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Chronology (Greek chronos time, logos, discourse), the science of time-measurement, has two branches:
It is thus for history what latitude and longitude are for geography. The first requisite in any system of historical chronology is an era, that is to say a fixed point of time, the distance from which shall indicate the position of all others. The term era, the derivation of which is not certainly known, appears first to have been employed in France and Spain to signify a number or rule. Since the need of a definite system of chronology was first recognized by mankind, many and various eras have been employed at different periods and by different nations. For practical purposes it is most important to understand those which affect Christian history.
Of the chronological systems previously in use it will be sufficient to briefly describe a few.
The Greeks dated events by Olympiads, or periods of four years intervening between successive celebrations of the Olympic games, and this mode of computation, having been largely adopted at Rome, continued to be frequently used in the first centuries of Christianity. The Olympiads started from 776 B.C., and consequently A.D. 1 was the fourth year of the 194th Olympiad.
The Romans frequently reckoned from the traditional foundation of their city (ab urbe conditâ—A.U.C.), which date, as has been said, coincided with 753 B.C. They likewise often designated years by the names of the consuls then in office (e.g. console Planco). Sometimes the Romans dated by post-consular years (i.e. so long after the consulate of a well-known man). Naturally the regnal years of Roman emperors presently supplanted those of consuls, whose power in later times was merely nominal, and from the emperors this method of describing dates was imitated by popes, kings, and other rulers, with or without the addition of the year A.D. It became in fact universal in the Middle Ages, and it subsists in documents, both ecclesiastical and civil, down to our own day.
The pontifical years of the popes are historically important (see chronological list in article POPE). Care must be taken, of course, in the case of such dates, to observe from what point of time each reign is reckoned. In an elective monarchy like the papacy there is necessarily an interval between successive reigns, which is occasionally considerable. Moreover, the reckoning is sometimes from the election of a pontiff, sometimes from his coronation.
In determining dates by the regnal years of other sovereigns there are of course various points to which attention must be paid. Confining ourselves to English history, the earlier kings after the Norman Conquest dated their reigns only from their coronation, or some other public exhibition of sovereignty, so that there was sometimes an interval of days or even weeks between the close of one reign and the commencement of the next. Only from the accession of Richard II (22 June, 1377) was the reign of a monarch held to begin with the death or deposition of his predecessor. Even subsequently to this it was reckoned sometimes from the day itself upon which the preceding monarch ceased to reign, sometimes from the day following. Not till the first year of Queen Elizabeth was it enacted that the former should be the rule. In certain particular instances the matter was still further complicated. King John dated his reign from his coronation, 27 May, 1199, but this being the Feast of the Ascension, his years were counted from one occurrence of this festival to the next, and were accordingly of varying length. Edward I dated from noon, 20 November, 1272, and in consequence this day in each year of his reign was partly in one regnal year and partly in another. In the civil wars of York and Lancaster, Henry VI and Edward IV equally ignored the period during which his rival assumed or recovered power, and counted their years continuously onwards from the time when they mounted the throne. Charles II, though he began to reign de facto only at the Restoration (29 May, 1660), reckoned his years, de jure, from his father's execution, 30 January, 1648-9, ignoring the Commonwealth and Protectorate. Queen Mary Tudor reckoned her reign from the death of Edward VI, 6 July, 1553, but the interval until 19 July of the same year being occupied by the abortive reign of Lady Jane Grey, public documents in her name commence only with the latter date. William III and Mary II began to reign 13 Feb., 1688-9, as "William and Mary". Mary died 28 December, 1694, when the style was altered to "William" alone; but no change was made in the computation of regnal years. Within the year, it was long usual to specify dates by reference to some well-known feast in the ecclesiastical calendar, as, for instance, "the Friday before Pentecost" or "the day of St. John the Baptist".
