# why do we have leap years every 4 years

Nearly every four years, we add an extra day to the calendar in the form of February 29, also known as Leap Day. Put simply, these additional 24 hours are built into the calendar to ensure that it stays in line with the EarthБs movement around the Sun. While the modern calendar contains 365 days, the actual time it takes for Earth to orbit its star is slightly longerБroughly 365. 2421 days. The difference might seem negligible, but over decades and centuries that missing quarter of a day per year can add up. To ensure consistency with the true astronomical year, it is necessary to periodically add in an extra day to make up the lost time and get the calendar back in synch with the heavens. The Egyptians were among the first to calculate the need for a leap year, but the practice didnБt arrive in Europe until the reign of the Roman dictator Julius Caesar. Before then, the Roman calendar had operated on a muddled lunar model that regularly required adding an extra month to maintain celestial consistency.

Finally, in 46 B. C. , Caesar and the astronomer Sosigenes revamped the Roman calendar to include 12 months and 365 days. This БJulian CalendarБ also accounted for the slightly longer solar year by adding a leap day every four years. CaesarБs model helped realign the Roman calendar, but it had one small problem. Since the solar year is only. 242 days longer than the calendar year and not an even. 25, adding a leap year every four years actually leaves an annual surplus of roughly 11 minutes. This minute discrepancy meant that the Julian Calendar drifted off course by one day every 128 years, and by the 14th century it had strayed 10 days off the solar year. To fix the glitch, Pope Gregory XIII instituted a revised БGregorian CalendarБ in 1582. In this model, leap years occur ever four years except for years evenly divisible by 100 and not by 400.

For example, the year 1900 was not a leap year because it was divisible by 100, but not 400. The PopeБs updated calendar remains in use to this day, but itБs still not perfectБexperts note that the remaining discrepancies will need to be addressed in around 10,000 years.

In the, a year ending in "00" that is by 400 is a century, with the of yielding 366 days instead of 365. Century years (divisible by 100) that are not divisible by 400 are (with 365 days) and not leap years. For example, the years 1600, 2000, and 2400 are century leap years since those numbers are divisible by 400, while 1700, 1800, 1900, 2100, 2200, and 2300 are common years despite being divisible by 4. Leap years divisible by 400 always ; thus the leap day February 29 in those years always falls on a Tuesday ( BA). The Gregorian calendar yields an average year that currently tracks the annual revolution period of the Earth more closely than the older, in which every fourth year (including end-of-century years) is a leap year.

The Julian formula adds too many leap days (3 every 400 years), causing the Julian calendar to drift gradually with respect to the astronomical seasons. Over time, natural events such as the spring equinox began to occur earlier and earlier in the Julian calendar. The Gregorian calendar was introduced in 1582, but was adopted by various countries over several centuries, with the result that some countries still used the older Julian calendar while others used the Gregorian calendar. Dates prior to 1582 are generally calculated using the Julian calendar, and different countries have different conventions about dates between 1582 and their adoption of the Gregorian calendar. (See, for example,. )

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