U

epact

noun

the difference in days between a solar year and a lunar year.
the number of days since the new moon at the beginning of the calendar year, January 1.

noun

the difference in time, about 11 days, between the solar year and the lunar year
the number of days between the beginning of the calendar year and the new moon immediately preceding this
the difference in time between the calendar month and the synodic month

1550s, “number of days by which the solar year exceeds a lunar one of 12 moons;” also “number of days into the moon on which the solar year begins;” from French épacte (12c.), from Late Latin epacta “an intercalary day,” from Greek epaktos, literally “brought in, inported,” verbal adjective of epagein “to intercalate, add, bring forward,” from epi “on” (see epi-) + agein “to bring, to lead” (see act (v.)).