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Thirty Days Hath September", or "Thirty Days Has September", [1] is a traditional verse mnemonic used to remember the number of days in the months of the Julian and Gregorian calendars. It arose as an oral tradition and exists in many variants. It is currently earliest attested in English, but was and remains common throughout Europe as well. Full:
February 30 or 30 February is a date that does not occur on the Gregorian calendar, where the month of February contains only 28 days, or 29 days in a leap year. However, from a historical perspective February 30 has been used at least once and appears in some reform calendars. The thirteenth-century scholar Johannes de Sacrobosco claimed that ...
Since each lunation is approximately 29 + 1 ⁄ 2 days, [1] it is common for the months of a lunar calendar to alternate between 29 and 30 days. Since the period of 12 such lunations, a lunar year, is 354 days, 8 hours, 48 minutes, 34 seconds (354.36707 days), [1] purely lunar calendars are 11 to 12 days shorter than the solar year.
The ancient Athenian calendar was a lunisolar calendar with 354-day years, consisting of twelve months of alternating length of 29 or 30 days. To keep the calendar in line with the solar year of 365.242189 days, an extra, intercalary month was added in the years: 3, 6, 8, 11, 14, 17, 19 of the 19-years Metonic cycle.
The Gregorian calendar, like the Julian calendar, is a solar calendar with 12 months of 28–31 days each. The year in both calendars consists of 365 days, with a leap day being added to February in the leap years. The months and length of months in the Gregorian calendar are the same as for the Julian calendar.
The actual length of a synodic month varies from about 29 days 6 hours and 30 minutes (29.27 days) to about 29 days and 20 hours (29.83 days), a variation range of about 13 hours and 30 minutes. Accordingly, for convenience, the Hebrew calendar uses a long-term average month length, known as the molad interval , which equals the mean synodic ...
Since the Islamic calendar is a purely lunar calendar of about 354 or 355 days, its year count increases faster than that of solar and lunisolar calendars. S.H. (or SH) [citation needed] is used by the Iranian calendar to denote the number of solar years since the Hijra. The year beginning at the vernal equinox equals the number of the ...
It has now been 30 days since Flight 370 presumably crashed in the southern Indian Ocean; the minimum battery life of the ULBs on Flight 370's flight recorders. The manufacturer of the ULBs predicts the maximum battery life is about 40 days. [19]: 11 6–16 April