An Etymological Dictionary of Astronomy and Astrophysics

Of or relating to a → leap year or to the extra day falling in a leap year.

Etymology (EN): L.L. bissextlis (annus) “year containing an intercalary day,” from bisextus, from bis “twice, two, doubled” + sextus “sixth,” because in the → Julian calendar the sixth day before the Calends of March was doubled every four years. Same as → leap and
→ intercalary day.

Etymology (PE): Andarheli, of or relating to andarhel→ intercalation.

Note. Some authors have used the Pers. term behizaki for “bissextile,” however this is problematic as far as the
calendar history is concerned. The term comes from Mid.Pers. vihezagig or vihezakik “movable,” which was the name of the additional whole month of 30 days which Iranians used in their calendar system in the Sasanid era. In fact they did not use an intercalary day every four years, but instead added a whole month after a period of 120 years. The operation was called vihezag. The term originates from Mid.Pers. vihez-
“to move, progress,” vihezag(k) “movement, progression.” Therefore, this dictionary cautions against equating behizaki and bissextile. → embolismic month.