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96-407: A blue moon is an astronomical phenomenon. Blue Moon may also refer to: Blue moon The calendrical meaning of "blue moon" is unconnected to the other meanings. It is often referred to as “traditional”, but since no occurrences are known prior to 1937 it is better described as an invented tradition or “modern American folklore”. The practice of designating the second full moon in

192-399: A Pruett blue moon (as occurred on December 31, 2009 in time zones west of UTC+05) is after one Metonic cycle, in 2028 in time zones west of UTC+08. At that time there will be a total lunar eclipse . Date of Easter As a moveable feast , the date of Easter is determined in each year through a calculation known as computus ( Latin for 'computation'). Easter

288-501: A century year). This is a correction to the length of the tropical year, but should have no effect on the Metonic relation between years and lunations. Therefore, the epact is compensated for this (partially – see epact ) by subtracting one in these century years. This is the so-called solar correction or "solar equation" ("equation" being used in its medieval sense of "correction"). However, 19 uncorrected Julian years are

384-475: A day earlier than it would normally be, in order to keep Easter before April 26, as explained below. In the year 2100, the difference will increase by another day. The epacts are used to find the dates of the new moon in the following way: Write down a table of all 365 days of the year (the leap day is ignored). Then label all dates with a Roman numeral counting downwards, from "*" (0 or 30), "xxix" (29), down to "i" (1), starting from 1 January, and repeat this to

480-556: A different date from that of the Eastern and Oriental Orthodoxy (which follow the Julian calendar). It was the drift of 21 March from the observed equinox that led to the Gregorian reform of the calendar , to bring them back into line. Easter commemorates the resurrection of Jesus , which Christians believe to have occurred on the third day (inclusive) after the beginning of Passover . In

576-466: A leap month every two or three years, before the lunar new year on 1  Nisan . Later Jews adopted the Metonic cycle to predict future intercalations . A possible consequence of this intercalation is that 14 Nisan could occur before the equinox, which some third-century Christians considered unacceptable (this cannot happen in the fixed calendar in use today). Consequently, it was decided to separate

672-557: A little longer than 235 lunations. The difference accumulates to one day in about 308 years, or 0.00324 days per year. In one cycle, the epact decreases due to the solar correction by 19 × 0.0075 = 0.1425 on average, so a cycle is equivalent to 235−0.1425/30 = 234.99525 months, whereas there are actually 19 × 365.2425 / 29.5305889 ≈ 234.997261 synodic months. The difference of 0.002011 synodic months per 19-year cycle, or 0.003126 days per year, necessitates an occasional lunar correction to

768-436: A month as "blue" originated with amateur astronomer James Hugh Pruett in 1946. It does not come from Native American lunar tradition , as is sometimes supposed. The moon - not necessarily full - can sometimes appear blue due to atmospheric emissions from large forest fires or volcanoes, though the phenomenon is rare and unpredictable (hence the saying “once in a blue moon”). A calendrical blue moon (by Pruett's definition)

864-504: A month, so I interpret it, was called Blue Moon”. In 1980 the term was used (with Pruett’s definition) in a U.S. radio program, Star Date , and in 1985 it appeared in a U.S. children’s book, The Kids' World Almanac of Records and Facts (“What is a blue moon? When there are two full moons in a month, the second one is called a blue moon. It is a rare occurrence.”) In 1986 it was included as a question in Trivial Pursuit (likely taken from

960-442: A multiple of 30. This is a problem if compensation is only done by adding months of 30 days. So after 19 years, the epact must be corrected by one day for the cycle to repeat. This is the so-called saltus lunae ("leap of the moon"). The Julian calendar handles it by reducing the length of the lunar month that begins on 1 July in the last year of the cycle to 29 days. This makes three successive 29-day months. The saltus and

1056-456: A new cycle. At the time of the reform, the epacts were changed by 7, even though 10 days were skipped, in order to make a three-day correction to the timing of the new moons. The solar and lunar corrections work in opposite directions, and in some century years (for example, 1800 and 2100) they cancel each other. The result is that the Gregorian lunar calendar uses an epact table that is valid for

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1152-423: A period of from 100 to 300 years. The epact table listed above is valid for the 20th, 21st and 22nd century. As explained below, the dates of Easter repeat after 5.7 million years, and over this period the average length of an ecclesiastical month is 2,081,882,250/70,499,183 ≈ 29.5305869 days, which differs from the current actual mean lunation length (29,5305889 d: see Lunar month#Synodic month ) in

