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Astro*Dictionary by Michael Erlewine





3 articles for "Julian Calendar"

Julian Calendar [Astro*Index]

Established in 46 BC by Julius Caesar with the advice of Sosigenes of Alexandria, the Julian Calendar was a revision of the ancient local calendar of the city of Rome. It reached its final form about 8 AD, was widely spread by the growth of the Empire, and remained in general use in the West until 1582 when it was further modified into the Gregorian calendar (which is almost universally used for civil purposes).

Originally, the Roman calendar was a lunar calendar with intercalation of months arbitrarily made by the pontifical authorities. Under Julius Caesar, this intercalation was neglected with such frequency that the calendar became about two months out of step with the solar year. To correct the discrepancy, Caesar inserted intercalations into the year 46 BC that increased its length to 445 days, and instituted his reformed calendar beginning with 45 BC. The year 45 BC was a Julian intercalary or leap year; but because of misunderstanding and confusion during the period following the adoption of the revised calendar, the intercalations were incorrectly made until rectified in 8 BC by Augustus, who omitted further intercalations until 8 AD. Adjustments made before the Augustan reform cannot be determined with certainty, but after 8 AD the Julian calendar was used without further change until the Gregorian reform in 1582. The Christian era as the chronological reckoning of the years was first used by the Roman abbot Dionysius Exiguus to designate the years in a table for determining the date of Easter that he prepared as a continuation of previous table in which the years had been designated according to the era of Diocletian. In extending the table, he adopted the equivalence: 532 AD = 248 Diocletian. He prepared the table six years before this in 525 AD but how he determined the correspondence is unknown. His method for designating the years was adopted by others. Over the course of the next few centuries the calendar became established in western Europe as a chronological era. In this system, the Christian era begins with year 1 AD. The immediately preceding year is designated BC1; there is no year 0 in the chronological reckoning. For astronomical purposes, the year immediately preceding 1 AD is designated 0; the other BC years are denoted by negative numbers, each numerically one less than the designation in the historical reckoning. Thus, in the astronomical system, the year -1 corresponds to 2 BC. The year 0 was a leap year.

The Christian era was adopted at different times in different countries with a variety of dates for the beginning of the year. The most common initial dates were December 25, January 1, March 1, and March 25. The different reckonings of the year were known as styles. In the Roman calendar, March had been the first month of the year, as reflected in the numerical names which still survive for the months September to December and in the position of the intercalary day at the end of February; but in 153 BC, January 1 became the first day of the official year with a change in the date of entry into office of the consuls and other magistrates. In Italy, however, down to the 18th century, the years of the Christian era began on March 1 in the Venetian style, on the preceeding March 25 in the Pisan style, and in the Florentine style on the following March 25, while at Rome different styles were used for different purposes. In 14th century England, the Christman-style beginning of December 25 was superseded by the Annunciation-style beginning on March 25. The Circumcision-style beginning on January 1 was substituted in 1752 by the act that introduced the Gregorian calendar. In Scotland, the year had begun officially on January 1 since 1600. The names "old style" and "new style" were used to distinguish not the different dates for the beginning of the year, but the Julian and Gregorian calendars, each of which has been used with different initial dates. The intercalary day was always inserted in February if it fell in a year whose number was divisible by 4. Consequently, when the actual beginning of the year was in March, the years divisible by 4 were not the leap years. Preceding the Christian era, the rule that when the New Year is January 1 the years divisible by 4 are leap years is valid only if the astronomical designations of the year by negative numbers are used.

See also:
♦ Gregorian Calendar ♦ Lunar Calendar ♦ Intercalation ♦ Leap Year
Julian Calendar [Munkasey M.]

A calendar which counts the days forward one byone from a very ancient date. The beginning date is Jan 1, 4713 BC. It is very easy using this calendar to calculate the number of days between dates. Planetary positions are calculated in the Julian Calendar.

See also:
♦ Gregorian Calendar ♦ Lunar Calendar ♦ Intercalation ♦ Leap Year
Julian Calendar [DeVore]

v. Calendar. (Roman Calendar)

After Julius Caesar conquest Egypt, he brought to Rome a Greek astronomer, Sosigines, who with the aid of Marcus Fabius accomplished the first great calendar reform, the Julian Calendar, named after himself, which went into effect through the civilized world in 45 B.C., and continued in use until 1582 A.D. These reforms consisted of the following:

(1) The equinox was returned to March, by inserting two months between November and December of 46 B.C., creating what was thereafter known as "the last year of confusion."

