The Muslim isha prayer is the last of five required prayers that make up Muslims' obligatory daily prayer ritual. Although the isha prayer is required in both Sunni and Shi'a branches of Islam, rules vary as to how the ritual should be performed.

Isha and Salat

As noted in the Encyclopedia of Islam, Islamic law establishes two general categories of prayer: The first is salat, the five obligatory daily prayers that collectively constitute the second of the Five Pillars of Islam. The salat al-isha, or late evening prayer, is the final required prayer of the day. Sunni Muslims schedule each prayer of the salat at a different time of day, with the maghrib prayer performed at sunset and the isha prayer later at night. Shiite Muslims, however, combine the noon and afternoon prayers and also combine the late evening prayer with the fourth obligatory prayer, the salat al-maghrib.

Isha and Voluntary Prayer

The second category of prayer is voluntary, or dua. Despite not being required, it is not uncommon for voluntary prayers to be customarily recited after the isha prayer. For example, Ayatollah Sayyid Mustafa Musawi advises the recitation of certain verses of the Quran following the isha if a believer is seeking divine help for repaying debts. Similarly, according to Shaykh Sohail Hanif, offering two voluntary prayers after the isha prayer is a way to make up for any deficiencies when performing the required ritual.

Specific Rules for Isha

The late evening prayer consists of four sequences of ritualized prostration and recitation, with the first two recitations spoken aloud. The proscribed timing of the late evening prayer varies not only between Sunnis and Shiites, who combine the late evening with the sunset prayer, but also among different schools of Sunni interpretation. Further complicating the calculation of the proper timing of the isha prayer are accounts of incidents and sayings from the life of Muhammad in the Hadith include two different interpretations. One account indicates that correct time for the late evening prayer starts with the disappearance of red sky at twilight. Another states that the correct time for isha does not start until the onset of complete darkness.

The Mathematics of Prayer

Besides principles from the Quran and the Hadith, or sayings of Muhammad, the proper performance of the late evening prayer relies on mathematical advances by medieval Islamic scholars. As the new edition of the Encyclopaedia of Islam observes in its article on Maqat ("appointed or exact time"), Islamic scholars determining the proper times of day for salat initially relied on basic ancient Greek math based on shadow position, but they later developed more sophisticated trigonometric calculations and astronomical analysis.