1248-558: A question in the Trivial Pursuit game in 1986. Several songs have been titled "Blue Moon", seen as a "symbol of sadness and loneliness." The moon (and sun ) can appear blue under certain atmospheric conditions – for instance, if volcanic eruptions or large-scale fires release particles into the atmosphere of just the right size to preferentially scatter red light. According to the Encyclopaedia Britannica, scattering

1344-399: A similar effect. The Antarctic diary of Robert Falcon Scott for July 11, 1911 mentioned "the air thick with snow, and the moon a vague blue". The key to a blue moon is having many particles slightly wider than the wavelength of red light (0.7 micrometer)—and no other sizes present. Ash and dust clouds thrown into the atmosphere by fires and storms usually contain a mixture of particles with

1440-463: A single year (1915, 1961, 1999, 2018, 2037, 2094). 1915 had four blue moons (two Pruett, one Maine, one astro-seasonal). 1934 and 2048 have three (one of each type). Despite the 187 blue moons appearing across the 200 years in this table, only 146 years have any of these 3 types of blue moons, leaving 54 years (thus averaging just over 1 year in every 4) which have none of the 3 rules represented in that calendar year. While not totally unexpected (given

1536-423: A slender crescent in the western sky after sunset) on the first day of the lunar month. The conjunction of Sun and Moon ("new moon") is most likely to fall on the preceding day, which is day 29 of a "hollow" (29-day) month and day 30 of a "full" (30-day) month. Historically , the paschal full moon date for a year was found from its sequence number in the Metonic cycle, called the golden number , which cycle repeats

1632-473: A wide range of sizes, with most smaller than 1 micrometer, and they tend to scatter blue light. This kind of cloud makes the moon turn red; thus red moons are far more common than blue moons. Blue moon as a calendrical term originated with the 1937 Maine Farmers’ Almanac , a provincial U.S. magazine that is not to be confused with the Farmers' Almanac , Old Farmer's Almanac , or other American almanacs . There

1728-524: A year. For this to add up to another full month would take 1/0.368 years. Thus it would take about 2.716 years, or 2 years, 8 months, and 18 days for another Pruett blue moon to occur. Or approximately once in 32.5 months on an average. When there are two Pruett blue moons in a single year, the first occurs in January and the second in March or April. The next time New Year's Eve falls on

1824-534: A “blue moon / Low in the west.” It was written at a time when the eruption of Mount Tambora was causing global climate effects, and not long before the first recorded instances of “blue moon” as a metaphor. The OED cites Pierce Egan’s Real Life in London (1821) as the earliest known occurrence of “blue moon” in the metaphorical sense of a long time. (“How's Harry and Ben?—haven't seen you this blue moon.”) An 1823 revision of Francis Grose’s ‘’Classical Dictionary of

1920-405: Is about 11 days shorter than the calendar year, which is either 365 or 366 days long. These days by which the solar year exceeds the lunar year are called epacts ( Ancient Greek : ἐπακταὶ ἡμέραι , romanized :  épaktai hēmérai , lit.   'intercalary days'). It is necessary to add them to the day of the solar year to obtain the correct day in the lunar year. Whenever

2016-420: Is an exception. The month ending in March normally has 30 days, but if 29 February of a leap year falls within it, it contains 31. As these groups are based on the lunar cycle , over the long term the average month in the lunar calendar is a very good approximation of the synodic month , which is 29.530 59 days long. There are 12 synodic months in a lunar year, totaling either 354 or 355 days. The lunar year

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2112-676: Is called the Blue Moon. It is said that when a person sees a blue moon and makes a wish, he will be granted a second chance in things.” In 1999 folklorist Philip Hiscock presented a timeline for the calendrical term. First, the August page of the 1937 Maine Farmers' Almanac ran a sidebar claiming that the term was used “in olden times” for an extra full moon in a season, and gave some examples (21 November 1915, 22 August 1918, 21 May 1921, 20 February 1924, 21 November 1934, 22 August 1937, and 21 May 1940). Six years later, Laurence J. Lafleur (1907-66) quoted

2208-594: Is celebrated on the first Sunday after the Paschal full moon (a mathematical approximation of the first astronomical full moon , on or after 21 March – itself a fixed approximation of the March equinox ). Determining this date in advance requires a correlation between the lunar months and the solar year , while also accounting for the month, date, and weekday of the Julian or Gregorian calendar . The complexity of

2304-413: Is further supported by the spurious explanation the almanac gave: The Moon usually comes full twelve times in a year, three times in each season... However, occasionally the moon comes full thirteen times in a year. This was considered a very unfortunate circumstance, especially by the monks who had charge of the calendar. It became necessary for them to make a calendar of thirteen months, and it upset

