Men did not begin counting years all at the same time. So while Westerners are in their 2009th year since the birth of Christ,(...) the Chinese are in the 4645th year, because they've been counting their years since the reign of the Emperor Quin Shi Huang Di.
Counting days is easy. Day and Night are the result of the Earth‘s rotation on its own axis. When one face is in darkness, at night, the other face is lit up by the Sun and it is daytime.
The lengths of months are tied to the moon’s cycle of phases. The time that passes between two full moons is about 29 and a half days.
The year itself corresponds to the length of one full revolution of the Earth around the Sun, which is on average 365 days and six hours.
Humans made different observations according to where on Earth they lived, So in North Africa, where the sky is often clear, the cycles of the Moon and stars are easier to observe. The use of a lunar calendar in these countries is easy. But in Europe the four seasons are very different, and a solar calendar, which better corresponds to the times of agricultural activity, was preferred.
This may explain the different kinds of calendars in the world.
As time passed, it was often necessary to make corrections when observed natural phenomena and the calendar dates that were supposed to correspond to them got out of sync.
Our current calendar comes from a Roman calendar called “Julian” which iitself came from an ancient Egyptian calendar.
The months of July and August both carry the names of Roman Emperors – Julius Caesar and Augustus. These two reformed the calendar by creating a leap year every four years. But this calendar was not yet completely satisfactory. In 1582, accumulated lags caused the Vernal Equinox (the first day of Spring) to be out of synchrony with its official calendar date by ten days, making it difficult to set the date of Easter. So Pope Gregory XIII imposed the cancellation of ten days. And to make sure this loss of synchrony didn’t happen again, the rules governing leap years were refined so as to cancel three leap years every four hundred years.
This new calendar was called the “Gregorian Calendar”. It is the calendar we use today, and the one most widely used around the world.
But the calendar still isn’t perfect. Did you know that an additional second was added at the end of 2008 in order to correct a new loss of synchrony?