Intercalary:
- interpolated; interposed.
- inserted or interpolated in the calendar, as an extra day or month.
- having such an inserted day, month, etc., as a particular year. [Dictionary.com]
Noted in “Yazılıkaya: A 3000-year-old Hittite mystery may finally be solved,” Colin Barras, NewScientist (22 June 2019, paywall):
But performing these two operations alone isn’t enough to make an accurate calendar, because 12 lunar months add up to only about 354.36 days. The calendar can be brought roughly back in line with the solar year – about 365.24 days – by adding a 13th “intercalary” month every third year, meaning a total of six additional months are added over an 18-year period. Even then, however, the calendar still drifts.
This drift can be largely removed by adding an additional intercalary month every 19th year – making the timepiece run on what is known as the Metonic cycle. Zangger and Gautschy suggest that the Hittites used the procession of 19 deities on the eastern wall to keep track of this. They think a stone marker was moved along this procession once a year to help the Hittites work out when to add extra months over a 19-year cycle (Journal of Skyscape Archaeology, DOI: 10.1558/jsa.37641).
