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Mercedonius, also known as Intercalaris, was the intercalary month added in leap years of the Roman calendar. The resulting year was either 377 or 378 days long. The exact mechanism by which this was done is not clearly specified in ancient sources. Modern scholarship holds[1] that Februarius was truncated to 23 or 24 days, to be followed by an intercalary month of 27 days. Earlier scholars such as Ideler held the view that in intercalary years February's length was fixed at 23 days and it was followed by a variable length mensis intercalaris with 27 or 28 days. Whatever the details, the five days a.d. VI Kal. Mart. to Prid. Kal. Mart., normally located at the end of February, are the concluding days of the mensis intercalaris in intercalary years. As a result, the year was lengthened by a total of 22 or 23 days. Romans believed that the month had been added to the Roman calendar, along with Ianuarius and Februarius, by King Numa Pompilius in the 7th century BC. The name Mercedonius comes from merces, meaning wages, as workers were paid at that time of year.

This month was supposed to be inserted every two or three years to align the 355-day common year with the tropical year.[2] The decision whether to insert the intercalary month was made by the pontifex maximus, supposedly based on observations to ensure the best possible correspondence with the seasons.[3] Unfortunately the pontifex maximus (whose office was generally held by a politician or soldier, notably Julius Caesar during the so-called Years of Confusion) often neglected to insert the month at the proper time, or deliberately inserted it early or late to allow some officials to stay in office longer or force others out early. Such unpredictable intercalation meant that dates following Februarius could not be known in advance; and Roman citizens living outside of Rome would often not know the current date.

The month was eliminated by Julius Caesar when he introduced the Julian calendar in 46 BC.


  1. ^ Following the discussion of intercalation in A. K. Michels, The Calendar of the Roman Republic (Princeton, 1967) 145–172, the standard reference on the pre-Julian calendar. Specialist studies of the pre-Julian calendar published since 1967 which endorse this view include papers and books by A. E. Samuel, P. S. Derow, P. Brind'Amour, V. M. Warrior, J. Rüpke, R. Hannah, and C. J. Bennett. Against: H. Chantraine, dismissed by Brind'Amour as special pleading. The earlier view described below is still repeated in some non-specialist works, such as the Encyclopædia Britannica or generalist surveys of calendrical history such as those of D. E. Duncan, G. R. Richards or A. Aveni.
  2. ^ "The lunar year of 354 days fell short of the solar year by 1114 days;—this in 8 years amounted to 90 days. These 90 days he divided into 2 months of 22 and 2 of 23 days, and introduced them alternately every second year for two octennial periods: every third octennial period, however, Numa intercalated only [...] 3 months [...] because he adopted 355 days as the length of his lunar year"
    Daniel Spillan, Livy's History of Rome, Book I. 19. Footnote 24.
    This is the theory of Macrobius in Saturnalia (c. AD 430).
  3. ^ "Their management was left to the pontiffs—ad metam eandem solis unde orsi essent—dies congruerent; 'that the days might correspond to the same starting-point of the sun in the heavens whence they had set out.'"
    D. Spillan, Livy's History of Rome, Book I. 19. Footnote 24.

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