A modulus is a divisor. New moon, quarter moon and full moon phases occur in predictible intervals, or periods. Any function to predict them would be classed as a periodic function. The modulus is the length of the period.
The corresponding, or counterparrt of the modulus is the modulo, which is the remainder when the quotient is reduced to a whole number.
5 ) 17 | 3
- 15 |
The quotient is
2 is the modulo.
By the modulo we are always able to predict what phase the moon will be in for a given modulus, or day in its cycle.