What's the point of leap year?

Answer 1

A leap year (or intercalary year) is a year containing an extra day (or, in case of lunisolar calendars, an extra month) in order to keep the calendar year synchronised with the astronomical or seasonal year. For example, February would have 29 days instead of just 28. Seasons and astronomical events do not repeat at an exact number of days, so a calendar which had the same number of days in each year would over time drift with respect to the event it was supposed to track. By occasionally inserting (or intercalating) an additional day or month into the year, the drift can be corrected. A year which is not a leap year is called a common year.

Leap year rules:
In order to get a closer approximation, it was decided to have a leap day 97 years out of 400 rather than once every four years. To implement the model, it was provided that years divisible by 100 would be leap years only if they were divisible by 400 as well. [2] [3] So, in the last millennium, 1600 and 2000 were leap years, but 1700, 1800 and 1900 were not. In this millennium, 2100, 2200, 2300, 2500, 2600, 2700, 2900 and 3000 will not be leap years, but 2400 and 2800 will be. The years that are divisible by 100 but not 400 are known as "exceptional common years". By this rule, the average number of days per year will be 365 + 1/4 - 1/100 + 1/400 = 365.2425.

Leap year algorithms:
Standard:

if year modulo 400 is 0 then leap
else if year modulo 100 is 0 then no_leap
else if year modulo 4 is 0 then leap
else no_leap
Vectorized:

mask400 = year modulo 400 EQ 0 ; this is a leap year
mask100 = year modulo 100 EQ 0 ; these are not leap years
mask4 = year modulo 4 EQ 0 ; this is a leap year
return mask4 and (~mask100 or mask400)
where ~ is the bitwise logical NOT operator

C++:

bool isLeapYear(int pY)
// Given: some year, pY
// Returns true if pY is a leap year, false if it is not
{
bool result=false;
if (pY % 400 == 0) result = true;
else if (pY % 100 == 0) result = false;
else if (pY % 4 == 0) result = true;
else result = false;
return result;
} // isLeapYear()

Leap day:
The Gregorian calendar is a modification of the Julian calendar first used by the Romans. The Roman calendar originated as a lunisolar calendar and named many of its days after the syzygies of the moon: the new moon (Kalendae or calends, hence "calendar") and the full moon (Idus or ides). The Nonae or nones was not the first quarter moon but was exactly one nundinae or Roman market week of nine days before the ides, inclusively counting the ides as the first of those nine days. In 1825, Ideler believed that the lunisolar calendar was abandoned about 450 BC by the decemvirs, who implemented the Roman Republican calendar, used until 46 BC. The days of these calendars were counted down (inclusively) to the next named day, so 24 February was ante diem sextum Kalendas Martii ("the sixth day before the calends of March") often abbreviated a. d. VI Kal. Mar. The Romans counted days inclusively in their calendars, so this was actually the fifth day before March 1 when counted in the modern exclusive manner (not including the starting day).

The Republican calendar's intercalary month was inserted immediately after Terminalia (a. d. VII Kal. Mar., February 23) or immediately after Regifugium (a. d. VI Kal. Mar., February 24). This intercalary month, named Intercalaris or Mercedonius, contained 27 days, 22 additional days to which the last five days of February were added. Because only 22 or 23 days were effectively added, not a full lunation, the calends and ides of the Roman Republican calendar were no longer associated with the new moon and full moon.

When Julius Caesar developed the Julian calendar in 46 BC, becoming effective in 45 BC, in addition to distributing an extra ten days among the months of the Roman Republican calendar he replaced the intercalary month by a single intercalary day, located where the intercalary month used to be. To create the intercalary day, the existing ante diem sextum Kalendas Martii (February 24) was doubled, producing ante diem bis sextum Kalendas Martii. Hence, the year containing the doubled day was a bissextile (bis sextum, "twice sixth") year. Originally, the first was regarded as bis sextum, the intercalary day, but in 238 Censorinus stated that the intercalary day was followed by the last five days of February, a. d. VI, V, IV, III and pridie Kal. Mar. (which would be those days numbered 24, 25, 26, 27, and 28 from the beginning of February in a common year), hence he regarded the bissextum as the first half of the doubled day. All later writers, including Macrobius about 430, Bede in 725, and other medieval computists (calculators of Easter), continued to state that the bissextum (bissextile day) occurred before the last five days of February.

