Golden Number , the place of a given year in the lunar cycle. It is used to determine on what day the paschal moon falls, and thus to find Easter day. The mean length of the lunar cycle agrees exactly with 19 Julian years. (See Calendar, Lunar Cycle, and Year.) The new moons were indicated before the reformation of the calendar by means of the lunar cycle, which restores them to the same days of the civil month, and places them on the same days in any two years that occupy the same rank in the cycle. Consequently a table of the full moon's phases for 19 years will serve for any year whatever, when we know its number in the cycle. The year preceding the commencement of our era, when the new moon fell on the 1st of January, is supposed to be the beginning of the cycle, which gives this rule for finding the golden number: Add 1 to the date and divide the sum by 19; the quotient is the number of cycles elapsed, and the remainder is the golden number. When the remainder is 0, the proposed year is the last or 19th of the cycle.
The new moons determined in this manner may, however, differ from the astronomical new moons as much as two days, because the sum of the solar and lunar inequalities, compensated in the whole period, may in certain cases amount to 10°, and thereby cause the new moon to arrive on the second day before or after the mean time. The Gregorian calendar rejects the golden numbers, as they are only adapted to the Julian calendar; the suppression of the ten days rendered it necessary to place them ten lines lower, and the centenary intercalation required them to be changed every century. Their place is supplied by another set of numbers called epacts. (See Epact.) - The golden numbers were introduced into the calendar about the year 530, but were disposed as they would have been if they had been inserted at the time of the council of Nice. It was usual to mark them | in the calendar with red or gold.