A year's position in the nineteen-year lunar cycle is called its golden number, and the name is literal: medieval calendars picked the figure out in gold leaf.

It is the key to the whole Easter calculation, and the epact — the Moon's age on the first of January — is derived from it. Working out the date of Easter was the most demanding arithmetic most medieval clergy ever did, and the reason the Church employed computists at all. The word computer descends from that job.