2400-467: Is given by the formula That is, the year number Y in the Christian era is divided by 19, and the remainder plus 1 is the golden number. (Some sources specify that you add 1 before taking the remainder; in that case, you need to treat a result of 0 as golden number 19. In the formula above we take the remainder first and then add 1, so no such adjustment is necessary.) Cycles of 19 years are not all

2496-401: Is no evidence of “blue moon” having been used as a specific calendrical term before 1937, and it was possibly invented by the magazine’s editor, Henry Porter Trefethen (1887-1957). As a term for the second full moon in a calendar month it began to be widely known in the U.S. in the mid-1980s and became internationally known in the late 1990s when calendrical matters were of special interest given

2592-406: Is no problem since they are the same. This does not move the problem to the pair "25" and "xxvi", because the earliest epact 26 could appear would be in year 23 of the cycle, which lasts only 19 years: there is a saltus lunae in between that makes the new moons fall on separate dates. The Gregorian calendar has a correction to the tropical year by dropping three leap days in 400 years (always in

2688-500: Is one day less: in years 1, 6, and 17 of the cycle the date is only 18 days later, and in years 7 and 18 it is only 10 days earlier than in the previous year. In the Eastern system , the Paschal full moon is usually four days later than in the West. It is 34 days later in 5 of the 19 years, and 5 days later in years 6 and 17, because in those years, the Gregorian system puts the Paschal full moon

2784-444: Is predictable and relatively common, happening 7 times in every 19 years (i.e. once every 2 or 3 years). Calendrical blue moons occur because the time between successive full moons (approximately 29.5 days) is shorter than the average calendar month. They are of no astronomical or historical significance, and are not a product of actual lunisolar timekeeping or intercalation . A 1528 satire, Rede Me and Be Nott Wrothe , contained

2880-485: Is the cause of “that epitome of rare occurrences, the blue Moon (seen when forest fires produce clouds composed of small droplets of organic compounds).” A Royal Society report on the 1883 Krakatoa eruption gave a detailed account of “blue, green, and other coloured appearances of the sun and moon” seen in many places for months afterwards. . The report mentioned that in February 1884 an observer in central America saw

2976-456: Is the first one in the year to have its fourteenth day (its formal full moon ) on or after 21 March. Easter is the Sunday after its 14th day (or, saying the same thing, the Sunday within its third week). The paschal lunar month always begins on a date in the 29-day period from 8 March to 5 April inclusive. Its fourteenth day, therefore, always falls on a date between 21 March and 18 April inclusive (in

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3072-422: Is to use the astronomical seasons, which are of unequal length. There is also reference in modern popular astrology to “zodiacal blue moons”. The table below has blue moon dates and times ( UTC ) calculated according to Pruett’s “calendar” rule (second full moon in a calendar month) and two versions of the “seasonal” rule (third full moon in a season with four). The Maine rule uses equal-length seasons defined by

3168-480: Is too late: The full moon would fall on 19 April, and Easter could be as late as 26 April. In the Julian calendar the latest date of Easter was 25 April, and the Gregorian reform maintained that limit. So the paschal full moon must fall no later than 18 April and the new moon on 5 April, which has epact label "xxv". 5 April must therefore have its double epact labels "xxiv" and "xxv". Then epact "xxv" must be treated differently, as explained in

3264-451: The computus would be the procedure of determining the first Sunday after the first ecclesiastical full moon falling on or after 21 March. The earliest known Roman tables were devised in 222 by Hippolytus of Rome based on eight-year cycles. Then 84-year tables were introduced in Rome by Augustalis near the end of the 3rd century. Although a process based on the 19-year Metonic cycle

3360-585: The Maine Farmers' Almanac used the term in a slightly different sense from the one now in common use. According to the OED, “Earlier occurrences of the sense given in the Maine Farmers' Almanac have not been traced, either in editions of the Almanac prior to 1937, or elsewhere; the source of this application of the term (if it is not a coinage by the editor, H. P. Trefethen) is unclear.” The conjecture of editorial invention

3456-526: The algorithm arises because of the desire to associate the date of Easter with the date of the Jewish feast of Passover which, Christians believe, is when Jesus was crucified. It was originally feasible for the entire Christian Church to receive the date of Easter each year through an annual announcement by the pope . By the early third century, however, communications in the Roman Empire had deteriorated to

3552-434: The 14th day fall between 21 March and 18 April inclusive, thus spanning a period of (only) 29 days. A new moon on 7 March, which has epact label "xxiv", has its 14th day (full moon) on 20 March, which is too early (not following 20 March). So years with an epact of "xxiv", if the lunar month beginning on 7 March had 30 days, would have their paschal new moon on 6 April, which

