736. If a small stick or something similar in shape—called a gnomon—is driven perpendicularly into a flat ground, when the sun rises in the morning, the shadow of the stick will point towards the West. The more the sun rises, its shadow decreases. In most cities, the shadow reaches its shortest point at the beginning of canonical mid-day (Ûuhr al-shar‘Ð) after which the growth of the shadow will be from the East. The further the sun declines towards the West, the more its shadow will increase.
Hence, when the shadow reaches its shortest point, and thereafter starts to increase, it will indicate that the time for canonical mid-day has entered. However, in certain cities, the shadow completely disappears. When it appears once again, it will indicate that the time for canonical mid-day has entered.
737. The time for Ûuhr and ‘aÒr is between the time the sun begins to decline from its zenith and sunset. If a person deliberately prays ‘aÒr before Ûuhr, his prayer is invalid, except in the case where there is only sufficient time to perform one prayer. In such a case, if one has not yet performed his Ûuhr prayer, he should first pray his ‘aÒr and then his Ûuhr with the intention of qaÃÁ. However, if a person forgetfully prays his ‘aÒr before his Ûuhr, his prayers are valid. He consider it as his Ûuhr prayer and then pray another four rak‘ah with the intention of ma fi dhimmah, based on obligatory precaution.
738. If a person forgetfully begins ‘aÒr prayers before Ûuhr and realizes his mistake whilst praying, he must change his intention from Ûuhr to ‘aÒr prayers. That is, he should make the intention that: all that which I have recited up till now and that which I have occupied myself with has been Ûuhr prayer. After completing his prayer, he should pray the ‘aÒr prayer.
739. Friday prayer is wÁjib al-ta‘ayyuni (a wÁjib without alternatives) in the presence of an infallible imam (Peace be upon him) or somebody appointed by the Imam. In his occultation however, the mukallaf is free to choose between offering Ûuhr prayer or Friday prayer, provided its conditions are met. Although the precaution is Ûuhr prayer, it is better to offer the Friday prayer.
740. The time for Friday prayer is constricted. Based on obligatory precaution it should not be delayed from the beginning of the time for Ûuhr prayer, after it has been established with certainty, satisfaction or the conjectural proofs authorized by the sharia.