# Leap year

a calendar had historically the purpose to let the beginning of the seasons fall year by year on the same date in order to be able to determine the time for example from celebrations or recurring events. A tropical year, thus thatPeriod between two spring-day-and-night-same, takes however about 365 days and somewhat less than 6 hours (365d 5:48: 45.98, not constantly!), so that a simple allocation, which gives the same number of days to each year, is impossible, without itself at the same time thatBeginning of the seasons shifts. In the calendars of the different cultures there are therefore methods, in order to approximate the respective duration of the yearly the tropical year.

The beginning of the seasons can be held for example on the same date, if in suitable distancesone** leap year**, one year with an additional day, which **intercalary day** is inserted. A simple method is to raise each year with one smoothly by 4 divisible year to the leap year. Thus it was inserted in the Julian calendar, in thatFebruary 29 instead of 28 days has. More exact calendars bring the tropical year closer better on: Since somewhat less than six hours per year are to be adjusted, (Julian ones) the intercalary day falls sometimes out (like z. B. in the year 1900 in the Gregorian calendar).

Today the Gregorian calendar applies in nearly all parts of the world, in particular in Germany, Austria and Switzerland.

## Table of contents |

## Julian calendar

46 v. Chr. used the Romans a calendar with twelve moon months with alternating 29 each and/or. 30 days, together 354 days. Because straight numbersbut, struck the Romans still another one day brought misfortune drauf. But nevertheless were to the sun year still more than ten days too few. First Julius Caesar distributed these ten days on the different months, on now 30 and/or.31 days increased. Only the then last month February went out as a misfortune month empty and remained thus with 29 and/or in the leap year at that time at 30 days. The myth that February lost one day, because emperors Augustus itself in Augustwhen monthly designation wanted to perpetuate and February still another one day took away, since August like the July, which should have month of its predecessor Caesar, 31 days, and because a straight number brought also misfortune, becomes in the article the monthAugust contradicted. But the Romans had a tiny, but on the centuries seen, effective inaccuracy to leave. 11 minutes and 14 seconds per year the then Julian year shot beyond the goal. Every 128 years therefore one leap year became toomuch begun.

## Gregorian calendar

of Pope Gregor XIII. therefore 1582 the following regulation was introduced (Gregorian calendar):

- Every year, which by 4 without remainder are divisible, are leap years.
- Every year, which by 100 without remainder divisiblyare, are
*no*leap years. - Every year, which by 400 without remainder are divisible, are again leap years.

So for example the secular years are not 1800, 1900 and 2100 leap years, the secular years 2000 and 2400 however already. Thus the Gregorian lastsCalendar year on the average 365.2425 days, and agrees more exact with the length of the tropical yearly than the Julian calendar year.

Since the first council of Nicäa in the year 325, with that the spring beginning on the 21. March was specified, amounted tothe accumulated difference already 10 days, and left Gregor on the 4 in such a way. October equal the 15. Octobers 1582 follow, in order to adjust this time.

Not in all countries this conversion took place on the same day. There were special difficulties in Sweden, where in the year 1712* two* intercalary days were inserted and thus this year 30. February had.

Quite unusually and little admits is the fact that not the 29. February **the intercalary day** of the Gregorian calendar is. Rather that becomes24. February doubles: the 24. and the 25. February fall after Roman counting method, which was still usual also during the introduction of the Gregorian calendar, on the same date. In the common year is called the 24. February “A. D. VI Kal. March. “(sixth daybefore beginning of March); the double day in the leap year is called likewise. There this sixth (lat. „sextus “) doubly (“to”) is counted, is called leap year and/or. - day “bissextilis” (Latin), “bissextile” (English), “bisestile” (Italian) etc. After today's counting shift the 24. and the furtherDays in February around one day; Meaning has this today still in the catholic church with the counting of the church conferences: church holidays and also the name days are affected by this regulation.

### the switching rules

- after that in principle furtheryear, whose numbers by 4 divide natural numbers result in, points valid Julian switching rule in February 29. Up and thereby 366 days cover day (“intercalary day “) as leap years.

- The Gregorian switching rule determines of it an exception:
*No*leap years are thoseSäkular-years (years, whose number at the end exhibits two zeros), whose number by 400 divide*no*natural number results in. (Therefore the years 1700, 1800 and 1900 were not leap years, and also the years 2100, 2200 and 2300*will not be*leap years.)

or:

- If the year is divisible by 4, but not by 100, then it is one leap year with 366 days. Examples: 1980, 1972, 1720.
- If the year is divisible by 100, but not by 400, then the year is a usual common yearand have only 365 days, z. B. in the years 1700, 1800 and 1900 or furthermore 2100.
- If the year is divisible by 400, the year is one leap year. The years 1600 and 2000 were - in agreement with thatJulian switching rule - leap years to 366 days.

or

rule which can be noticed simply:

- One leap year is every four years (1992 and 1996 e.g. were. Leap years)
- every hundred years not (1700 and 1800 were not leap years)
- and every four hundred years nevertheless (2000nevertheless leap year was)

## orthodox church calendar

the Gregorian year is approximately 27 seconds too long. As result the Gregorian calendar limps after 1,600 years around a half day afterwards. Therefore the Greek-orthodox church struck a somewhat more complicated switching ruleforwards: From the Gregorian calendar the centuries are deviating leap year only if they result in the remainder of 2 or 6 divided by 9. Thus the year 2800 would not be a leap year, but only the year 2900. The deviation to the sun year amounts to with orthodox church calendar only scarcely 3 seconds.

## French revolution calendar

in the time, in which the French revolution calendar had validity, were the years 3, 7 and 11 leap years.

The original regulation of the calendar was that the first dayeach yearly on the autumn Tag-und-Nacht-Gleiche and each fourth year falls one leap year to be is. These demands contradict themselves however, and so Charles Rommes, the author of the calendar system suggested, the Gregorian calendar similar a regulation. It pushed howeverfor political reasons thereby on resistance. The discussions were however generic term-oiled by the short validity of the revolution calendar.

## Jewish calendar

in the Jewish calendar, a calendar aligned after the moon, which is adapted to the sun year by complicated regulations,becomes in all years, which during a division by 19 a remainder inserted of 0, 3, 6, 8, 11, 14, or 17 to have, one switching month. It is called* Adar* II.

## Islamic calendar

in the Islamic calendar is the determination of one leap year extremely with difficulty. After a common system, which is used among other things in the GNU Emacs, every year, those are with a division by 30 a remainder of 2, 5, 7, 10, 13, 16, 18, 21, 24, 26,or 29 has, leap years.

After this system have in the Islamic calendar all even-numbered months 30 days and all odd-number months 29 days. In leap years the last odd-number month one day is added, so that it has then 30 days.

## calendar if leap years

in the traditional work on, 13 months have lunisolaren Chinese calendar instead of the usual twelve; with 383, 384 or 385 days instead of 353, 354 or 355 days in normal years.

Among the computation one ranks the number of Neumondebetween the elften month of a yearly (the month of the winter sun turn) and the elften month of the following yearly. If 13 Neumonde fall into this period, then one switching month is inserted. The first month, which does not contain *zhong* qì, receives the same numberlike the previous month with an additive as a switching month.

## Aztec calendars

in the Aztec calendar are switched on at the end of each 52-Jahres-Zyklus 13 days. From it average yearly length of 365 days and 6 hours results. That yields from the tropical yearmore than 11 minutes off. The accuracy of this leap year method is comparable with the Julian calendar.

## atomic time

today, in the age of the atomic time measurement, must be inserted occasionally one switching second, in order to adapt the time the middle Sonnenzeit.

See also:* Summer time, year, common year*