Using astronomical calculations for prayer times

Is using an app like Athan allowed to know when to pray?

Salam Alaikum,

I have found 2 Similar Fatwas but they are with regards to the time of Iftar.

Question:
I live in a rural area somewhat far from the city, and the sound of the adhan does not reach us. Is it permissible for us to determine fasting and breaking the fast in this area based on prayer times from Google (via the internet)?

Answer:
What is obligatory is to observe the actual onset of night, which is by the sun setting below the horizon and the disappearance of the twilight. This becomes clear by the appearance of darkness on the tops of hills from the eastern side, or by the appearance of light on the face of the moon.As for dawn (fajr), there is about one hour and ten minutes between the rising of dawn and sunrise. The matter of dawn is easier, as the fasting person can exercise caution and refrain from things that break the fast a few minutes earlier. May Allah grant you success.

 Question:
In our area, they give the adhan for Maghrib earlier than it should be. I do not break my fast except upon the adhan of Masjid al-Hadi, which is broadcast on Al-Masirah. What is your opinion, what is required of me regarding them, and is it valid to rely on prayer times specified in calendars?

Answer:
The sign of the entrance of night is the appearance of darkness from the eastern direction and the appearance of light on the face of the moon, or the sighting of a star from the northern half of the sky. If any one of these occurs, then night has entered.

My advice would be to use such applications only as a reference, ensuring their timings align with our madhhab. Do not rely on them independently. Ensure that the Adhan timings coincide or are close to our Madhabs prayer timings, then wait a while for until you are sure that the prayer time has definitely started and is present.

The preferred time for ẓuhr begins when the sun passes the meridian—its sign being the shadow’s inclination toward the east after reaching its shortest length—and it ends when the shadow of an object equals its length from the time of that inclination, which is the beginning of the time of ʿaṣr.

The end of ʿaṣr is when the shadow of an object becomes twice its length.

The time of maghrib begins with the sighting of a night star and lasts until the disappearance of the red twilight, which marks the beginning of ʿishāʾ; its end is the passing of one-third of the night.

The beginning of fajr is from the appearance of the spreading light rising from the direction of ʿAdan toward the qiblah until just before sunrise by the amount of time needed to perform one rakʿah.
 

What do I do if I cannot see the spreading light or the disappearance of the red twilight because of buildings, clouds, or light pollution?

Fajr time is the same time as the Sunnis so that would not be an issue.

As for Maghrib, you can check if the Moon has a glow, if the 3 signs are not possible then you can definitely combine it with Isha Salah.