ఒక [[కాలెండరు సంవత్సరం]]లో అదనంగా ఒక [[రోజు]] గానీ లేక ఒక [[నెల]] గాని అదనంగా ఉంటే, దానిని '''లీపు సంవత్సరం''' అంటారు. [[ఖగోళ సంవత్సరం]]తో, కాలెండరు సంవత్సరానికి వచ్చే తేడాను సరిచేయడానికి లీపు సంవత్సరాన్ని అమలుచేసారు. ఖగోళ సంవత్సరంలో ఘటనలు కచ్చితంగా ఒకే వ్యవధిలో పునరావృతం కావు. కాబట్టి ప్రతి ఏడూ ఒకే సంఖ్యలో రోజులుండే కాలెండరు, ఖగోళ ఘటనలను సరిగా ప్రతిఫలించక, ఏళ్ళు గడిచే కొద్దీ తేడాలు చూపిస్తూ ఉంటుంది. సంవత్సరానికి అదనంగా ఒక రోజునో లేక ఒక నెలనో చేర్చి ఈ తేడాను నివారించవచ్చు. లీపు సంవత్సరం కానిదానిని సాధారణ సంవత్సరం, లేదా మామూలు సంవత్సరం అంటారు.
ఈ గ్రాఫ్ పటములో సీజనల్ సంవత్సరానికి కేలండర్ సంవత్సరానికి తేడాను చూపబడింది.
Leap years (which keep the calendar in sync with the year) should not be confused with [[leap second]]s (which keep clock time in sync with the day).
The [[Gregorian calendar]], the current standard calendar in most of the world, adds a 29th day to [[February]] in all years evenly divisible by 4, except for century years (those ending in -00), which receive the extra day only if they are evenly divisible by 400. Thus 1996 was a leap year whereas 1999 was not, and 1600, 2000 and 2400 are leap years but 1700, 1800, 1900 and 2100 are not.
The reasoning behind this rule is as follows:
*The Gregorian calendar is designed to keep the [[vernal equinox]] on or close to [[March 21]], so that the date of [[Easter]] (celebrated on the Sunday after the 14th day of the Moon that falls on or after [[21 March]]) remains correct with respect to the vernal equinox.
*The vernal equinox year is currently about 365.242375 days long.
*The Gregorian leap year rule gives an average year length of 365.2425 days.
This difference of a little over 0.0001 days means that in around 8,000 years, the calendar will be about one day behind where it should be. But in 8,000 years' time the length of the vernal equinox year will have changed by an amount we can't accurately predict (see below). So the Gregorian leap year rule does a good enough job.
|[[బొమ్మ:Gregoriancalendarleap.png]]<BR>''This graph shows the variation between the seasonal year versus the calendar year due to unequally spaced 'leap days' rules. See [[Iranian _calendar#Calendar _seasonal error|Iranian calendar]] to contrast with a calendar based on 8 leap days ever 33 years.''
===Which day is the leap day?===
The Gregorian calendar is a modification of the Julian calendar first used by the Romans. The [[Roman calendar]] originated as a [[lunar calendar]] (though from the [[5th century BC]] it no longer followed the real moon) and named its days after three of the phases of the moon: the new moon (''calends'', hence "calendar"), the first quarter (''nones'') and the full moon (''ides''). Days were counted down (inclusively) to the next named day, so [[24 February]] was ''ante diem sextum calendas martii'' ("the sixth day before the calends of March").
Since [[45 BC]], February in a leap year had ''two'' days called "the sixth day before the calends of March". The extra day was originally the second of these, but since the [[third century]] it was the first. Hence the term '''bissextile day''' for [[24 February]] in a '''bissextile year'''.
Where this custom is followed, anniversaries after the inserted day are moved in leap years. For example, the former feast day of [[Saint Matthias]], [[24 February]] in ordinary years, would be [[25 February]] in leap years.
This historical nicety is, however, in the process of being discarded: The [[European Union]] declared that, starting in 2000, [[29 February]] rather than [[24 February]] would be leap day, and the [[Roman Catholic Church]] also now uses [[29 February]] as leap day. The only tangible difference is felt in countries that celebrate [[feast day]]s.
The [[Julian calendar]] adds an extra day to February in years divisible by 4.
This rule gives an average year length of 365.25 days. The excess of about 0.0076 days with respect to the [[vernal equinox year]] means that the vernal equinox moves a day earlier in the calendar every 130 years or so.
==Revised Julian Calendar==
The [[Revised Julian calendar]] adds an extra day to February in years divisible by 4, except for years divisible by 100 that do not leave a remainder of 200 or 600 when divided by 900. This rule agrees with the rule for the Gregorian calendar until 2799. The first year that dates in the Revised Julian calendar will not agree with the those in the Gregorian calendar will be 2800, because it will be a leap year in the Gregorian calendar but not in the Revised Julian calendar.