3648-548: The 16th century the lunar calendar was out of phase with the real Moon by four days. The Gregorian Easter has been used since 1583 by the Roman Catholic Church and was adopted by most Protestant churches between 1753 and 1845. German Protestant states used an astronomical Easter between 1700 and 1776, based on the Rudolphine Tables of Johannes Kepler , which were in turn based on astronomical positions of

3744-401: The 3 types appearing. One lunation (an average lunar cycle) is 29.53  days . There are about 365.24 days in a tropical year . Therefore, about 12.37 lunations (365.24 days divided by 29.53 days) occur in a tropical year. So the date of the full moon falls back by nearly one day every calendar month on average. Each calendar year contains roughly 11 days more than

3840-463: The 30) epact labels assigned to it. The reason for moving around the epact label "xxv/25" rather than any other seems to be the following: According to Dionysius (in his introductory letter to Petronius), the Nicene council, on the authority of Eusebius , established that the first month of the ecclesiastical lunar year (the paschal month) should start between 8 March and 5 April inclusive, and

3936-501: The 4th century. Victorius of Aquitaine tried to adapt the Alexandrian method to Roman rules in 457 in the form of a 532-year table, but he introduced serious errors. These Victorian tables were used in Gaul (now France) and Spain until they were displaced by Dionysian tables at the end of the 8th century. The tables of Dionysius and Victorius conflicted with those traditionally used in

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4032-486: The 6th figure after the decimal point. This corresponds to an error of less than a day in the phase of the moon over 40,000 years, but in fact the length of a day is changing (as is the length of a synodic month), so the system is not accurate over such periods. See the article ΔT (timekeeping) for information on the cumulative change of day length. This method of computation has several subtleties: Every other lunar month has only 29 days, so one day must have two (of

4128-578: The British Isles. The British tables used an 84-year cycle, but an error made the full moons fall progressively too early. The discrepancy led to a report that Queen Eanflæd , on the Dionysian system – fasted on her Palm Sunday while her husband Oswiu , king of Northumbria, feasted on his Easter Sunday. As a result of the Irish Synod of Magh-Lene in 630, the southern Irish began to use

4224-545: The Dionysian tables, and the northern English followed suit after the Synod of Whitby in 664. The Dionysian reckoning was fully described by Bede in 725. It may have been adopted by Charlemagne for the Frankish Church as early as 782 from Alcuin , a follower of Bede. The Dionysian/Bedan computus remained in use in western Europe until the Gregorian calendar reform, and remains in use in most Eastern Churches, including

4320-513: The Easter cycles of Theophilus as a "Paschal computus ." By the end of the 8th century, computus came to refer specifically to the calculation of time. The calculations produce different results depending on whether the Julian calendar or the Gregorian calendar is used. For this reason, the Catholic Church and Protestant churches (which follow the Gregorian calendar) celebrate Easter on

4416-476: The Gregorian Easter, were delayed one week so they were on the same Sunday as the Gregorian Easter. Germany's astronomical Easter was one week before the Gregorian Easter in 1724 and 1744. Sweden's astronomical Easter was one week before the Gregorian Easter in 1744, but one week after it in 1805, 1811, 1818, 1825, and 1829. Two modern astronomical Easters were proposed but never used by any Church. The first

4512-452: The Gregorian or Julian calendar, for the Western and Eastern system, resp.), and the following Sunday then necessarily falls on a date in the range 22 March to 25 April inclusive. However, in the Western system Easter cannot fall on 22 March during the 300-year period 1900–2199 (see below). In the solar calendar Easter is called a moveable feast since its date varies within a 35-day range. But in

4608-478: The Hebrew lunisolar calendar, Passover begins at twilight on the 14th day of Nisan . Nisan is the first month of spring in the northern hemisphere , with the 14th corresponding to a full moon. By the 2nd century, many Christians had chosen to observe Easter only on a Sunday. The Hebrew calendar does not have a simple relationship with the Christian calendars : it resynchronizes with the solar year by intercalating

4704-478: The Maine rule is sometimes called the “seasonal”, “true” or “traditional” rule (though of course no tradition of it exists prior to 1937). Blue moons by Pruett’s definition are sometimes called “calendar blue moons”. The "seasonal" blue moon rule is itself ambiguous since it depends which definition of season is used. The Maine rule used seasons of equal length with the ecclesiastical equinox (March 21). An alternative

