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CHAPTER IX.

Divisions of Time.

imal parts.

TIME, as measured by the celestial luminaries, is divided into periods, cycles, years, months, weeks, days, hours, minutes, seconds, and sometimes farther sexages

Periods, in astronomical reckoning, are large divisions of time. The Chaldean Period is a circle of( 25,858 years. This period respects the motion of the terrestrial poles. At the termination of it the axis of the Earth points to the same stars as at the beginning. ?

The Julian Period is formed by multiplying together the cycles 28, 19, and 15. It consists of 7,980 years. The creation of the world, according to the common computation, was on the 706th year, and the Dionysian era of Christ's birth, on the 4,713th year, of this period. According to some, the birth of Christ was earlier by four years. The Julian period is found of use in comparing the dates of ancient events.

The Dionysian Period, or circle of Easter, consists of 532 years, formed by multiplying the cycle of the Sun, 28, by that of the Moon, 19.

CYCLES ARE REVOLUTIONS OF TIME.

The Cycle of the Sun consists of 28 years. By this cycle the days of the week are brought to the same days of the month the Sun to the same signs and degrees of the ecliptic, with little variation ; and the leap years to the same state as at. the commencement of the cycle. Each of these returns, separately, in a much shorter period. But, by the cycle, they are brought to coincide. The Cycle of the Moon is the Golden Number. It

is a period of 19 years, at the expiration of which, the changes and fulls, with the other aspects of the Moon, return to the same months, and days of the month, as at the beginning, or within a day of the same time.

The Roman Indiction is a period of 15 years, established by Constantine, in the year 312, for indicating the times of certain payments, made by the subjects to the government.

For finding the cycle of the Sun, golden number, and indiction, add 4,713 to the year of the Christian era, and divide the sum by 28, 19, and 15, respectively; the remainders are the numbers sought for

Required the cycle of the Sun, golden number, and indiction, for the year 1831.

the year.

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The Epact is the excess of the solar above the lunar year of 354 days, or 12 mean lunations. It is taken for the age of the Moon, on the first day of January

For finding the Julian epact, multiply the golden number of the year by 11 ; the product, if less than 30, is the Epact. But, if the product exceed 30, divide it by 30 ; the remainder is the epact.

To find the Gregorian epact, the Julian epact must be first found. From this subtract 12, the number of days between the old and new style in the present century; the remainder is the epact required.* If

* The rule of Mr. Pike and some others, to deduct 11 for the difference between the Julian and Gregorian epact, applies to the last

nothing remain, 29 is the epact. If the subtraction cannot be made, add 30 to the Julian epact, and subtract as before.

The golden number and epact are little used at the present time ; especially where accuracy is required. The Roman indiction, still less important, is retained

our almanacs ; why, is difficult to be conceived, unless as it is used in forming the Julian period.

in

A Year.

A complete revolution of the seasons constitutes a year. The difference in the years, the tropical, the sidereal, and anomalistic, has been considered. The civil solar year consists of( 365 days) and in bissextile, of 366. In this manner it is used in the United States, and most European nations. The lunar year consists of (12 lunar months, or mean lunations, computed at 354 days, the surplus arising from the minutes and seconds of the lunation being generally dropped in the computation. In this calendar a (month is added every third year, to make the lunar coincide with the solar year. This month is intercalary, or embolimic.

The Jews computed their time by lunar years. “But, by intercalating no more than a month of thirty days, which they called Ve-Adarkevery third year, they feli 3 days short of the solar year in that time"

The year of the Greeks consisted of 12 months, of 29 and 30 days, alternately taken, comprising 354 days, or about 12 mean lunations. This lunar year was with difficulty connected with the solar year, or the revolution of the seasons, so as to make a particular month fall at the same season in successive years.

century only. The difference between the styles ought, in all computations of the kind, to be deducted. Hence the practice of celebrating the birth-day of Washington on the 22d of February, in the present century, must be erroneous. The celebration ought to be on the 23d of February.)

“ The Olympic games were celebrated every fourth year, during the full Moon, next after the summer solstice, and the year of the Greeks was so regulated as to make this full Moon the first month.) This purpose was effected by intercalations; but these were managed so injudiciously, that, in the time of Meton, the calendar and the celebration of the festivals had fallen into great confusion."

The ancient Romans computed their time by the Lustrum, a period of four years. ) They also reckoned by lunar years, as established by Romulus, till Julius Cæsar reformed the calendar, introducing the system of computation known as the Julian calendar to the present time. In this calenday three years were common, consisting of 365 days each.) Every fourth year, the 24th day of February was twice reckoned, making it consist of 366 days. T'his, being the 6th of the calends of March, was called bis sextus dies, denominated by us bissextile. The intercalary day is now added to the last of February, and from it the year is called bissextile, or leap year. The Julian calendar long prevailed in Europe. But, from observations on the time of Easter, the civil year was found to be too long for the tropical, and another attempt was made to reform the calendar.

The vernal equinox fell on the 21st of March, at the time of the Council of Nice, 325 of the Christian

In 1582, Pope Gregory XIII. observed, that the same equinox happened ten days earlier in the year than it had done at the time of the Nicene Council.) To correct the style, he altered the calendar ten days, ordering, that the 5th day of October should be called the 15th. Thus amended, the style was called the Gregorian, or new style. Though adopted in several European countries, it was not received into England till the year 1752. The Julian calendar, or old style, still prevails in Russia. In the present century, the difference between the old style and the new is twelve days, as before stated.

era.

Pope Gregory stopped not at the alteration of the style. He endeavored to establish a principle, by which the civil year and the tropical would in future coincide. By this principle, bissextile is to be omitted times in

When the centuries of the Christian era are divided by four, if there be a remainder, the year at the end of the century is to be reckoned common; but if nothing remain, the leap year is to be retained, or the last year of the century is to be reckoned bissextile. Though the year 1800 would have been a leap year in the Julian calendar, yet it was considered common in all our almanacs on the Gregorian principle. Our computations, to the present time, are made on the same principle. Thus, at the end of the 19th century, the leap year is to be omitted, there being a remainder, when 19 is divided by 4; but the year 2000 will be considered bissextile; because there is no remainder when 20 is divided by 4.

The omission of three bissextiles in 400 years does not bring the civil year exactly to coincide with the tropical, as computed by La Place. The former still exceeds the latter 20 seconds, 24 thirds. This excess will amount to a day in about 4236 years. The omission of one bissextile in 129 years, would bring the different computations to great nearness.

Months are the principal divisions of a year. These are lunar, solar, and civil. The sidereal lunar month is the time the Moon is passing from a point in the heavens to the same again ; as from a star to the same star, as before stated. But the principal lunar month is a lunation ; or the time the Moon is passing from one change to another. This seems to have given the name to this division of time; or to be the foundation of months. The solar month is the time the Sun is passing one of the signs of the ecliptic, or the 12th part

of a year.

Civil months are of two kinds. The weekly month,

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