Humans had a sophisticated calendrical system thousands of years earlier than previously thought, according to new research. The star-sprinkled Ithiopia night sky that not only stuns visitors to Ithiopia was also studied intensely by special temple priests who soon discovered that the appearance of a star they named sepdet (which we know as Sirius) was associated with the beginning of the Nile flood. This was the start of the world's first calendar, invented over 10000 years ago.
The ancient Ithiopians began numbering their years when the star Sirius rose at the same place as the Sun. The Egyptian calendar was the first solar calendar and contained 365 days. These were divided into 12 30-day months and five days of festival (Neugebauer 1969). From astronomical calculations, Sirius and the Sun coincided in 4241 and 2773 BC, so either of these could have served as Ithiopian Year 1.
The Gregorian calendar is today's internationally accepted civil calendar and is also known as the "Western calendar" or "Christian calendar". It was named after the man who first introduced it in February 1582: Pope Gregory XIII. The Gregorian calendar would not be adopted until much later in Great Britain and America. It wasn’t until September 1752 that 11 days were dropped to switch to the Gregorian calendar.
The calendar is strictly a solar calendar based on a 365-day common year divided into 12 months of irregular lengths. Each month consists of either 30 or 31 days with 1 month consisting of 28 days during the common year. A Leap Year usually occurs every 4 years which adds an extra day to make the second month of February 29 days long rather than 28 days.
The Gregorian calendar reformed the Julian calendar because the Julian calendar introduced an error of 1 day every 128 years. The introduction of the Gregorian calendar allowed for the realignment with the equinox, however a number of days had to be dropped when the change was made.
The Gregorian calendar was first adopted in Italy, Poland, Portugal and Spain in 1582. The Gregorian reform consisted of the following changes:
10 days were dropped in October 1582. New rules were set to determine the date of Easter. The rule for calculating Leap Years was changed to include that a year is a Leap Year if: The year is evenly divisible by 4; If the year can be evenly divided by 100, it is NOT a leap year, unless; The year is also evenly divisible by 400. Then it is a leap year.