4800-604: The Sun and Moon observed by Tycho Brahe at his Uraniborg observatory on the island of Ven , while Sweden used it from 1739 to 1844. This astronomical Easter was the Sunday after the full moon instant that was after the vernal equinox instant using Uraniborg time ( TT + 51 ) . However, it was delayed one week if that Sunday was the Jewish date Nisan   15, the first day of Passover week, calculated according to modern Jewish methods. This Nisan   15 rule affected two Swedish years, 1778 and 1798, that instead of being one week before

4896-474: The Vulgar Tongue’’, edited by Egan, included the definition: “Blue moon. In allusion to a long time before such a circumstance happens. ‘O yes, in a blue moon.’” An earlier (1811) version of the same dictionary had not included the phrase, so it was likely coined some time in the 1810s. "Once in a blue moon" is recorded from 1833. The use of blue moon to mean a specific calendrical event dates from 1937, when

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4992-445: The almanac in the U.S. magazine Sky & Telescope (July 1943, page 17) in answer to a reader’s question about the meaning of “blue moon”. Then James Hugh Pruett (1886-1955) quoted it again in Sky & Telescope (March 1946, p3), saying “seven times in 19 years there were — and still are — 13 full moons in a year. This gives 11 months with one full moon each and one with two. This second in

5088-519: The approaching millennium. It created a misapprehension that the calendrical meaning of “blue moon” had preceded the metaphorical one, and inspired various folk etymologies , e.g. the “betrayer” speculation mentioned earlier, or that it came from a printing convention in calendars or a saying in Czech. A 1997 Taiwanese movie, Blue Moon , had the log line “There is usually only one full moon every month, but occasionally there are two – and that second full moon

5184-404: The astronomical Easter one month before the Gregorian Easter in 1924, 1943, and 1962, but one week after it in 1927, 1954, and 1967. The 1997 version would have placed the astronomical Easter on the same Sunday as the Gregorian Easter for 2000–2025 except for 2019, when it would have been one month earlier. The Easter cycle groups days into lunar months, which are either 29 or 30 days long. There

5280-437: The calendrical definition of “blue moon” that is now most commonly used, i.e. the second full moon in a calendar month. “A blue moon in the original Maine Farmers' Almanac sense can only occur in the months of February, May, August, and November. In the later sense, one can occur in any month except February." This later sense gained currency from its use in a United States radio programme, StarDate on January 31, 1980 and in

5376-455: The children’s book), and in 1988 a forthcoming blue moon received widespread press coverage. In 1999 U.S. astronomer Donald W. Olson researched the original articles and published the results in a Sky & Telescope article co-authored with Richard T Fienberg and Roger W. Sinnott. From the examples given by Trefethen in the 1937 Maine Farmers’ Almanac they deduced a “rule” he must effectively have used. “Seasonal Moon names are assigned near

5472-921: The crescent moon as “a magnificent emerald-green” while its ashen part was “pale green”. Venus, bright stars and a comet were also green. The report authors suspected that green moons were a contrast effect, since in those cases the surrounding sky was seen as red. People saw blue moons in 1983 after the eruption of the El Chichón volcano in Mexico, and there are reports of blue moons caused by Mount St. Helens in 1980 and Mount Pinatubo in 1991. The moon looked blue after forest fires in Sweden and Canada in 1950 and 1951, On September 23, 1950, several muskeg fires that had been smoldering for several years in Alberta, Canada, suddenly blew up into major—and very smoky—fires. Winds carried

5568-402: The cycle in use since 1900 and until 2199), then an epact of 25 puts the ecclesiastical new moon on April 4 (having the label "25"), otherwise it is on April 5 (having label "xxv"). An epact of 25 giving April 4 can only happen if the golden number is greater than 11. In which case it will be 11 years after a year with epact 24. So for example, in 1954 the golden number was 17, the epact was 25,

5664-599: The date of the Christian festival of Easter depended on an accurate computation of full moon dates, and important work was done by the monks Dionysius Exiguus and Bede , explained by the latter in The Reckoning of Time , written c725 CE. According to Bede, “Whenever it was a common year, [the Anglo-Saxons] gave three lunar months to each season. When an embolismic year occurred (that is, one of 13 lunar months) they assigned

5760-536: The dating of Easter from the Hebrew calendar, by identifying the first full moon following the March equinox. By the time of the First Council of Nicaea (AD 325), the Church of Alexandria had designated 21 March as an ecclesiastical date for the equinox, irrespective of actual astronomical observation. In 395, Theophilus published a table of future dates for Easter, validating the Alexandrian criteria. Thereafter,

5856-419: The day of the full moon . It is the day of the lunar month on which the moment of opposition ("full moon") is most likely to fall. The Gregorian method derives new moon dates by determining the epact for each year. The epact can have a value from * (0 or 30) to 29 days. It is the age of the moon in days (i.e. the lunar date) on 1 January reduced by one day. The "new moon" is most likely to become visible (as

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5952-567: The dynamical mean sun, and is presumed to have been the original rule of Trefethen. The “astro-seasonal” rule uses the unequal astronomical seasons defined by the apparent sun. All calculations are by David Harper. The fourth column shows blue moon dates that were actually printed in the Maine Farmers’ Almanac , as found by Olson, Fienberg and Sinnott in 1999. They studied issues published between 1819 and 1962, and found that all mentions occurred between 1937, when H.P. Trefethen introduced

6048-567: The eastern fringes of the Roman empire, by the tenth century all had adopted the Alexandrian Easter, which still placed the vernal equinox on 21 March, although Bede had already noted its drift in 725 – it had drifted even further by the 16th century. Worse, the reckoned Moon that was used to compute Easter was fixed to the Julian year by the 19-year cycle. That approximation built up an error of one day every 310 years, so by

6144-411: The ecclesiastical new moon was reckoned on April 4, the full moon on April 17. Easter was on April 18 rather than April 25 as it would otherwise have been, such as in 1886 when the golden number was 6. This system automatically intercalates seven months per Metonic cycle. Label all the dates in the table with letters "A" to "G", starting from 1 January, and repeat to the end of the year. If, for instance,

6240-425: The end of the year. However, in every second such period count only 29 days and label the date with xxv (25) also with xxiv (24). Treat the 13th period (last eleven days) as long, therefore, and assign the labels "xxv" and "xxiv" to sequential dates (26 and 27 December respectively). Add the label "25" to the dates that have "xxv" in the 30-day periods; but in 29-day periods (which have "xxiv" together with "xxv") add

6336-405: The epact for the year is entered. If the epact for the year is for instance 27, then there is an ecclesiastical new moon on every date in that year that has the epact label "xxvii" (27). If the epact is 25, then there is a complication, introduced so that the ecclesiastical new moon will not fall on the same date twice during a Metonic cycle. If the epact cycle in force includes epact 24 (as does

6432-511: The epact reaches or exceeds 30, an extra intercalary month (or embolismic month) of 30 days must be inserted into the lunar calendar: then 30 must be subtracted from the epact. Charles Wheatly provides the detail: "Thus beginning the year with March (for that was the ancient custom) they allowed thirty days for the moon [ending] in March, and twenty-nine for that [ending] in April; and thirty again for May, and twenty-nine for June &c. according to

6528-434: The epact. In the Gregorian calendar, this is done by adding 1 eight times in 2,500 (Gregorian) years (slightly more than 2500 × 0.003126, or about 7.8), always in a century year: this is the so-called lunar correction (historically called "lunar equation"). The first one was applied in 1800, the next is in 2100, and will be applied every 300 years except for an interval of 400 years between 3900 and 4300, which starts

6624-405: The epacts only from 8 March to 5 April. As an example, if the epact is 27, an ecclesiastical new moon falls on every date labeled xxvii . The ecclesiastical full moon falls 13 days later. From the table above, this gives new moons on 4 March and 3 April, and so full moons on 17 March and 16 April. Then Easter Day is the first Sunday after the first ecclesiastical full moon on or after 21 March. In

6720-441: The example, this paschal full moon is on 16 April. If the dominical letter is E, then Easter day is on 20 April. The label " 25 " (as distinct from "xxv") is used as follows: Within a Metonic cycle, years that are 11 years apart have epacts that differ by one day. A month beginning on a date having labels "xxiv" and "xxv" written side by side has either 29 or 30 days. If the epacts 24 and 25 both occur within one Metonic cycle, then

6816-402: The expression "Once in a Blue Moon". There is no evidence that an extra moon in a month, season or year was considered unlucky, or that it led to 13 being considered unlucky , or that the extra moon was called "blue", or that it led to the phrase "once in a blue moon". There is good reason to suspect that the 1937 article was a hoax, practical joke, or simply misinformed. It is however true that

6912-413: The extra month to summer, so that three months together bore the name ‘‘Litha’’; hence they called [the embolismic] year ‘‘Thrilithi’’. It had four summer months, with the usual three for the other seasons.” The name Litha is now applied by some Neo-Pagans to midsummer . The 1937 Maine Farmers' Almanac article was misinterpreted by James Hugh Pruett in a 1946 Sky and Telescope article , leading to

7008-540: The first Sunday of the year is on 5 January, which has letter "E", then every date with the letter "E" is a Sunday that year. Then "E" is called the dominical letter (DL) for that year – from dies dominica (Latin for 'the Lord's day'). The dominical letter cycles backward one position every year. In leap years, after 24 February, the Sundays fall on the previous letter of the cycle, so leap years have two dominical letters:

7104-419: The first for before, the second for after the leap day. In practice, for the purpose of calculating Easter, this need not be done for all 365 days of the year. For the epacts, March comes out exactly the same as January, because 31+28 days = 30+29 epacts, so one need not calculate January or February. To avoid the need to calculate the dominical letters for January and February, start with D for 1 March. You need

7200-447: The label "25" to the date with "xxvi". The distribution of the lengths of the months and the length of the epact cycles is such that each civil calendar month starts and ends with the same epact label, except for February and, one might say, for August, which starts with the double label "xxv"/"xxiv" but ends with the single label "xxiv". This table is called the calendarium . The ecclesiastical new moons for any year are those dates when

7296-598: The lines, “Yf they saye the mone is belewe / We must beleve that it is true.” The intended sense was of an absurd belief, like the moon being made of cheese . There is nothing to connect it with the later metaphorical or calendrical meanings of “blue moon”. However, a confusion of belewe (Middle English, “blue”) with belǽwan (Old English “to betray”) ) led to a false etymology for the calendrical term that remains widely circulated, despite its originator having acknowledged it as groundless. Percy Bysshe Shelley’s poem "Alastor" (1816) mentioned an erupting volcano and

7392-407: The lunar calendar, Easter is always the third Sunday in the paschal lunar month, and is no more "moveable" than any holiday that is fixed to a particular day of the week and week within a month, such as Thanksgiving . As reforming the computus was the primary motivation for the introduction of the Gregorian calendar in 1582, a corresponding computus methodology was introduced alongside

7488-430: The lunar month took the name of the Julian month in which it ended. The nineteen-year Metonic cycle assumes that 19 tropical years are as long as 235 synodic months. So after 19 years the lunations should fall the same way in the solar years, and the epacts should repeat. Over 19 years the epact increases by 19 × 11 = 209 ≡ 29 ( mod 30) , not 0 (mod 30) . That is, 209 divided by 30 leaves a remainder of 29 instead of being

7584-445: The lunar phase on January 1 every 19 years. This method was modified in the Gregorian reform because the tabular dates go out of sync with reality after about two centuries. From the epact method, a simplified table can be constructed that has a validity of one to three centuries. The date of the Paschal full moon in a particular year is usually either 11 days earlier than in the previous year, or 19 days later. In 5 out of 19 years it

7680-419: The new (and full) moons would fall on the same dates for these two years. This is possible for the real moon but is inelegant in a schematic lunar calendar; the dates should repeat only after 19 years. To avoid this, in years that have epacts 25 and with a Golden Number larger than 11, the reckoned new moon falls on the date with the label 25 rather than xxv . Where the labels 25 and xxv are together, there

7776-460: The new calendar. The general method of working was given by Clavius in the Six Canons (1582), and a full explanation followed in his Explicatio (1603). Easter is the Sunday following the Paschal full moon date. The Paschal full moon date is the ecclesiastical full moon date on or after the ecclesiastical equinox on 21 March. The fourteenth day of the lunar month is ecclesiastically considered

7872-463: The number of days in 12 lunar cycles, so every two or three years (seven times in the 19 year Metonic cycle ), there is an extra full moon in the year. The extra full moon necessarily falls in one of the four seasons (however defined), giving that season four full moons instead of the usual three. Given that a year is approximately 365.2425 days and a synodic orbit is 29.5309 days, then there are about 12.368 synodic months in

7968-585: The old verses: Impar luna pari, par fiet in impare mense; In quo completur mensi lunatio detur. "For the first, third, fifth, seventh, ninth, and eleventh months, which are called impares menses , or unequal months, have their moons according to computation of thirty days each, which are therefore called pares lunae , or equal moons: but the second, fourth, sixth, eighth, tenth, and twelfth months, which are called pares menses , or equal months, have their moons but twenty nine days each, which are called impares lunae , or unequal moons." Thus

8064-460: The overlapping frequencies of these 3 rules), it so happens there are not any 2 sequential years (at least within these 200) wherein none of the 3 types of blue moon occur. Conversely, despite the preponderance of years with blue moons (of at least 1 type) occurring in this 200-year range, there are no instances of more than 4 sequential years having a blue moon, of any of these 3 types -- i.e. at least 1 year out of every 5 sequential years has none of

