| A calendar is a system for reckoning the passing of time. The principal problem in drawing up calendars arises from the fact that the solar day, the lunar month and the tropical year – the most immediate natural time units- are not simple multiples of each other. In practice a solution is found in basing the system either on the phases of the Moon (lunar calendar) or on changing of the seasons (solar calendar). The difficulty that days eventually get out of step with the Moon or the seasons is got over by adding in (intercalating) one or more extra days or months at regular intervals in an extended cycle of months or years. |
| The earliest Egyptian calendar had a year of 12 months with 30 days each, though later 5 extra days were added at the end of each year so that it approximated the tropical year of 365.25 days. In classical times, the Greeks came to use a lunar calendar in which 3 extra months were intercalated every eight years (the octannial cycle). Through about 432BC the astronomer Meton discovered that 235 lunar months fitted exactly into 19 years (Metonic cycle), this becoming the basis of the modern Jewish and ecclesiastical calendars. |
| The Roman calendar was reformed under Julius Caesar in 46BC fixing the year at 365 days but intercalating an additional day every fourth year (this giving an average 365.25 day-year). The Christians, who called it Anno Domini, adopted this Julian calendar and it continued in use until 16th century when it had become about 10days out of step with the seasons, the tropical year in fact being less than 365.25 day-years. |
| In 1582 AD Pope Gregory 13 ordered to omit 10 days from that year and not to give a leap year for century years unless divisible by 400 so that there would be no recurrence of any discrepancy in future. But since the error of one day occurs in |
| about 127 years, this adjustment will not make it fully error-free. This calendar was called the Gregorian calendar. |
| This Gregorian calendar was only slowly adopted particularly in non- catholic countries, the reform waiting until 1752 AD in England and its American colonies by which time 11 days had to be dropped. But in spite of all these defects it is still in civil use throughout the world even today. Various proposals for further reforms of this calendar have come to nothing. |