This is a brief History and comparison between the most commonly used calendars.
Named after Pope Gregory XIII (read more about him here)who initiated it in 1582, the calendar is presently the most used in the world. A leap year is every year divisible by four. Centurial years divisible by 400 are leap years. For example, 2000 is a leap year while 1800 and 1900 are not.
The calendar was a correction of the Julian calendar. It shortened the average year by 0.0075 days to offset the drift of the calendar with the equinoxes. The change was first taken in by the Catholic countries of Europe. Three centuries later, the Eastern Orthodox countries and the protestant countries also adapted to this refined calendar. In 1923, Greece was the last European to accept the Gregorian calendar. Up to date, most non-Western countries have embraced the calendar for civil purposes.
Julius Caesar designed the Julian calendar in 46 BC. It was an improvement of the Roman calendar. On January 45 BC, it was the official calendar. It was the popular calendar of the Roman world, especially in Europe, in the Americas where there were European settlements and in other parts as well.
Pope Gregory replaced the Julian calendar with the more refined, Gregorian calendar. The latter was promulgated in 1582. However, it is used by the Berbers, in parts of the Eastern Orthodox Church, Anabaptist, and in the Eastern Orthodox Church.
In the 20th and 21st centuries, the dates in the Julian calendar are 13 days behind that of the Gregorian date. The Julian calendar has the ‘normal’ years and the ‘leap’ years consisting of 365 days and 366 days respectively. There is a cycle of three ‘normal’ years before a leap year. On average, the Julian year is 365.25 days long.
The Julian year is 10.8 minutes longer than the Gregorian year. The reason behind this is, there is a 0.002% difference between the two with Gregorian having 365.2425 days and Julian having 365.25 days.
Also known as the Jewish calendar, the Lunisolar calendar is widely used for Jewish religious practices. In addition, it determines the schedule for Jewish holidays and for the suitable public readings of the Torah, regular Psalm readings, and other ceremonial uses. It is also used for religious purposes, agricultural planning, and an official calendar for civil purposes in Israel. The use of the Hebrew calendar as a time frame for agriculture is on the decline since there is a shift to the Gregorian calendar.
The current Hebrew calendar evolved from Babylonian influence. The calendar considered a new crescent moon, with a new month commonly added after two or three years to rectify the difference between twelve lunar months together with the solar year.
The average Hebrew calendar is longer than the present mean tropical year by 6 minutes and 40 seconds. As a result, every 217 years of the Hebrew calendar will lag by a day when compared to the current mean tropical year; and almost every 238 years will be a day behind the mean Gregorian calendar year.
The Jewish holidays look like they shift around with respect to the Western – Gregorian, calendar. It is because the western calendar is solar while the Jewish calendar is lunar.