The
Month is taken from the cycles of the Moon phases, which takes about 29.5 days. The
Babylonians used 29 and 30 days alternately. The
Egyptians used all 30 day months, also the Greeks. The
Romans made the Julian calendar on one 28 day month and the others either 30 or 31. The month (which is
lunar) is not suitable for determining seasons, it has to be the
Sun. The Month is determined in two ways. First, the period taken to complete one orbit of the
Earth. Second, the time for the moon to complete a cycle of its phases, called the
Synodic month and is 29.53059 days, this is the basis of the calendar month.
In the year 46 BC the calendar was hopelessly confused. So Julius Caesar initiated a reform of the entire system. He appointed the Alexandrian astronomer Sosigenes to undertake the revision of the Calendar. Sosigenes did away with the lunar system and replaced it with the (tropical) solar year of 365.25 days. These changes resulted in the creation of the Julian calendar. Well now, this 365.25 days for the Julian calendar was off a bit, the tropical being 365.242199 days. The difference amounts to 11 minutes and 14 seconds per year. So.. by the year 1572 the calendar was in error by a full 10 days. Pope Gregory III issued a "papal bull" and the Jesuit astronomer Christopher Clavius went to work on this problem. The length of the year was redefined as 365.2422 days a difference of 0.0078 days per year from the Julian calendar. (we now have here a Gregorian calendar). This changed amount of error to 3.12 days every 400 years. Clavius had allowed for such discrepancy and suggested that three out of every four centennial years , which would ordinarily be leap years, should instead be regarded as common years. This lead to the practice that no centennial year could be a leap year unless it was divisible by 400. Following this rule 1700, 1800, and 1900 were common years, but the year 2000 would be a leap year. This Gregorian reform gives us an extremely accurate calendar system. The Gregorian calendar established January 1 as the beginning of the year and has been referred to as the "new style calendar" and the Julian referred to as the "old style calendar".