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Bissextile, Intercalary, Embolistic

One of the less edifying manifestations of the approaching millennium is the argument about which year it occurs on, a debate I do not intend to join, as it is accompanied by an almost religious conviction on either side which mere logic cannot affect. A less common and more subtle difficulty is deciding whether the year 2000 is a leap year or not. After all, 1900 was not one, nor was 1800, nor was 1700 in much of Europe, though it was in Britain and the then American colonies. This seems as good a time as any to look into the language of leap years and along the way try to work out how such a confusion could arise.

The calendar we use nowadays is called the Gregorian calendar, after Pope Gregory XIII. By his time, the calendar which had been instituted by edict of Julius Caesar in 46BC, called therefore the Julian calendar, was creaking badly. In the 1600 years since it had been instituted, it had drifted against the seasons until it was nearly a fortnight out. Gregory’s concern was not so much with the seasons as with the effect this was having on the dates of church festivals, especially Easter, whose date depended partly on the calendar and partly on the phases of the moon. It was well known that the problem was due to the Julian calendar overestimating the length of the year by about three-quarters of a day a century. The necessary calculations were carried out by Antonio Lilius and written up into a Papal bull by Christopher Clavius. He recommended that 10 days should be left out of the year 1582 to bring the calendar back into synchronism with the seasons and that centennial years should only be leap years if they were divisible by 400. He only took out 10 days to return the calendar to the position it had had in 325, when the rules for setting Easter had been first established at the Council of Nicaea (at the place which is now called Iznik, in Turkey).

The rule was implemented in Catholic Europe relatively quickly, but very spottily, with different countries adopting the change at different times. Protestant England and some other countries didn’t change. Sweden attempted a confused plan that left them out of step with all other countries for 12 years until they returned to the Julian calendar on the unique date of 30 February 1712, before finally converting in the usual way in 1753. Britain and its colonies only swapped over in 1752, by which time the error had grown to 11 days (we weren’t by a long way the last — that was Greece in 1923). Here is a bit of the Act of 1751 authorising the change:

Be it further enacted by the Authority aforesaid, That the several Years of our Lord, one thousand eight hundred, one thousand nine hundred, two thousand one hundred, two thousand two hundred, two thousand three hundred, or any other hundredth Years of our Lord, which shall happen in Time to come, except only every fourth hundredth Year of our Lord, whereof the Year of our Lord two thousand shall be the first, shall not be esteemed or taken to be Bissextile or Leap Years, but shall be taken to be common Years, consisting of three hundred and sixty-five Days, and no more; and that the Years of our Lord two thousand, two thousand four hundred, two thousand eight hundred, and every other fourth hundred Year of our Lord, from the said Year of our Lord two thousand inclusive, and also all other Years of our Lord, which by the present Supputation are esteemed to be Bissextile or Leap Years, shall for the future, and in all Times to come, be esteemed and taken to be Bissextile or Leap Years, consisting of three hundred and sixty-six Days, in the same Sort and Manner as is now used with respect to every fourth Year of our Lord.

From An Act for Regulating the Commencement of the Year; and for Correcting the Calendar now in Use, 1751.

The Act of Parliament is still in force in Britain and so authoritatively settles doubts concerning the length of the year 2000, as well as that of later quadricentennial years that are unlikely to concern any of us now living.

Though a bit archaic and longwinded, the language of the Act is clear enough, except that the word bissextile will probably be unfamiliar. By the time of the Act, this was merely an adjective describing a leap year but its origins lay in the way the Romans interpolated their extra day every four years. They didn’t add it at the end of February as we do, but six days before its end, that is, after the day we would now call the 24th of February. At this point in the month, the Romans counted days backwards from the start of the next month, so this additional day was called bis sexto, “the second sixth (day)” which evolved into our bissextile. Another term for such an interpolated day, now virtually obsolete, is embolismic, from mediaeval Latin embolismus, which itself comes from a Greek word meaning “insertion; interpolation” and which is also the source of our medical term embolism, “the obstruction (of an artery, etc) by a clot of blood, bubble of air etc”. Yet another, still current, is intercalary, from the Latin inter–, “between” and calare, “to proclaim”, because it was the custom to proclaim the fact of the inserted extra day to give people due warning of it. Our most usual term, leap year, which if you think about it is a rather strange idiom, may come from the fact that festivals after the intercalary day leap one day further forward compared with a normal year.

