Why you have leap year?

Answer 1

We have a leap year because it takes 365.25 days for the Earth to make one orbit of the sun. As we do not count quarter days in the calender, every 4 years we get 1 day so we add it to February and make it 29th of Feb a leap year.

That's the short answer. The longer answer is that it takes very slightly more than 365.25 days (365.256363004 days, an error of about 9 minutes a year) for the Earth to revolve around the Sun. Using the rule of every fourth year being a leap year, this adds up to an error of about 3 days (2.545 days) in 400 years, so the full rule is that every year evenly divisible by 4 is a leap year, except that century years not evenly divided by 400 are not leap years. Example, 2000 was a leap year, but 2100, 2200, and 2300 will not be leap years.

This system, the Gregorian Calendar, was put into place in 1582 by Pope Gregory XIII in order to correct for the prior Julian Calendar, which assumed 365.25 exactly. The change also included dropping the ten extra days that had accumulated, realigning the Vernal Equinox with March 21st, and it adjusted the Lunar Calendar as well, because it was off by about 4 days at that time.

It is interesting to note that these observations and calculation were made in the 16th century, using (by today's standards) not very accurate measurements. It is also interesting to note that England did not fully agree with this, due to the dichotomy between the Roman Catholic Church and the Church of England, so they kept the leap years for 1700, 1800, and 1900, stepping into alignment in 1923.

Answer 2

A:

The actual amount of days in a year isn't exactly 365. It is closer to 365.25 days, which means that a quarter of a day is left over. Those left over quarters add up to be an entire day every four years, and to account for this, we have February 29.

A:

Actually, the average number of days in one year is about 365.2425. This makes every four years a leap year, except for century years not divisible by 400, i.e. 2100, 2200, and 2300 will not be leap years, but 2400 will be. The Julian calendar did not account for this, which made it in error by 3 days every 400 years.

Answer 3

I'm not all that fond of leap years, so I don't really miss them when they aren't around.

The purpose of leap years is to keep the calendar in synchronization with the solar year and the seasons. The "year" - one orbit of the Earth around the Sun - isn't an exact multiple of "days" - the time that it takes for the Earth to rotate once. The "year" is actually 365.26 days long. The orbital period of the Moon is also not neatly divisible by either days or years.

Some cultures do not sync the year to the seasons; the Islamic calendar, for example. A holiday that happens in summer one year will happen in spring a few years later, and in winter a few years after that.

But most, especially in agricultural societies, do or did. If the calendar does nothing else, it should at least offer some standardization on when to plant, and when to harvest.

Because the year is not a precise integer number of days long, we will need to have "leap" years, or "intercalendary days" to tweak the calendar into correspondence with the year.

The current Gregorian calendar of 365 days per year adds one "leap" day every 4 years. If the year were exactly 365.25 days long, this would keep everything accurate forever, but the year is actually a bit over that; 365.26 days. So once per century, in century years, the Gregorian calendar skips a leap day. Even that isn't QUITE perfect, so on years divisible by 400, we do have a "leap day".

So 1900 (a century year) was NOT a leap year, even though it was divisible by 4. 1996, 2000, and 2004 were all leap years. But the year 2100 will not be.