In papal and other documents, another epoch is often added, namely, the Indiction. This had originally been a period of fifteen years, at the close of which the financial accounts of the Roman Empire were balanced; but for purposes of chronology the indictions are conventional periods of fifteen years, the first of which began in the reign of Constantine the Great. Unlike the Olympiads, the indictions themselves were not numbered, but only the place of a year in the indiction in which it fell. Thus indictione quartâ; signifies not "in the fourth indiction", but "in the fourth year of its indiction", whatever this was. It was obvious that such an element of computation could serve only to verify more precisely the date of a year already approximately known. Moreover, the indictions were calculated on different systems, which have to be understood and distinguished:
The date at which the year commenced varied at different periods and in different countries. When Julius Caesar reformed the calendar (45 B.C.) he fixed 1 January as New Year's Day, a character which it seems never quite to have lost, even among those who for civil and legal purposes chose another starting point. The most common of such starting points were 25 March (Feast of the Annunciation, "Style of the Incarnation") and 25 December (Christmas Day, "Style of the Nativity"). In England before the Norman Conquest (1066) the year began either on 25 March or 25 December; from 1087 to 1155 on 1 January; and from 1155 till the reform of the calendar in 1752 on 25 March, so that 24 March was the last day of one year, and 25 March the first day of the next. But though the legal year was thus reckoned, it is clear that 1 January was commonly spoken of as New Year's Day. In Scotland, from 1 January, 1600, the beginning of the year was reckoned from that day. In France the year was variously reckoned: from Christmas Day, from Easter eve, or from 25 March. Of all starting points a movable feast like Easter is obviously the worst. From 1564 the year was reckoned in France from 1 January to 31 December. In Germany the reckoning was anciently from Christmas, but in 1544 and onwards, from 1 January to 31 December. In Rome and a great part of Italy, it was from 25 December, until Pope Gregory XIII reformed the calendar (1582) and fixed 1 January as the first day of the year. The years, however, according to which papal Bulls are dated still commence with Christmas Day. Spain, with Portugal and Southern France, observed an era of its own long after the rest of Christendom had adopted that of Dionysius. This era of Spain or of the Cæsars, commenced with 1 January, 38 B.C., and remained in force in the Kingdom of Castile and Leon till A.D. 1383, when a royal edict commanded the substitution of the Christian Era. In Portugal the change was not made till 1422. No satisfactory explanation has been found of the date from which this era started.
The introduction of the Gregorian Calendar entailed various discrepancies between the dates which different people assigned to the same events. The Julian system of time-measurements, introduced by Cæsar, was not sufficiently accurate, as it made the year slightly too long, with the result that by the sixteenth century it had fallen ten days in arrear, so that, for instance, the day of the vernal equinox, which should have been called 21 March, was called 11 March. To remedy this, besides substituting an improved system which should prevent the error from operating in future, it was necessary to omit ten full days in order to bring things back to the proper point. Pope Gregory XIII, who introduced the reformed system, or "New Style", ordained that ten days in October, 1582, should not be counted, the fourth of that month being immediately followed by the fifteenth. He moreover determined that the year should begin with 1 January, and in order to prevent the Julian error from causing retardation in the future as in the past, he ruled that three leap years should be omitted in every four centuries, viz. those of the centennial years the first two figures of which are not exact multiples of four, as 1700, 1800, 1900, 2100, etc. The New Style (N.S.) was speedily adopted by Catholic States, but for a long time the Protestant States retained the Old (O.S.), from which there followed important differences in marking dates according as one or other style was followed. In the first place there was the original difference of ten days between them, increased to eleven by the O.S. 29 February in A.D. 1700, to twelve days in 1800, and to thirteen in 1900. Moreover, the period from 1 January to 24 March inclusive, which was the commencement of the year according to N.S., according to O.S. was the conclusion of the year previous. From want of attention to this, important events have sometimes been misquoted by a year. In illustration may be considered the death of Queen Elizabeth. This occurred in what was then styled in England 24 March 1602, being the last day of that year. In France and wherever the N.S. prevailed, this day was described as 3 April, 1603. To avoid all possible ambiguity such dates are frequently expressed in fractional form as 24 March/3 April, 1602/3. In our modern histories years are always given according to N.S., but dates are otherwise left as they were originally recorded. Thus Queen Elizabeth is said to have died 24 March, 1603. Not till 1700 was the Gregorian reform accepted by the Protestant States of Germany and the Low Countries, and not till 1752 by Great Britain, there being by that time a difference of eleven days between O.S. and N.S. Sweden, after some strange vacillation, followed suit in 1753. O.S. was still followed by Russia and other Eastern Orthodox countries well into the twentieth century, and their dates consequently were thirteen days behind those of the rest of Christendom.
The Christian Era has this disadvantage for chronological purposes, that dates have to be reckoned backwards or forwards according as they are B.C. or A.D., whereas in an ideally perfect system all events would be reckoned in one sequence. The difficulty was to find a starting point whence to reckon, for the beginnings of history in which this should naturally be placed are those of which chronologically we know least. At one period it was attempted to date from the Creation (A.M. or Anno Mundi), that event being placed by Christian chronologists, such as Archbishop Usher, in 4004 B.C., and by the Jews in 3761 B.C. But any attempt thus to determine the age of the world has been long since abandoned. In the year 1583, however—that following the Gregorian reform—Joseph Justus Scaliger introduced a basis of calculation which to a large extent served the purpose required, and, according to Sir John Herschel, first introduced light and order into chronology. This was the Julian Period—one of 7980 Julian years, i.e. years of which every fourth one contains 366 days. The same number of Gregorian years would contain 60 days less. For historians these commence with the midnight preceding 1 January, 4713 B.C., for astronomers with the following noon. The period 7980 was obtained by multiplying together 28, 19, and 15, being respectively the number of years in the Solar Cycles the Lunar Cycle, and the Roman Indiction, and the year 4713 B.C. was that for which the number of each of these subordinate cycles equals 1. The astronomical day is reckoned from noon to noon instead of from midnight to midnight. Scaliger calculated his period for the meridian of Alexandria to which Ptolemy had referred his calculation.
