Best Answer: The jump year (leap year) is a contrivance so that the calendar 12 months (usually 365 days) does not get too a long way away from the solar (astronomical) year. You say: huh? Well, the astronomical year - the time it takes the earth to go exactly once around the solar - is no longer precisely 365 days. The ancients estimated it as 365.25 days. That wasn't bad as calculations go; it's simply 365 days, 5 hours, forty-eight minutes, and 46 seconds.
Now, you may additionally suppose that crummy little fraction (almost 6 hours or 1/4 of a day) would not remember much. But every four years, the calendar would lose a full day towards the seasonal year. Christmas (Dec. 25) would start to come a little formerly every year. After about 20 years it would come before the iciness solstice; after 200 years or so, Christmas would come in the autumn (since the seasons are tied to the astronomical year because they rely on the earth's slant relative to the sun) . . . and then in the summer season . . . and . . . To stop this drift between the calendar 12 months and the astronomical (seasonal) year, we add one extra day each and every 4 years. Thus, over the four 12 months period, we have 1461 days, not 1460, for a common of 365.25 days per year. That quite a whole lot makes it come out right. This innovation was imposed in the year 709 AUC (ab urbe condita, after the founding of the city), when Julius Caesar regulated the calendar. Nowadays, we refer to it as forty-five BC. The Nicaean Council in 325 AD adopted that calendar for Christendom. But it still wasn't precisely right. As mentioned above, the astronomical yr is not 365 days 6 hours (365.25 days), it is 365 days 5 hours 48 minutes and forty-six seconds (365.2422 days). So as the calendar went alongside with its jolly add-a-day-every-four-years pattern, it won about 11 minutes 14 seconds each year. After every 128 years, that was a full day. Note it's going the other route - Christmas would fall LATER in the season every year. This anomaly used to be corrected by way of Pope Gregory in March 1582. By that time, the calendar 12 months used to be 10 days off the seasonal year. ( The real concern was now not Christmas, however, Easter which had to manifest near the vernal equinox and in accordance to the lunar cycle, but it truly is another story.) They made two corrections. The first was that they just dropped ten days. The day after October 5, 1582, grew to be October 15, 1582. (Some countries adopted this alternate later, in some cases centuries later.) This restored the equinox to its rightful place. The 2d exchange was once to reform the calendar to forestall slippage in the future; and we use that same calendar system today, known as the Gregorian. The Russian Orthodox Church nonetheless uses the Julian calendar. Christmas comes out about January 7 in their calendar. About each century, the Orthodox Christmas slips one greater day towards the solar calendar. Currently, there is a thirteen-day lag that by using 2100 will come to be a 14-day lag.) How does the Gregorian device work? We nonetheless have a jump year every 4 years, to accommodate the nearly 6-hour distinction that was once recognized in Julius Caesar's time. The Gregorian correction is that every hundred years, we make it NOT a soar year. Thus, 1700, 1800, and 1900 were not soared years, even-even though they would have been in the everyday 4 yr cycle. Thus, every a hundred years, there are 24 bounce years, now not 25. So that lets the calendar year common 365.24 days each year. Does that do it? Sadly, no. There are nonetheless these greater seconds - the astronomical year is 365.2422 days. So every 400 years, we DON'T NOT add the greater day (double poor intended). So 1700, 1800, 1900 were NOT leaped years, but 2000 was. If you have observed the math, that gets us very close. Over a 400 yr length, the calendar will include a common of 365.2425 days per year. Every 4,000 years (the first will be the year 4000, then 8000, etc.) we make the century years NOT jump years again. And that gives us an average of 365.24225 days per 12 months over a 4 yr period. Still no longer exact, however, the calendar year won't fluctuate by way of greater than a day from its modern location in the seasonal (astronomical) yr in two hundred centuries - shut sufficient for sensible purposes.
So the rule is:
except for the last year of each century, such as 1900, which is not a solar year . . .
except when the number of the century is a multiple of 4, such as 2000, which is a bounce year . . .
except for the year 4000 and its later multiples (8000, 12000, etc) which are not leaping years.
Conclusion:
