Times are taken from the HM Nautical Almanac Office:
(Astronomical Twilight used for Imsaak and Isha).
The most common values for both Islamic evening and morning twilight’s are 18º and 18º solar depression. There are however some variations of this being used in different places. According to Dr D A King (1975) who has done considerable historical work on Islamic times in the Medieval period, for the evening and morning twilights 16º or 17º and 19º or 20º were employed. According to him the parameters 20º and 16º are those of Ibn Yunus and the parameters 19º and 17º were used by various later Egyptian astronomers and the early tables prepared in Cairo and Damascus have been in use until rather recently. Naser(1976) also refers to a value of 19º for the twilights. King in a later note (personal communication, 1982) has confirmed that the medieval Muslim astronomers generally used 18º/18º or 19º/17º for Fajr/Isha whereas other values such as 20º/16º are also attested but not any value smaller than 16º.
This is of specific interest to us because a few years back, a value of 15º depression for both twilights was proposed by Bagvi (1972; p23 and personal communication, 1983) on the authority of Mualana Rashid Ahmed Ludhianvi of Karachi who apparently made personal observations (discussed by the latter in Ahsan ul Fatwa Vol II, Karachi). I myself followed Bagvi (Ilyas, 1976a; 1976b) and used it for preparing the first South Australian Time Table (Ilyas and Hurrem, 1976) but in light of new information, 18º seems a correct value. This 15º proposition has however met strong objections from various circles in Pakistan. Of special mention is the work by Latiff (1982) who has compiled a lot of astronomical information including a discussion on zodical lights viz astronomical twilights as well as information on observation exercise. Latiff argues for 18º/18º for the evening and morning twilights. For interest, Sheikh Tahir, a renowned astronomer of Malaysia employed 20º/18º for morning and evening and the practice carries on.
The fact that the beginning of morning twilight also marks the beginning of the fasting period, the value for the morning is more critical than the evening one. It is not surprising then that there would be a great concern for any suggestion of a value which appears to be too (small) late that it may even nullify the Fast. Therefore extreme care has to be employed in this regard and very reliable observational data would be required to accept a value which is less than 18º for the morning twilight. The practice of evening/morning asymmetry is attested by Mr Abdul Hafiz Maniar of Surat for India and Dr MM Qurashi of Islamabad (Pakistan) for Yemen (personal communications, 1977; 1982). Presently it seems reasonably proper to adopt the general 18º/18º practice and perhaps make some allowance for the beginning of fast (a usual practice is to allow 10 minutes). From: A modern Guide to Astronomical Calculations of Islamic Calendar, Times & Qibla, by Dr Mohammad Ilyas (Berita Publishing SDN BHD). In this timetable, Isha is Astronomical Twilight (as defined by the Nautical Almanac)
Note For Shi’a Ithna’shari Muslims, Maghrib times:
According to Ayatullah Seestani, he says:
“The obligatory precaution is that as long as the redness in the eastern sky appearing after sunset has not passed overhead, Maghrib Salaat should not be performed.”
Ayatullah Fadhullah says:
“Sunset takes effect when the sun’s disk disappears from the horizon. Thus one can at such a time say one’s Maghrib prayer, and also break his fast if one fasts.”
Ayatullah Khui says,
“It is ihtiyat wajib to wait until the redness that appears in the eastern horizon after sunset has passed overhead.”
Shaykh Sudduq says,
“Sunset (Maghrib) is validated by the fall of the sun’s disk.”
Shaykh Tusi says,
“Maghrib starts at the setting of the sun.”