This rule gives an average year length of 365.242222… days. This is a very good approximation to the ''mean'' [[tropical year]], but because the ''vernal equinox'' tropical year is slightly longer, the Revised Julian calendar does not do as good a job as the Gregorian calendar of keeping the vernal equinox on or close to [[21 March]].
The Chinese calendar is [[lunisolar calendar|lunisolar]], so a leap year has an extra ''month'', often called an ''embolismic'' month after the Greek word for it. In the [[Chinese calendar]] the [[leap month]] is added according to a complicated rule, which ensures that month 11 is always the month that contains the northern winter [[solstice]]. The intercalary month takes the same number as the preceding month; for example, if it follows the second month then it is simply called "leap second month".
The Hebrew calendar is also [[lunisolar calendar|lunisolar]] with an embolistic month. In the [[Hebrew calendar]] the extra month is called ''Adar Alef'' ([[Adar_1|first Adar]]) and is added before ''[[Adar]]'', which then becomes Adar Sheni ([[Adar_2|second Adar]]). According to the [[Metonic cycle]], this is done seven times every nineteen years, specifically, in years, 3, 6, 8, 11, 14, 17, and 19.
In addition, the Hebrew calendar has postponement rules that postpone the start of the year by one or two days. The year before the postponement gets one or two extra days, and the year whose start is postponed loses one or two days. These postponement rules reduce the number of different combinations of year length and starting day of the week from 28 to 14, and regulate the location of certain religious holidays in relation to the Sabbath.
In the [[Hindu calendar]], which is a [[lunisolar calendar]], the embolismic month is called [[adhika maas]] (extra month). It is the month in which the sun is in the same sign of the stellar zodiac on two consecutive dark moons.
The [[Iranian calendar]] also has a single intercalated day once in every four years, but every 33 years or so the leap years will be five years apart instead of four years apart. The system used is more accurate and more complicated, and is based on the time of the March equinox as observed from [[Teheran]]. The 33-year period is not completely regular; every so often the 33-year cycle will be broken by a cycle of 29 or 37 years.
==Long term leap year rules==
The accumulated difference between the Gregorian calendar and the vernal equinoctial year amounts to 1 day in about 8,000 years. This suggests that the calendar needs to be improved by another refinement to the leap year rule: perhaps by avoiding leap years in years divisible by 8,000.
(The most common such proposal is to avoid leap years in years divisible by 4,000 [http://www.google.com/search?q=%22gregorian+calendar%22+error+%22leap+year%22+4000]. This is based on the difference between the Gregorian calendar and the ''mean tropical year''. Others claim, erroneously, that the Gregorian calendar itself already contains a refinement of this kind [http://www.straightdope.com/mailbag/mleapyr.html].)
However, there is little point in planning a calendar so far ahead because over a timescale of tens of thousands of years the number of days in a year will change for a number of reasons, most notably:
#[[Precession of the equinoxes]] moves the position of the vernal equinox with respect to [[perihelion]] and so changes the length of the vernal equinoctial year.
#[[Tidal acceleration]] from the sun and moon slows the rotation of the earth, making the day longer.
In particular, the second component of change depends on such things as [[post-glacial rebound]] and [[sea level rise]] due to [[climate change]]. We can't predict these changes accurately enough to be able to make a calendar that will be accurate to a day in tens of thousands of years.
There is a [[tradition]], said to go back to [[Saint Patrick]] and [[Brigid of Ireland|Saint Bridget]] in [[5th century]] [[Ireland]], whereby women may only make marriage proposals in leap years.
===Saint Patrick and the leap year===
:[[Saint Patrick]], having driven the frogs out of the bogs was walking along the shores of Lough Neagh, when he was accosted by Saint Bridget in tears, and was told that a mutiny had broken out in the nunnery over which she presided, the ladies claiming the right of proposing for marriage.
:Saint Patrick said he would concede them the right every seventh year, when Saint Bridget threw her arms round his neck, and exclaimed, "Arrah, Pathrick, jewel, I daurn't go back to the girls wid such a proposal. Make it one year in four." Saint Patrick replied, "Bridget, acushla, squeeze me that way again, an' I'll give ye leap-year, the longest of the lot." Saint Bridget, upon this, popped the question to St Patrick himself, who, of course, could not marry: so he patched up the difficulty as best he could with a kiss and a silk gown.
(Source: Evans, Ivor H, ''Brewer's Dictionary of Phrase and Fable'', Cassell, London, 1988)
According to a [] law in [[Scotland]], fines were levied if the proposal was refused by the man; compensation ranged from a kiss to a silk gown to soften the blow. Because men felt that put them at too great a risk, the tradition was in some places tightened to restricting female proposals to [[29 February]].
A person who was born on [[29 February]] may be called a "[[leapling]]". In non-leap years they usually celebrate their birthday on [[28 February]] or [[1 March]].
There are many instances in children's literature where a person's claim to be only a quarter of their actual age turns out be based on counting their leap-year birthdays. A similar device is used in the plot of the [[Gilbert and Sullivan]] [[operetta]] ''[[The Pirates of Penzance]]''.
== ఇవీ చూడండి ==