the conjunction: it is not directly perceptible to the senses, but requires calculation of the sun’s movement through its stations. What the senses perceive is only an approximation. The end of winter and the arrival of the season the Arabs call summer and others call spring are plainly perceptible. By contrast, the sun’s alignment with a particular part of the celestial sphere called such-and-such a zodiacal sign, or with either of the points known as the Head and the Tail, requires calculation.

Since there are twelve zodiacal signs, when the lunar month has recurred twelve times, all of them have been traversed, thus constituting a full year. To this year are tied the rulings of our religion concerning periods fixed by revealed law or by stipulation. These include periods prescribed directly by revealed law, as with fasting and Hajj; those occasioned by a person’s own action, such as the waiting period, the period of sworn abstinence from marital relations, and fasting in expiation or fulfilment of a vow; and those fixed by stipulation, such as the term of a debt, the period for exercising a contractual option, oaths, and so forth.