Intercalation is the insertion of a leap day, week or month into some calendar years to make the calendar follow the seasons or moon phases. Lunisolar calendars may require intercalations of both days and months. The solar year does not have a whole number of days (it's about 365.24 days), but a calendar year must have a whole number of days. The only way to reconcile the two is to vary the number of days in the calendar year.

In solar calendars, this is often done by adding to a common year of 365 days, an extra day (leap day or intercalary day) about every four years: this makes a leap year of 366 days.

The Decree of Canopus, which was issued by the pharaoh Ptolemy III, Euergetes of Ancient Egypt in 239 BC, decreed a solar leap day system.

In the Julian Calendar as well as in the Gregorian Calendar that improved it, intercalation is done by adding an extra day to February in each leap year. In the Julian Calendar this was done every four years. In the Gregorian calendar years whose number is evenly divisible by 100 but not 400, were exempted in order to improve accuracy. Thus, 2000 was the first century year in 400 years to be a leap year.

The solar year does not have a whole number of lunar months (it is about 12.37 lunations), so a lunisolar calendar must have a variable number of months in a year. This is usually 12 months, but a 13th month (an intercalary or embolismic month) is added to the year every two or three years.

ISO 8601 includes a specification for a 52-week year. Any year that has 53 Thursdays has 53 weeks; this extra week may be regarded as intercalary.

The determination of whether a year has intercalation may be calculated (Julian, Gregorian and Hebrew calendars), or determined by observation (Iranian calendar).

ee also

*Bahá'í calendar and Ayyám-i-Há
*Chinese calendar
*Hebrew calendar
*Hindu calendar
*Islamic prohibition of intercalation
*Iranian calendar
*Thai lunar calendar
*Bengali calendar

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  • intercalation — [ ɛ̃tɛrkalasjɔ̃ ] n. f. • XVe; lat. intercalatio 1 ♦ Rare Action d intercaler. 2 ♦ Didact. Addition d un jour au mois de février, les années bissextiles. 3 ♦ Math. ⇒ insertion. Intercalation de termes intermédiaires dans une série. ⇒… …   Encyclopédie Universelle

  • Intercalation — In*ter ca*la tion, n. [L. intercalatio: cf. F. intercalation.] [1913 Webster] 1. (Chron.) The insertion of a day, or other portion of time, in a calendar. [1913 Webster] 2. The insertion or introduction of anything among others, as the insertion… …   The Collaborative International Dictionary of English

  • intercalation — Intercalation. s. f. v. Addition d un jour dans le mois de Février aux années bissextiles. L année qu on fait l intercalation, le mois de Février a vingt neuf jours …   Dictionnaire de l'Académie française

  • intercalation — intercalation. См. интеркаляция. (Источник: «Англо русский толковый словарь генетических терминов». Арефьев В.А., Лисовенко Л.А., Москва: Изд во ВНИРО, 1995 г.) …   Молекулярная биология и генетика. Толковый словарь.

  • intercalation — index insertion, intervention (interference) Burton s Legal Thesaurus. William C. Burton. 2006 …   Law dictionary

  • intercalation — 1570s, from L. intercalationem (nom. intercalatio) insertion of an intercalary day, noun of action from pp. stem of intercalare (see INTERCALATE (Cf. intercalate)) …   Etymology dictionary

  • intercalation —  Intercalation  Интеркаляция   Обратимое включение молекулы или группы между другими молекулами или группами …   Толковый англо-русский словарь по нанотехнологии. - М.

  • Intercalation — Cette page d’homonymie répertorie les différents sujets et articles partageant un même nom. Sur les autres projets Wikimedia : « Intercalation », sur le Wiktionnaire (dictionnaire universel) Le mot intercalation est employé dans… …   Wikipédia en Français

  • intercalation — (in tèr ka la sion ; en vers, de six syllabes) s. f. 1°   Action d intercaler ; résultat de cette action.    Proprement, l addition d un jour dans le mois de février, aux années bissextiles. L année où l on fait l intercalation, le mois de… …   Dictionnaire de la Langue Française d'Émile Littré

  • INTERCALATION — s. f. Action d intercaler, ou Le résultat de cette action. Il se dit, proprement, de L addition d un jour dans le mois de février, aux années bissextiles. L année où l on fait l intercalation, le mois de février a vingt neuf jours.   Il se dit,… …   Dictionnaire de l'Academie Francaise, 7eme edition (1835)

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