What is Gregorian Calendar?
A Human’s urge to make astronomical observations and record these to make exact calculations of time led to making of calendars. The Lunar calendar was the first to get formalized. It was based on the phases of the Moon. But, it was proved inaccurate and required many corrections. This led to observing the Sun’s movements and making of a solar based calendar. The Julian Year was introduced by Julius Caesar in 45 B.C.E. and had 365.25 days in a year and had a leap year too. This was based on the astronomical observations of Sun’s position with respect to the Earth. But, accurate measurement of 365.24219879 days as length of a year created inaccuracies in the Julian calendar. The calendar period did not match with seasons by the time it was 16th Century A. D.
In 1576 A.D. Pope Gregory XIII composed a commission of mathematicians, astronomers and priests who adopted the suggestions of Luigi Lilio. To make corrections for the slippages, the Pope announced that October 4, 1582 which was a Thursday would be followed by October 15, 1582 as a Friday. This created a 11 days corrections to slippages and everyone gradually adopted to this calendar, known as Gregorian calendar. Today, we follow this calendar the world over.
But, small errors persist even now and by 13000 A.D. (much after our lives), further corrections would be required to this calendar. By then the slippage will be of 10 days. The speed of Earth’s rotation around the Sun and that of its spinning around its own axis is slowly decreasing. Taking this into account, the present calendar will have to make corrections by subtracting one day after every 2000 years.
We do not know what other factors will come into being as the Human’s quest to get more information about the Galactic movements and Universal phenomenon may lead to newer findings. But, as of now, we are following the Gregorian calendar.