Science behind leap years

At the time, such changes were considered controversial, but not nearly as controversial as the plan to put the calendar back into sync with the seasons.

The term ‘leap year’ didn’t come about until the 14th century. The ‘leap’ refers to the effect that leap days have on particular dates. For example, take any day, say March 9 — In 2014, it fell on a Sunday. In 2015, it was a Monday, but in 2016, it was Wednesday. Because 2016 had an extra day — February 29 —  it caused the example date to ‘leap over’ Tuesday. This works for any date, though dates in January and February will leap over a day in the year following a leap year, e.g. 2017.

At the time, such changes were considered controversial, but not nearly as controversial as the plan to put the calendar back into sync with the seasons. The Pope only had the authority to reform the calendar of Spain, Portugal, the Polish–Lithuanian Commonwealth and most of Italy.

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In those regions, the calendar was advanced by 10 days — Thursday, October 4, 1582 (of the Julian calendar), was followed by Friday, October 15, 1582 (of the Gregorian calendar).

Many Catholic countries and colonies soon followed, but several protestant nations objected to losing 10 days because they didn’t want to indicate fellowship with the Catholic Church. Some nations wouldn’t switch over another hundred or more years. The British Empire (including the American colonies) did not adopt the change until 1752. Japan finally adopted the Gregorian calendar in 1873 and Korea in 1895. Many Eastern European nations chose to opt out until the early 20th century. Greece, in 1923, was the last European country to change.

Today, the Gregorian calendar is accepted as an international standard, although several countries have not adopted it, including Afghanistan, Ethiopia, Iran, Nepal and Saudi Arabia.

Many countries use the Gregorian calendar alongside other calendars, and some use a modified Gregorian calendar. Some Orthodox churches use a revised Julian calendar, which results in them celebrating Christmas (December 25 in the Julian calendar) on January 7 in the Gregorian calendar.