Modern calendar was decreed in circa 1582 (Published in Times of India in the first week of Januray 2010)
What is the Gregorian calendar?

The current international calendar and the world’s most widely used one is the Gregorian calendar, first proposed by Italian doctor Aloysius Lilius and decreed on Feb 24, 1582 by Pope Gregory XIII, after whom it was named. It was adopted to rectify the errors in the older Julian calendar. Due to its obvious connotations of Christianity, sometimes it’s used by replacing the traditional era notations AD and BC (Anno Domini and Before Christ) with CE and BCE (Common Era and Before Common Era).

What was the Julian calendar?

Julius Caesar introduced the Julian calendar in 45 BC. The Julian calendar was an improvement on the Roman calendar and was probably designed to approximate the tropical year, or the time taken by the Sun, as seen from the Earth, to return to the same position along its path. It has a regular year of 365 days divided into 12 months, and a leap day is added to February every four years. Hence the Julian year is on average 365.25 days. It was in common use in Europe until the 1500s, when countries in that region started changing to the Gregorian calendar. What was the leap year error? Although the new calendar was much simpler than the pre-Julian calendar, the algorithm for leap years was mistaken. It added a leap day every three years, instead of every four years. According to Roman grammarian and philosopher Macrobius, the error was the result of counting inclusively, so that the four-year cycle was considered as including both the first and fourth years. This resulted in too many leap days. Augustus remedied this discrepancy after 36 years by restoring the correct frequency. He also skipped several leap days in order to realign the year and it earned him a place in the calendar as the eighth month was named after him.

What long-term problems did the use of the Julian calendar cause?

On average, the astronomical solstices (the time when the Sun appears to reach its northernmost and southernmost extremes) and the equinoxes (when the Sun is positioned directly over the Earth’s equator) advance by about 11 minutes per year against the Julian year. As a result, the Julian calendar introduced an error of one day every 128 years. So every 128 years, the year shifted one day backward with respect to the calendar. By 1582, that meant that the world was running 10 days ahead of time. To get rid of inconvenience made by the Julian calender, the Gregorian calendar was introduced.

How are there 97 leap years in 400 years?

Every year divisible by 4 is a leap year, except for most of the years divisible by 100. Among the latter, every year divisible by 400 is a leap year. So, 1700, 1800, 1900, 2100, and 2200 are not leap years. But 1600, 2000, and 2400 are leap years.

Is there a 4,000-year rule?

It has been suggested by the astronomer John Herschel that a better approximation to the length of the tropical year would be 365.24225 days. This would mean 969 leap years every 4000 years, rather than the 970 leap years mandated by Gregorian calendar. This could be achieved by dropping one leap year from Gregorian calendar every 4000 years. This rule, however, has not been officially adopted.

How did countries shift to the Gregorian calendar?

Pope Gregory XIII decreed that 10 days should be dropped from October 1582 so that October 15, 1582 should follow immediately after October 4 of that year. Most catholic countries including Spain, Portugal and most of Italy soon adopted it. But Protestants were a bit reluctant. The British Empire and Sweden adopted it in 1752 and 1753 respectively. Russia, however, remained on the Julian calendar until 1917, after the Russian Revolution (which is thus called the ‘‘Oct Revolution’’ though it occurred in Gregorian Nov) and Greece continued to use it until 1923.

What are the other calendars?

Most Muslims use the Hijri calendar to determine the proper Islamic holy days. It is a lunar calendar with 354 days and hence a year is about 11 days shorter than the solar year. As a result, Islamic holy days usually shift 11 days earlier in the Gregorian year. Similarly, the Saka calendar, which is a lunisolar calendar, was adopted as the official calendar of India. In this calender, the year zero starts in the year 78 of the Christian era. To determine leap years, add 78 to the Saka year — if the result is a leap year in the Gregorian calendar, then the Saka year is also a leap year. The Persian calendar is used in countries such as Iran and Afghanistan.