Determining sunset in mountainous area
2-7-2016 | IslamWeb
Question:
The time of Maghrib starts when the sun has completely set. This means that when the disk of the sun goes below the horizon, then the time for Maghrib starts. However, if a person lives in a very hilly area and tries to look at the sun with his eyes to determine when it sets, it will appear to set earlier behind the hills. A note on this can be found on the following site: http://www.thenakedscientists.com/HTML/questions/question/2794/ Does this mean that if one lives in a very hilly area and tries to determine whether the sun has set or not, he should wait a little while before praying the Maghrib or break the fast since the sun will appear to have set earlier?
Answer:
All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger.
We apologize for not browsing the attached website or issue a judgment about the information in it. However, we shall explain the general ruling in this regard:
Sunset, according to the sharee'ah, starts when the disk of the sun has completely disappeared in an open place where there are no barriers blocking the sight of the sun, such as a mountain or the like. If there is a barrier obstructing sight of the sun, then sunset is marked by the appearance of darkness in the direction of the East.
An-Nawawi 
said:
“Our scholars held that the sunset is established when the whole disk of the sun falls below the horizon. This is visible in the desert. Shaykh Abu Haamid and other scholars said, 'There is no significance attached to the sun rays that remain visible in the horizon after the sun has completely set. Rather, its time (i.e. the Maghrib prayer) starts even with the remaining sun rays. In urban areas with buildings or mountains, the criterion is that the observer does not see any of the sun rays on the walls and mountain tops and that darkness advances from the East.” [Al-Majmoo’]
Mawaahib Al-Jaleel reads, “The Maghrib prayer becomes due when the disk of the sun has completely disappeared in an open place where there are no mountains. As for a place where it sets behind a mountain, one should look towards the East; if it is dark, then that is proof that the sun has set.”
Allaah knows best.