Various eras employed by historians and chroniclers may be briefly mentioned, with the dates from which they were computed.
At the French Revolution it was determined to introduce an entirely new system of chronology, dating from that event and having no affinity with any previously adopted. In the first form this was the Era of Liberty, commencing 1 January, 1789. This was soon replaced by the Republican Era, at first appointed to commence 1 January, 1792, and afterwards 22 September, 1792. This was the date of the proclamation of the Republic, which coincided with the autumnal equinox, calculated on the meridian of Paris. The year was divided into twelve months of thirty days each, and the days into decades, weeks being abolished. The months had names given to them according to their seasonal character.
As these months contained only 360 days, five jours complémentaires were added at the end of Fructidor, officially called Primidi, Duodi, Tridi, Quartidi, Quintidi, but commonly known as Sans-culottides. Olympic or leap years occurred every fourth year of the Republic, and had a sixth intermediary day called Sextidi. The period thus terminated was called Franciade. This calendar was enforced in France till 1 January, 1806, when it was abolished by Napoleon, and the use of the Gregorian calendar resumed.
The days of the week are numbered according to their natural order, viz. Sunday=1, Monday=2, Tuesday=3, Wednesday=4, Thursday=5, Friday=6, Saturday=7. (At the time from which the Christian Era starts there were of course no weeks, such a measure of time not being known among the Greeks and Romans. Counting backwards, however, according to our present system, we can divide all time into weeks, and it is to be noted that in the Christian period the order of days of the week has never been interrupted. Thus, when Gregory XIII reformed the Calendar, in 1582, Thursday, 4 October, was followed by Friday, 15 October. So in England, in 1752, Wednesday, 2 September, was followed by Thursday, 14 September. What we style 14 August, 1907, the Russians style 1 August, but both call it Wednesday.) For our present purpose the year commences with March; January and February being reckoned as the 11th and 12th months of the preceding year; thus 29 February, when it occurs, is the last day of the year and causes no further disturbance.
As a matter of fact, it is found by computation that 1 March of the year known as A.D. 1 was a Tuesday. Assigning to this year the figure 1 as its year number, to March the figure 1 as its month number, and adding these to 1, the day number of 1 March, we get 3, indicating Tuesday the third day of the weeks. From this first datum all the rest follows. The succeeding days of March increase their figures each by 1, on account of the increased day number. When 7 is passed it is only the figures which remain, after division by that number, which are to be considered; thus 11 may be treated as 4 (7+4) and 30 as 2 (28+2). In general, any exact multiple of 7 (14, 21, 28) may be added or subtracted when convenient without affecting the result. Instead of adding any number (e.g. 1 or 4) we may subtract its difference from 7 or a multiple of 7 (e.g. 6 or 3). The remainder 0 in a division is equivalent to 7, and thus in calculating for the day of the week it signifies Saturday.
As the days of the leading month, so those of the months preceding it follow naturally. As March contains 31 days (i.e. 28+3), April necessarily begins with a day 3 places later in the weekly sequence, and its month number instead of 1 is 4. So of other months, according to the number of days in that which preceded. The following are the month numbers throughout the year which never change:—March 1; April 4; May 6; June 2; July 4; August 0; September 3; October 5; November 1; December 3; January 6; February 2. A.D. 1, being a common year of 365 days (or 52 weeks+1 day), ends with the same day of the week—Tuesday—with which it commenced. Consequently the next year, A.D. 2, commences a day later, with Wednesday for 1 March, and as its year number is increased to 2, we get 2+1+1=4. So in A.D. 3, the year number becomes 3, and 1 March is Thursday. But on account of 29 February preceding 1 March, A.D. 4, this day falls 366 days (or 52 weeks+2 days) after 1 March, A.D. 3, or on Saturday, and its year number must be increased to 5; 5+1+1=7. Thus, to find the number belonging to any year within its own century, we must find how many days beyond an exact number of weeks there have been since that century commenced. As every common year contains one day more than fifty-two weeks, and every leap year two days more, by adding at any period the number of leap years which there have been in the century to the total number of years in the same, we obtain the number of days required. To obtain the number of leap years, we divide the last two figures of the date (i.e. those in the tens and units place) by four. The quotient (neglecting any remainder) shows the number of leap years; which, added to the same two figures, gives the number of days over and above the sets of fifty-two weeks which the years contain. Thus, for example, the year '39 of any century (939, 1539, 1839, 1939) will have 6 for its year number; for in such year 48 extra days will have accumulated since the corresponding day of the centurial year (00), viz. 1 day for each of the 30 common years, and 18 days for the 9 leap years.