Until 1970, the Roman Catholic Church always celebrated the feast of Saint Matthias on a. d. VI Kal. Mar., so if the days were numbered from the beginning of the month, it was named February 24 in common years, but the presence of the bissextum in a bissextile year immediately before a. d. VI Kal. Mar. shifted the latter day to February 25 in leap years. The practice is still observed by those who use the older calendars.

Answer 2

The time for the earth to orbit the sun, measured from spring equinox to spring equinox, is something like 365.22424 days, and it is the fraction that is the gotcha. To keep the seasons from moving, one would like the calendar to match the actual length of the year as closely as possible, but no one is interested in a calendar with fractional days. Since this is approximately 365.25 days, you can keep things fairly well in step if you add a day every four years, and this started being done in Roman times. By the 1500's, the error had accumulated to something like eleven days, and Pope Gregory's astronomers proposed tweaking the system to make century year not be leap years unless the year were divisible by 400. So, 2000 was a leap year, and 2096 will be also, but 2100 won't. Most of the world adopted the new scheme when it was proposed, but Britain was on the outs with the Catholic Church at the time and didn't fall into line until about 1751. The American colonies followed suit. The previous responder's remarks about the moon are not correct.

Answer 3

'rhsaunde....' - congrats!!!
You stuck to the point, gave some seriously good info, and if you would stay away from 'binary' math-arithmatic or whatever it was(unless you're required to use it literally), and stop 'tweaking' answers with tidbits about the relative shorter(miniscule) actual distance travelled using 2 miles at the 45th parallel as an example(the odometer doesn't have a mind of it's own either!!), when the poor guy simply asked in the 'vernacular'(simple-speak) what was all the fuss over 1+1=2 possibly not being true(so-to-speak) and he was at least curious. Later he stated that he didn't want any part of 'binary' or any other tricks! I even gave you a thumbs up for this one! I still can't figure out(ya I've got a bit of insomnia this am, by the way - it's 4:45am here,but I'm 62, retired and can sleep my brains out this am to make up for it) how you can amass a gazillion points, yet 'only' have 11% 'best answers'? With your persistence and obvious deducing powers, you could do better than that! G**Z, as I said(in the vernacular), you must be strapped to the comp. seat!! The best - Mark B.

My apologies to the asker - I had to digress to this - many of your answers are bang on - no need for me to be redundant. I just HAD to take the opportunity to give praise where due as opposed to the stinging I gave this gent on another question. Thanks for the space. The 2 points means nothing to me!

Answer 4

A leap year is a year in which an extra day is added to the calendar in order to synchronize it with the seasons. Since the tropical year is 365.242190 days long, a leap year must be added roughly once every four years (four times the fractional day gives ). In a leap year, the extra day (known as a leap day) is added at the end of February, giving it 29 instead of the usual 28 days.

Answer 5

One year is the time period taken by our earth to complete its one rotation around the Sun. This period is approximately equal to 365 +1/4 days. But our calender is only for 365 days. To compensate 1/4 day per year = 4 × 1/4 = 1 day adjusted after every 4 years. and in that year a year is calculated for 366 days by taking 1 day extra in the month of February as 29 February.
Leap is the year in which the number indicating year is exactly divisible by 4.
But in case of century years i. e. number ending with two zeroes must be divisible by 400.
Years 1500, 1700, 2100 are not leap years as these numbers are not divisible by 400.

Answer 6

leap year is when a year has 366 days . Earth completes a revolution in 365 1/4 days than after 4 revolution 1/4 is multiplied in 4 than it gets 1day more .Then we add it as 29 feb

Answer 7

earth completes its one round around the sun not in 365 days but in 365.25 days, so four .25 days make full one day and that full one day is added after every four year to give a leap year having 366 days.

Answer 8

fake funds she lives in like the southern factor of the international the region its wintry climate in july and summer season in December The yolk of the egg is yellow A outdoor stick A adventure Fishing line secret Gloves A sponge N, elementary, its the 1st letter of each and each month beginning up from march to November so it may be N for November.

Answer 9

Simply put. It's to do with the cycle of the the moon. The start to the full moon to the starting point and from that point on usually takes 365 days, once in a while it takes shorter so we drop a day. The moon's cycle varies every year that is why the Chinese New Year, Easter etc that is based on the moon calendar is different every year.

Answer 10

The earth rotates around the sun every 365.25 days, so every 4th year, we make up for the accumulated quarter-days.