Julian Period

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Related to Julian Period: Modified Julian date, Julian Day Numbers

Julian Period


a time interval of 7,980 years that is used in astronomical and chronological calculations. The system of continuous reckoning of time in Julian days (JD), proposed in the late 16th century by the French scholar Joseph Justus Scaliger, is extensively used in science. In the system, every moment of time is designated by the number of days (with allowance for fractions of a day) that have elapsed since the start of the current Julian period, which is taken to be Greenwich mean noon (12:00 universal time) on Jan. 1, 4713 B.C. Tables published in astronomical yearbooks and handbooks are used to determine the number of full days that have elapsed up to the time of interest.

If the calculations are made to an accuracy of a day, it is convenient to treat the figures in the tables as the numbers of the corresponding dates. In this case, to determine the number of days that have elapsed between two historical events, one need only subtract the number of the date of the first event from the number of the date of the second event.

In the 1960’s the numbering of days in modified Julian days (MJD) was introduced. In this system, the start of each day is at Greenwich midnight, which is more convenient in practical calculations. Modified Julian days are related to Julian days by the formula MJD = JD − 2,400,000.5.

The Julian period is the least multiple of three cycles: the 28-year solar cycle, at the end of which the days of the month return to the same days of the week; the 19-year lunar cycle (the me-tonic cycle), at the end of which the phases of the moon return to the same dates; and the 15-year cycle of indiction, on completion of which an extraordinary tax was collected in ancient Rome (the last fact, however, has no practical significance for reckoning time in Julian days).

Table 1. Number of days elapsed from the start of the period to noon on January 0 of a leap year
YearJanuary 0YearJanuary 0

In order to determine the number of a given date in the 20th century in the New Style (the Gregorian calendar), the following are added: (1) the number of the standard January 0 of the leap year preceding the given year (Table 1), (2) the number of days elapsed from this day to day 0 of the given month and year (Table 2), and (3) the given day of the month. For example, June 22, 1941 = 2,429,629 + 517 + 22 = 2,430,168 JD and May 9, 1945 = 2,431,090 + 486 + 9 = 2,431,585 JD. Subtracting the first number from the second, we find that 1,417 days have elapsed between the Great Patriotic War and Victory Day. To determine the number of days in the 19th, 18th, and 17th centuries from the number obtained from Table 1 for the corresponding year of the 20th century, it is necessary to subtract 36,524 once, twice, and three times, respectively. For 1900 (see Table 1) and for the years 1800 and 1700, which are nonleap years in the New Style, in Table 2 1 and 32 should be selected for January 0 and February 0 in the first column instead of 0 and 31, respectively.