[4:103] Once you complete your Contact Prayer (Salat),
you shall remember GOD while standing, sitting, or lying down. ... the Contact
Prayers (Salat) are decreed for the believers at specific times.
[6:96] At the crack of dawn, He causes the morning to emerge. He made the night still, and He rendered the sun and the moon to serve as calculation devices. Such is the design of the Almighty, the Omniscient.
For the Afternoon (Asr) contact prayer, we use the definition that is followed by most muslims (length of any object's shadow equals the length of the object itself plus the length of that object's shadow at noon).
To compute Dawn and Night, the default calculation method is when the sun is 18 degrees below the horizon. This is based on the scientific definition of when dawn and night begin.
Astronomical dawn is formally defined as when the sun is 18 degrees below the horizon in the morning. Similarly, astronomical twilight ends when the sun is 18 degrees below the horizon in the evening (i.e., then Night begins).
According to astronomers: Theoretically, the dimmest stars ever visible to the naked eye—those of the sixth magnitude—will become visible in the evening once the sun falls more than 18° below the horizon (i.e. when astronomical dusk ends) and become invisible when the sun moves to within 18° of the horizon in the morning (when astronomical dawn begins).
This matches the Quranic definition of dawn seen in:
[52:49] Also during the night glorify Him, and at dawn as the stars fade away.
Sometimes, in far northern or southern latitudes, the 18 degree computation leads to an overlap between Night and Dawn. For those cases, we provide an alternative method of computation: using 120 minutes (2 hours) based on information in Appendix 15 of Quran, the Final Testament .
Note: Traditional muslims (for a variety of reasons, including convenience during long summer days) have come up with multiple definitions of when Dawn and Night begin (e.g., degrees below horizon: 14 , 15, 17, 17.5, 17.7, 18, 18.5, 19.5).