The Solar Cycle is another of these pe riods, the inventor of which present, however, unknown. It consists of 28 years, at the expiration of which the sun returns to the sign and degree of the ecliptic which he had occupied at the conclusion of the preceding period, and the days of the week correspond to the same days of the month as at that time. It is used to determine the Sunday, or dominical, letter, which we shall briefly explain.
In our present calendars the days of the week are distinguished by the first seven letters of the alphabet : A, B, C, D, E, F, G ; and the rule for applying these letters is, invariably, to put, A for the first day of the year, whatever it be, B for the second, and so in succession to the seventh. Should the first of January be Sunday, the dominical, or Sunday, let ter fur that year will be A, the Monday letter B, &c. and as the number of the letters is the same as that of the days of the week, A will fall on every Sunday, B on every Monday, &c. throughout the year. Had the yearconsisted of 364 days, making an exact number of weeks, it is obvious that A would always have stood for the dominical letter : the year con taining, however, one day it follows that the dominical letter of the succeed ing year will be G. For Sundayibeing the first day of the preceding year will be also the last, and the first Sunday in the next year will fall on the seventh day, and will be marked by the seventh letter, or C. This retrocession of the letters will, from the same cause, continue every year, so as to make the domini cal letter of the third, &c. If every year were common, the process would con tinue regularly, and a ofSeVell years would suffice to restore the same letters to the same days as before. But the in tercalation of a day, every bissextile or fourth year, has occasioned a variation in this respect. The bissextile year con taining 566, instead of 565 days, will throw the dominical letter of the follow, ing year back two letters ; so that, as in the present year (1808), if the dominical letter at the beginning of the year be C, the dominical letter of the next year wilt be, not B, but A. This alteration is not effected by dropping a letter altogether, but by changing the dominical letter at the end of February, where the interca lation of a day takes place. Thus, in the present year, C is the dominical letter in January and February, but B is substi tuted for it in March, and continues to be the dominical letter through the re mainder of the year. In consequence of this change every fourth year, twenty eight years must elapse, before a com plete revolution cab take place in the dominical letter, and it is on this &cum, stance that the period of the solar cycle is founded.. A table constructed to chew the dominical letters, for any given years of one of these cycles, will answer for the corresponding years in every successive cycle. The first year of the Christian
era corresponds to the ninth of this cy cle; if, therefore, to any given year of the Christian wra nine be added, and the sum be divided by 28, the quotient will denote the number of the revolutions of the cycle since the ninth year B. C. and the remainder will be the year of the cy cle. if there be no remainder, the year of the cycle will be the last, or twenty eight. e. g. Nine being added to 1808, makes 1817 ; this sum being divided by 28, gives a quotient of 64 for the revolu tions of the cycle, and a remainder of 25 for the year of the cycle. There is ano ther cycle in use, called The Cycle of Indiction. It consists of fifteen years, and is derived from the Ro. mans. Learned men are not agreed as to the origin of it, bin. the most probable opi nion is, that the return of this period was appointed for the payment of some public taxes or tributes. The first year of this cycle is made to correspond to the year 3 B C. If therefore to any given year of the Christian xigt. 3 be added, and the sum be divided by 15, the remainder trill he the year of this cycle. There is however another mode of calculating it. This cycle was established by Constantine A. D.312; if therefore from the given year of the Christian era 312 be subtracted, and the remainder be divided by 15, the year of this cycle will be obtained. In either of these ways, if there be no remainder, the indiction will be 15. We subjoin an ex ample, calculated the methods above specified_ c.../ The Allan Period, some acquaintance with which is indespensable in the study of chronology, will be easily undertood from the preceding account of the cycles. It is formed by the combination of the three, by multiplying the numbers 28, 19, and 15, of the cycles of the suit, moon, and indiction, into each other. The total of years thus produced is 7980, of which the Julian period consists, at the expira tion of which, and not sooner, the first years of each of those cycles will again come together. This period was invented by Joseph Scaliger, as one by which all xras, epochs, and computations of time, might readily be adjusted. The first year of the Christian xra corresponds to the 4714th of the Julian period, andit extends as far hack as 706 years beyond the com mon date of the creation 4004. The year of the Julian period, corresponding with any given year before or since the com mencement of the Christian ;era, may easily be found by the following rule, If the year required be of the latter kind, add to it 4713, the number of years of the Julian period elapsed before the Christian xra, and the sum will be the year required. If it be of the former, subtract the year B. C. from 4714, and the difference will give it.