In this article, we shall elucidate the use of shadow lengths for
regulating prayer times among Zoroastrians, focusing on a part of a Zoroastrian religious book entitled Shāyest Nāshāyest , written in Middle Persian (Pahlavī) language probably around the 9th c. A.D. Then, we will compare this text with a part of another Persian work entitled Yawāqīt al-þUlūm wa Darārī al-Nujūm which is written in the second half of the 6th century after Hijra (the 12th c. A.D.) by Abū Mu¬ammad al-Najjār in Islamic civilization. For this purpose, we will recalculate the given values for the shadow lengths of the gnomon – taken as the height of a man – in both texts and find the best fitted latitudes for them. Finally, we will mention the similarities of the two texts to investigate the possibility of a historical relationship between them. Moreover, the transmission of this timekeeping method among some civilizations will be discussed