One more element of calculation remains to be considered — the Century. We begin with the Julian system, or Old Style (O.S.)—according to which all centuries contain 75 common years of 365 days, and 25 leap years of 366, and accordingly 125 days in all, over and above 5200 weeks. But 125 days=17 weeks+6 days. Therefore a Julian century ends with the day of the week two days previous to that with which if began, and the succeeding century will begin with the day of the week, one day earlier than its predecessor. Thus, A.D. 1 March, 1300, being Tuesday, in 1400 it would be Monday, in 1500 Sunday, in 1600 Saturday. Having obtained the centurial number for any century, we add to it the year numbers of the years which follow to the close of that century. Centurial numbers O.S. are obtained by subtracting the centurial figure or figures (viz. those preceding 00) from the multiple of 7 next above, the remainder being the number required. Thus for A.D. 1100 the centurial number is 3 (14-11), for 1500, 6 (21-15), for 1900, 2 (21-19).
Under the N.S. three centuries in every four contain 76 common years and 24 leap years, and thus have only 124 days over 5200 weeks, or 17 weeks and 5 days, and end with the day of the week three earlier than they began. The following century, beginning two days earlier than that which it follows, has its centurial number less by 2. Thus 1 March, A.D. 1700, was Monday, and the centurial number 0 (or 7). 1 March, 1800, was Saturday, and the centurial number 5. Every fourth centurial year N.S., being a leap year (1600, 2000, 2400, etc.), has 366 days; and the century to which it belongs, like those of the O.S., diminishes its centurial number only by 1 from the preceding. N.S. having been introduced in the sixteenth century, it is only for dates 15— and upwards that N.S. centurial numbers are required. They are as follows: for 1500=3; 1600=2; 1700=7; 1800=5; 1900=3; 2000=2. It will be seen that the same figures constantly recur. Leap year centuries (with the first two figures exactly divisible by 4) having the centurial number 2, and the three centuries following having 7 (or 0), 5, and 3 respectively, after which 2 comes round again. The centurial number N.S. can be obtained from that of O.S. if the difference of days between O.S. and N.S. be allowed for. This is done by subtracting the said difference from the O.S. centurial number, increased by as many times 7 as the subtraction requires. As we have seen, for the sixteenth and seventeenth centuries, the difference was 10 days; for the eighteenth, 11; for the nineteenth, 12; for the twentieth and twenty-first, 13. Thus:
Examples. – (1) King John was crowned 27 May, 1199. What day?
Therefore the day was Thursday.
Therefore the day was Sunday, as readers of "Vanity Fair" will recollect.
Therefore the day was Friday.
Therefore the day was Sunday.
It was O.S.
According to N.S. it would be: –
GIRY, Manuel de Diplomatique (Paris, 1894); MAS-LATRIE, Trésor de chronologie (Paris, 1889), Introduction; GROTEFEND, Taschenbuch der Zeitrechnung (Hanover and Leipzig, 1905); GROTEFEND, Zeitrechnung des d. Mittelalters und der Neuzeit (3 vols., Leipzig, 1891-8). Other valuable works of reference are the Benedictine Art de vérifier les dates (Paris, 1818-42); BOND, Handy Book for Verifying Dates (1875); IOELER, Handbuch der Chronologie (Berlin, 1825-6); HAMPSON, Medii Ævi Kalendarium (London, 1841); KRUSCH, Studien zur christlich-mittelalterlichen Chronologie (Leipzig, 1881); SACHSE, Das Aufkommen der Datierungen nach dem Festkalender in Urkunden, etc. (Erlangen, 1904); NILLES, Calendarium Utriusque Ecclesiæ (Innsbruck, 1896); MARZI, La Questione della Riforma del Calendario nel quinto concilio Lateranensi (Florence, 1896); MACDONALD, Chronologies and Calendars (London, 1897); GOLDSCHNEIDER, Ueber die Einfuhrung der neuen Kalender in Danemark und Schweden (Berlin, 1898); LEOOUBLE, La Connaissance des Années det des Jours (Soissons, 1887), a convenient little handbook; ARBUTHNOT, The Mysteries of Chronology (London, 1900).