# Golden number (time)

The golden number (Latin numerus aureus) indicates the position of the calendar year of the Christian era within the 19-year lunar circle. From this position, the date of Easter depends. She was assigned consecutively to the so-called Golden numbers 1 to 19, wherein the number 1 has been set for the year 1 BC. The year 1 AD, the golden number 2, and 18 AD, the 19th with the Golden Number 1 for 19 AD, the count starts over again.

The most commonly specified formula for determining the Golden Number GZ for the year and the year j is:

The year 2009, the Golden Number ( 2009 1 ) mod 19 = 15 In 2013, the conversion rule (0 → 19) is applied: The 2013 has the golden number (2013 1 ) mod 19 = 0, ie 19

The following formula leads except for the year 1 BC to the same result without having to apply the conversion rule in exceptional years:

Because of their importance in the calculation of Easter ( Computus ( Easter calculation ) ) - the most important festival in Christianity - was this auxiliary variable in late medieval years calendars often written with golden color, thus the name Golden Number to have originated. It is also possible that they already came to this name because of their outstanding importance.