(2) The lunar year and the intercalary month were abolished.

(3) The length of the mean solar year was fixed at 365.25 days, the length at which the ancients had figured it.

(4) To compensate for the accumulation of these fractions into a day every four years, the extra day was inserted at the end of February, then the last month of the year, making it a "leap year" of 366 days.

(5) Renamed Quintilis, the fifth month, after himself, calling it Juli.

(6) Evenly distributed the days among the months, 30 days to the even months, and 31 days to the odd months, except February which had 30 days only in leap year.

(7) Ordered it to take effect January 1, 45 B.C.

However, despite the fact that the Julian calendar went into effect on January 1st, the civil year continued to date from March 25th.

The system was slightly disarranged by Augustus, who renamed Sextilis as August, but refusing to be honored by a shorter month than Julius, ordered it increased to 31 days, reducing February to 28 days except on leap years. Hence, to him we owe the irregular arrangement of the 30 and 3i day months, and the poem we moderns must recite in order to tell which are which. He did, however, render one important service, not without its droll aspects, by suspending leap years for some eleven years to correct a 3-day error which had progressively accumulated because the pontiffs had been intercalating every third instead of every fourth year for some 36 years, and this error of from 1 to 3 days in the chronology of the period has never been corrected.

See also:
♦ Calendar ♦ Gregorian Calendar ♦ Lunar Calendar ♦ Intercalation ♦ Leap Year
Julian Date   JD [Astro*Index]

Direct equivalent of a moment of universal time (UT), espressed using Julian Day Numbers (corresponding to the previous noon) and a decimal fraction of the day elapsed since that previous noon. Thus, for 1970 JAN 14,18:00:00 UT the previous noon was: 1970 JAN 14,12:00:00 UT which corresponds to: JD 2440600.0 Six hours (0.25 days) have elapsed between that noon and the given moment. Therefore, the requested Julian Date is: JD 2440600.250000

See also:
♦ Julian Day Number ♦ Universal Time
Julian Date [Munkasey M.]

A date in the Julian calendar.

A day number from the Julian calendar. For example, day number 2,440,588 is equal to Jan. 1, 1970 in the Gregorian calendar.

See also:
♦ Julian Day Number ♦ Universal Time
Julian Day [DeVore]

For calculating long intervals of time it was found desirable to eliminate the months and years, and number the days consecutively. Hence one such numbered day is identified by the prefix J.D., meaning Julian Day. (v. Day.)

This table of Julian Days is a convenience in computing the number of elapsed days between any two dates. Merely set down the J.D. for the two days and subtract: the difference will be the number of elapsed days. For dates that are separated by only 2 or 3 years, it will be necessary to use only the last three or four digits. These tables can be applied to any years, if care is exercised to see that a table for a bissextile year is used for a bissextile year, and one for a common year is used for a common year.

See also:
♦ Julian Day Number ♦ Universal Time
Julian Day Number [Astro*Index]

To facilitate astronomical calculations, days (beginning at noon) are numbered consecutively from an epoch sufficiently far in the past to precede the historical period. The number assigned to a day in the continuous count is the Julian Day Number, which is defined to be 0 (zero) for the day starting at Greenwich Noon on 4713 BC January 1 of the Julian proleptic calendar. The Julian day number, therefore, denotes the number of days that has elapsed, at Greenwhich Noon on the day designated, since the above epoch.

See also:
♦ Scaliger, Joseph Justus ♦ Epoch
Julian Ephemeris Date   JED [Astro*Index]

Direct equivalence of the Ephemeris Time, expressed using Julian Day Numbers. The fundamental epoch: 1900 JAN 00,12:00:00 ET is equated to: JED 2415020.0

See also:
♦ Ephemeris Time
Julian Year [Astro*Index]

Mean length of a year in the Julian Calendar

See also:
♦ Year ♦ Julian Calendar


Astro*Index Copyright © 1997 Michael Erlewine


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