Time of Fajr According to Arabic Linguists
Before elaborating on this issue further, let us look at the meaning of the word fajr as used in the Arabic language:
“Fajr means the light of the morning; because it is the cleaving of darkness from before the light: i.e. the redness of the sun in the darkness of the night; the fajr in the end of the night is like the shafaq in the beginning. It is twofold: the first is called al-fajr al-kadhib (the false dawn); that which rises without extending laterally, which appears black, presenting itself like an obstacle (on the horizon); the second is called al-fajr al-sadiq (the true dawn); which is the rising and spreading (dawn) which appears to rise and fills the horizon with its whiteness, and this is what is called ‘amud as-ssubh; rising after the former has disappeared, and by its rising the day commences, and everything by which fasting would be broken becomes unlawful to the faster..”[2]
Time of Fajr According to the Qur’an
The above definitions, as compiled from various authoritative Arabic lexicons, are directly based on the Qur’an and the Sunnah. Thus, while stipulating the rules of abstinence as applicable to the person fasting, Allah says:
{It is lawful for you to go in unto your wives during the night preceding the [day’s] fast: they are as a garment for you, and you are as a garment for them.
God is aware that you t you acted unfaithfully to yourselves, and so He has turned unto you in His mercy and removed this hardship from you.
Now, then, you may lie with them skin to skin, and avail yourselves of that which God has ordained for you, and eat and drink until you can discern the white streak of dawn against the blackness of night, and then resume fasting until nightfall; but do not lie with them skin to skin when you are are in retreat in the mosques.
These are the bounds set by God: do not, then, offend against them.
It is thus that God makes clear His messages unto mankind, so that they might remain conscious of Him.} (Al-Baqarah 2: 187)
The verse mentioned above has been further explained in a number of traditions. When one of the Companions, namely, `Adiyy ibn Hatim, mistakenly thought this to be an order to determine whether he can distinguish a white thread from a black thread, the Prophet (peace and blessings be upon him) pointed out to him that what is meant is the whiteness of the day as distinct from the blackness of the night, and he said,
“Eat and drink and do not be misled by the ascending white light. Eat and drink until the redness (of the dawn) appears.” (Ahmad)
Dawn According to Natural Sciences
It may be expedient here to refer to the various usages and definitions of the dawn as applied in natural sciences:
“Dawn is the time that marks the beginning of the twilight before sunrise. It is recognized by the presence of weak sunlight, while the sun itself is still below the horizon. …Dawn may easily be determined by observing a thread. When the color of a thread can be determined, changing from black to the thread’s distinctive color, the ‘crack of dawn’ has occurred. This same measure of ambient light can be used to determine, conversely, the instant of dusk.”
There are also more technical definitions of dawn, including the following:
“Astronomical dawn: the moment after which the sky is no longer completely dark; formally defined as the time at which the sun is 18 degrees below the horizon in the morning.
Nautical dawn: the time at which there is enough sunlight for the horizon and some objects to be distinguishable; formally, when the sun is 12 degrees below the horizon in the morning.
Civil dawn: that time at which there is enough light for objects to be distinguishable, so that outdoor activities can commence; formally, when the sun is 6 degrees below the horizon in the morning.”[3]
[2] Cf. E.W. Lane’s Arabic-English Lexicon.
[3] Source: http://www.absoluteastronomy.com/topics/Dawn