That word calare also gave rise to another key term, the calends, which described the first day of each month, so called because at one time it was the custom to call people together on that day to proclaim the phases of the moon and hence of the festivals that depended on them. It is also the origin of the word calendar itself, which originally meant an account book, the calends being the days on which accounts became due for payment. Incidentally, the middle days of each month were the Ides, from Latin iduare, “to divide”, and the Nones were the ninth day before the Ides.

It took several decades before the changes introduced by the 1751 Act were fully accepted. The alteration in leap years was the least of them, because that had no impact until the year 1800, nearly half a century later. To correct the calendar the eleven days between 2 and 14 September 1752 were left out, one significant and immediate effect of which was to move the following Christmas Day. At the same time, the date of the New Year in England and Wales was changed from 25 March to 1 January to fit continental practice and to match the system in use in Scotland (a rare example of England falling in line with Scottish practice rather than the other way around). It was common even fifty years afterwards to refer to dates as either Old Style or New Style to distinguish pre-1752 ones from later ones and to refer to January 5 as Old Christmas Day.

One very important consequence was to change the way Easter was calculated and so to invalidate all the tables in prayer books that showed the date of that most important Christian festival. Easter Day had been fixed at the Council of Nicaea as being “the first Sunday after the first full moon following the vernal equinox, except that if the full moon falls on a Sunday, Easter shall be held the Sunday following”. Under the Julian calendar, the phases of the moon and hence the dates of Easter repeated roughly every 19 years, a period known as the Metonic cycle after the Greek astronomer Meton. Tables had been brought into use in 530 in which each year was given a golden number indicating its place in the cycle, most possibly so named because they were marked in gold in medieval calendars. (These numbers had been calculated by a Roman abbot named Dionysius Exiguus, “little Dennis”, who introduced at the same time our AD dating scheme, though his calculations of the years since the birth of Christ were a bit adrift.) Combining this value with a table of Sunday or dominical letters, Easter could be derived without calculation. The change to the Gregorian calendar required the golden number to be replaced by another using a 30-year cycle based on the epact, the number of days by which the solar year was longer than the lunar year, a word derived from the Greek meaning “to bring in”.

Associated with calendars we have the words almanac, “a calendar of days and months with astronomical data etc”, a word from medieval Latin which was once supposed to come from Arabic (because of the initial al, the definite article in Arabic, which also appears in such words as alcohol and alchemy), but is now known to be of Greek origin. Their construction was not a trivial matter, as this description from the first edition of the Encyclopaedia Britannica of 1771 shows:

The first thing to be done is, to compute the sun’s and moon’s place for each day of the year, or it may be taken from some ephemerides and entered in the almanac; next, find the dominical letter, and, by means therof, distribute the kalendar into weeks; then, having computed the time of easter, by it fix the other moveable feasts; adding the immoveable ones, with the names of the martyrs, the rising and setting of each luminary, the length of day and night, the aspects of the planets, the phases of the moon, and the sun’s entrance into the cardinal points of the ecliptic, i.e. The two equinoxes and solstices.

Note the spelling of kalendar, a form which was looking very old-fashioned by this date. The word ephemerides, an apparently plural word used as a singular, is an older form of what we would now more commonly call an ephemeris, “a table of the predicted positions of a celestial body”, which comes from a Greek word meaning “diary; calendar”, and from which we also derive the terms ephemeral, strictly “lasting only one day”, Ephemeroptera, the formal name for the short-lived mayflies, and ephemera, the museum term for items which were never designed to be retained.

Though the Gregorian calendar is so familiar as to almost escape comment, it is widely recognised as being far from perfect and a number of schemes have been put forward to improve or replace it, including the World Calendar and the International Fixed Calendar. The most famous attempt was in revolutionary France, when a completely new system was briefly adopted, though abandoned as impractical in 1805. The months of this calendar were: Vendémiaire (“vintage”, 22 Sep — 21 Oct), Brumaire (“mist”, 22 Oct — 20 Nov), Frimaire (“frost”, 21 Nov — 20 Dec), Nivôse (“snow”, 21 Dec — 19 Jan), Pluviôse (“rain”, 20 Jan — 18 Feb), Ventôse (“wind”, 19 Feb — 20 Mar), Germinal (“seed time”, 21 Mar — 19 Apr), Floréal (“blossom”, 20 Apr -19 May), Prairial (“meadow”, 20 May — 18 Jun), Messidor (“harvest”, 19 Jun — 18 Jul), Thermidor (“heat”, 19 Jul — 17 Aug), and Fructidor (“fruits”, 18 Aug — 16 Sep). All very Arcadian, but scarcely practical.

Page created 9 Nov. 1996
Last updated 24 Apr. 1999

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Last modified: 24 April 1999.