Gregorian
The Gregorian calendar is a solar calendar introduced by Pope Gregory XIII in 1582. It was designed as a reform of the Julian calendar, which had accumulated an error of approximately 10 days over the centuries due to its leap year rule. The reform consisted of skipping 10 days in October of 1582, and revising the leap year rule to more accurately reflect the length of a year.
Under the Julian calendar, leap years occurred every four years, resulting in an average year length of 365.25 days. However, the actual length of a solar year is approximately 365.2422 days, which means that the Julian calendar was accumulating an error of about 0.0078 days per year. This error added up over time, resulting in a significant discrepancy between the calendar and the actual position of the Sun in the sky.
To address this issue, the Gregorian calendar introduced a revised leap year rule. Under this rule, a leap year occurs in years that are divisible by 4, except for years that are divisible by 100 but not divisible by 400. This eliminates the leap year in centurial years (such as 1700, 1800, and 1900), but retains the leap year in years that are divisible by 400 (such as 1600 and 2000). This results in an average year length of 365.2425 days, which is much closer to the actual length of a solar year.
The Gregorian calendar was gradually adopted by Catholic countries, and then by other European countries over the following centuries. It was not adopted by Protestant countries until much later, with Great Britain and its colonies adopting it in 1752. The Gregorian calendar is now the most widely used calendar system in the world, and is the standard calendar in most countries.
One of the main advantages of the Gregorian calendar is its accuracy in reflecting the length of a solar year. This means that the date of the spring equinox (March 20 or 21) and the autumnal equinox (September 22 or 23) remain relatively stable from year to year. This stability is important for agricultural and astronomical purposes, as well as for the coordination of international events.
In conclusion, the Gregorian calendar is a widely used solar calendar that was introduced in 1582 as a reform of the Julian calendar. Its revised leap year rule makes it much more accurate than its predecessor, and it is now the standard calendar in most countries.