averaging the motions of the sun and moon. This averaging consists in taking their highest and lowest rates of motion, finding the mean, and adding it up.
Since the usual pattern among the months of the year is for the first to have thirty days and the second twenty-nine, all these forms of calculation and tabulation assume that the first month has thirty days, the second twenty-nine, and the year 354 days. Every few years, they must record an additional day, making that year 355 days; they add it to Dhū al-Hijjah, for example. This is the basis of their reckoning.
This reckoning agrees with reality most of the time, since this is the predominant pattern of the months. It is not, however, an invariable rule: two, three, or more consecutive months may each have thirty days, and two, three, or more consecutive months may each have twenty-nine days. Their tables and calculations are thus disproved, and their religion, which is not upright, is undermined. This is one of the reasons why tabulation and calculation must not be relied upon in determining the lunar crescents.
Such is the method of these renegade innovators who have departed from the law of Islam: they calculate the month in question from the months preceding it, either in every year or in some years, and record the results.
As for the second group, they are certain jurists of Basra who held that his words: