What Marks the Beginning of the ‘Ishaa’ prayer?
6-8-2018 | IslamWeb
Question:
Salaam ‘Alaykum. Is the disappearance of the red afterglow/twilight (i.e the time for the end of maghrib and beginning of ‘ishaa’) in exact accordance with the end time of nautical twilight or are they two different things (i.e. is the disappearance of the red afterglow/twilight and nautical twilight the same thing or two different things)?Jazaakum-Allaah Khayran.
Answer:
All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad 
is His slave and Messenger.
Islam clarified that every prayer has a beginning and an ending time; Allah Says (what means): {Indeed, prayer has been decreed upon the believers a decree of specified times.} [Quran 4:103]
Moreover, the Sunnah detailed the timings of all prayers, amongst which is the ‘Ishaa’ prayer. The ‘Ishaa’ prayer begins when the Maghrib prayer’s time ends, which is when the red twilight disappears. ‘Abdullah ibn ‘Amr 
narrated that the Prophet said: “The time of the Maghrib continues as long as the brightness of the twilight does not disappear, and the timing of the ‘Ishaa’ prayer continues until midnight.” [Muslim] Commenting on this narration, An-Nawawi said: “The twilight here refers to the red twilight, and this is the opinion of Ash-Shaafi’i
and the majority of the jurists and linguists.” [End of quote]
The nautical twilight is a term used by astronomers, however, we did not find any jurist mentioning it.
What matters in this issue is the redness of the twilight as we stated above. If this redness still appears after the nautical twilight disappears, then it is still the time of the Maghrib prayer and the time for the ‘Ishaa’ prayer did not begin. This is the opinion of the majority of the jurists.
There is a white twilight as well, and this appears after the red twilight disappears. The opinion of Abu Haneefah is that the disappearance of this white twilight is what marks the beginning of the ‘Ishaa’ prayer. We are not sure about the nautical twilight, but perhaps the one who used it wanted to refer to the white twilight, which is different from the red twilight.
To sum up, the sign used to indicate that the time of Maghrib is not finished yet is the red twilight, as we mentioned above, and once this disappears, then the time of ‘Ishaa’ begins; this is the opinion of the majority of the scholars.
Allah knows best.