8160-484: The point that the church put great value in a system that would allow the clergy to determine the date for themselves, independently yet consistently. Additionally, the church wished to eliminate dependencies on the Hebrew calendar , by deriving the date for Easter directly from the March equinox. In The Reckoning of Time (725), Bede uses computus as a general term for any sort of calculation, although he refers to

8256-441: The regular arrangement of church festivals. For this reason thirteen came to be considered an unlucky number. Also, this extra moon had a way of coming in each of the seasons so that it could not be given a name appropriate to the time of year like the other moons. It was usually called the Blue Moon... In olden times the almanac makers had much difficulty calculating the occurrence of the Blue Moon and this uncertainty gave rise to

8352-414: The same length, because they may have either four or five leap years. But a period of four cycles, 76 years (a Callippic cycle ), has a length of 76 × 365 + 19 = 27,759 days (if it does not cross a century division). There are 235 × 4 = 940 lunar months in this period, so the average length is 27759   /   940 or about 29.530851 days. There are 76 × 6 = 456 usual nominal 30-day lunar months and

8448-457: The same number of usual nominal 29-day months, but with 19 of these lengthened by a day on leap days, plus 24 intercalated months of 30 days and four intercalated months of 29 days. Since this is longer than the true length of a synodic month, about 29.53059 days, the calculated Paschal full moon gets later and later compared to the astronomical full moon, unless a correction is made as in the Gregorian system (see below). The paschal or Easter-month

8544-416: The seven extra 30-day months were largely hidden by being located at the points where the Julian and lunar months begin at about the same time. The extra months commenced on 1 January (year 3), 2 September (year 5), 6 March (year 8), 3 January (year 11), 31 December (year 13), 1 September (year 16), and 5 March (year 19). The sequence number of the year in the 19-year cycle is called the " golden number ", and

8640-682: The smoke eastward and southward with unusual speed, and the conditions of the fire produced large quantities of oily droplets of just the right size (about 1   micrometre in diameter) to scatter red and yellow light. Wherever the smoke cleared enough so that the sun was visible, it was lavender or blue. Ontario, Canada, and much of the east coast of the United States were affected by the following day, and two days later, observers in Britain reported an indigo sun in smoke-dimmed skies, followed by an equally blue moon that evening. Ice particles might have

8736-442: The spring equinox in accordance with the ecclesiastical rules for determining the dates of Easter and Lent . The beginnings of summer, fall, and winter are determined by the dynamical mean Sun . When a season contains four full Moons, the third is called a Blue Moon.” They termed this the “Maine rule” for blue moons, as distinct from Pruett’s 1946 definition that was seen to have been a misinterpretation. In popular astronomy

8832-514: The term, and 1956, when Trefethen’s editorship ended (consistent with it being Trefethen’s own invention). Occasional discrepancies between the Maine rule and the almanac’s printed dates can be ascribed to clerical errors or miscalculation. In one case (August 1945) Trefethen appears to have used the apparent rather than mean sun. The table shows that in 200 years there are 187 full moons that could be called "blue" by some definition - an average of nearly one per year. Two Pruett blue moons can occur in

8928-751: The vast majority of Eastern Orthodox Churches and Non-Chalcedonian Churches . The only Eastern Orthodox church which does not follow the system is the Finnish Orthodox Church, which uses the Gregorian. Having deviated from the Alexandrians during the 6th century, churches beyond the eastern frontier of the former Byzantine Empire, including the Assyrian Church of the East , now celebrate Easter on different dates from Eastern Orthodox Churches four times every 532 years. Apart from these churches on

9024-539: Was first proposed by Bishop Anatolius of Laodicea around 277, the concept did not fully take hold until the Alexandrian method became authoritative in the late 4th century. The Alexandrian computus was converted from the Alexandrian calendar into the Julian calendar in Alexandria around 440, which resulted in a Paschal table (attributed to pope Cyril of Alexandria ) covering the years 437 to 531. This Paschal table

9120-658: Was proposed as part of the Revised Julian calendar at a Synod in Constantinople in 1923 and the second was proposed by a 1997 World Council of Churches Consultation in Aleppo in 1997. Both used the same rule as the German and Swedish versions but used modern astronomical calculations and Jerusalem time ( TT + 2 21 ) without the Nisan   15 rule. The 1923 version would have placed

9216-521: Was the source which inspired Dionysius Exiguus , who worked in Rome from about 500 to about 540, to construct a continuation of it in the form of his famous Paschal table covering the years 532 to 616. Dionysius introduced the Christian Era (counting years from the Incarnation of Christ) by publishing this new Easter table in 525. A modified 84-year cycle was adopted in Rome during the first half of

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