Calendars of humanity

What one asks a year ahead? To begin with it is good, but also coinciding with the astronomical year (365 days, 5 hours, 48 minutes and 46 seconds). After all, a year is the time that it takes the Earth to complete a full orbit around the Sun, and one of the minimum requirements which should have a year of good quality is that its life and of the journey of the Earth in its orbit are equal. He is not a mere fad: is interesting stations to begin more or less always on the same date: autumn and spring (equinoxes) produced on March 21 and September 21, and the beginning of the summer and winter (solstices), on 21 December and June respectively. The subject of the stations was vitally important to ancient agricultural societies that were to determine the dates of sowing and harvesting.
The first primitive lunar calendars could not fit into the solar year: discrepancies are corrected both while adding a month or a few extra days. But in the century I before Christ, in Rome, the accumulated errors had managed to that calendar year and the solar were outdated in three months: the winter began in March and the fall in December, which was no doubt quite uncomfortable.
Julius Caesar introduced the first major reform. The universal use of the solar calendar imposed throughout the Roman world, set the duration of the year at 365 days and six hours, and for those six hours of difference is non-accumulating is sandwiched an extra day every four years: leap years have three hundred sixty-six days. The reform came into force on 10 January of the year 45 b.c. - 805 of the Foundation of Rome. Eventually, the custom of taking prevailed as leap years that are multiples of four.
But here did not finish the thing, since the Julian year of 365 days and six hours was a little longer (11 minutes and 14 seconds) than the actual astronomical year, and again the mistakes began to accumulate: at the end of the 16th century dates were running about ten days, and spring began on September 11: the Pope Gregorio XIII undertook a new reform to correct discrepancies and forcing stations to start when they should: a Pontifical Decree of March 1582, he abolished the Julian calendar and imposed the Gregorian calendar. Date, running it ten days was changed: September 11 (day in which occurred the spring equinox) became a 'de facto' September 21, which was eliminated the backlog in sixteen centuries and the calendar year and the astronomical returned to coincide.
But also the rule of leap years was modified: henceforth would be leap those years that are multiples of four, unless they end up in two zeros. These latter are leap only those that are multiples of four hundred (as 1600). Others (such as the 1700) do not. Thus, neither 1800 nor 1900 were leap years. The year 2000, however, it will be (because although ends in two zeros is a multiple of four hundred): the formula eliminates three days every four centuries, it is the difference that accumulated the Julian calendar during that time.
However, even the "Gregorian year" with all its corrections is 26 seconds long that astronomical year, which implies a difference day 3323-yearly. To correct this little discrepancy has been proposed to remove a day every four thousand years in such a way that the year 4000, the 8000 or the 16000 are not leap (although touched). In any case, the length of the year eight thousand, or sixteen thousand, you do not need to worry now: the years that we are using have one duration more than acceptable.
Article translated for educational purposes from:  Planeta Sedna