# Rambam's method for calculating the conjunction of the moon (and sun)

I think I'm missing something basic here...

The Rambam, in Hilchot Kiddush Hachodesh (6:3) says that the interval between one conjunction to the next is is 29 full days, 12 hours of the 30th day, and 793 units of the 13th hour. This, he says, is the length of the lunar month.

But later, after making the point in 6:5 that dividing a lunar month into groups of seven (i.e. weeks) yields a remainder of 1 day, 12 hours, and 793 units, he then goes on to say (6:6) that

When you know the time of the conjunction [of the sun and the moon] for any particular month, and add 1 - 12 - 793, you will arrive at the time of the conjunction of the following month.

Again, I think I'm missing something basic here: why do you have to add that remainder to calculate the next molad? I don't understand, why, in the first place we're dividing a lunar month into groups of seven? Seems arbitrary to me. So, if the interval from one molad to the next is 29 full days, 12 hours of the 30th day, and 793 units of the 13th hour, then why does he later to say to add 1 - 12 - 793?