s Day . What they might not have specify , however , is that it ’s not exactly 365 days — it ’s in reality closer to 365.2421 daytime .
So , if we want our calendar year to commence powerful when Earth begins a new gyration around the Lord’s Day , we have to answer for for ( close to ) an surplus quarter of a Clarence Shepard Day Jr. each year , or one day every four years . History.comreportsthat the Egyptians had already been doing this for a while before Europe ultimately bewitch on in 46 B.C.E. , when Roman dictator Julius Caesar and astronomer Sosigenes put their heads together to come up with what we now call the Julian calendar , which includes 12 month , 365 days , and an extra “ leap solar day ” every four years on February 29 .
But rounding 0.2421 up to 0.25 each twelvemonth create an issue , because it did n’t quite add up to a full daytime every four eld — and that tiny disagreement meant that after 128 year , the calendar year ended up starting a daylight before Earth had dispatch its rotation around the sun . By the fourteenth century , thecalendaryear was starting a banging 10 days before Earth finished its ambit .
In 1582 , Pope Gregory XIII seek tocorrectthe error by suggesting that we but hop aleap dayevery so often . His Gregorian calendar , which we still utilise today , mandate that we omit the leap day during years evenly divisible by 100 but not by 400 . For instance , the year 2000 included a leap day because it ’s divisible by 100 and 400 ; the yr 2100 , on the other hand , will not let in a leaping mean solar day , since it ’s equally divisible by 100 , but not by 400 .
Gregory XIII ’s fudge factor to Caesar ’s overcorrection is itself a bit of an under - correction , so we ’ll probably need to reassess our leap day protocol again in about 10,000 years .
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