Once the sun has set, it continues to descend below the horizon. At this point, the horizon bares a reddish colour. After this for a short period only, the sky adorns a whitish clear colour.
In Astronomy this time is called Dawn, or daybreak. This occurs as a result of the sun rays being broken and dispersed inside the atmosphere. Hence, the reddish and whitish colours that we see are a result of the sun rays being present in the atmosphere. With the use of modern astrological equipment, it is found that this occurs once the sun has descended -17 degrees below the horizon. From this point onwards, the sun’s rays cannot enter the atmosphere and thus night begins.
From midnight, the sun descends to its lowest position and then it starts to rise again. When the sun reaches -19 degrees below the horizon, the dawn takes place in the East, and night comes to an end.
According to Islamic scholars; The redness of the horizon ends when the sun reaches -17 degrees below the horizon, and thus, this is the start time for Isha.
When the sun has descended to this lowest position and starts to rise again, when it reaches the - 19 degrees point, the whiteness before the redness begins, and thus this is agreed upon as the time of Imsaq.