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Numbers
Chronological Approach
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Calendars
In the year of 46 before Christ, Julius Caesar, a Roman Emperor, invites Socigenes {Sosigenis ~ Σωσιγένης}, a Greek astronomer from Alexandria {Aleksandria ~ Αλεξάνδρεια}, in order to advise him on how to reform the Lunar calendar. Socigenes proposed the dismissal of the calendar used until then and this was the beginning of the Julian calendar which introduced a year of 365 and a quarter of a day days. These quarters of a day were added up and every fourth year and thus, February ended up having 29 days, instead of the usual 28. This special year was called a leap year.
However, by the middle of the 16th century, the vernal equinox had slipped by 10 days, because the Julian calendar which was based on a year of 365 and a quarter of a day days, was 11 minutes and 14 seconds earlier compared against the tropical year.
Pope Gregory the 13th, who was elected in 1572, invited Christopher Clavius, an astronomer who was also a member of the Jesuit Order, in order to advise him on how to correct the slippage. The correction resulted in the loss of 10 days during the month of October in the year of 1572. October 4th, was followed by October 15th and this was the beginning of the Gregorian calendar. Since then, every fourth centennial is leap, in contrast with the three centennials that precede it and that are not leap.
For the Greek State, February 16th, 1923 was followed by March 1st, 1923 and this was the beginning of the Gregorian calendar.
For the Greek Orthodox Church, March 10th, 1924 was followed by March 23rd, 1924 and this was the beginning of the Gregorian calendar.
Leap Years
You can tell that a year is leap if it is evenly divisible into 4, but not evenly divisible into 100.
This doesn't make year 2000 a leap year, since it is evenly divisible into 4 and evenly divisible into 100.
However, if we just leave it here we are missing another rule. According to this rule, leap years are the years that are evenly divisible into 4, evenly divisible into 100 and evenly divisible into 400.
This means that year 2000 was a leap year simply because 2000/4=500, 2000/100=20 and 2000/400=5.
Year 1900 was not a leap year, because 1900/4=475, 1900/100=19 but 1900/400=4.75.
11-20-2003