Allaah, Ta'aala, says:
أُحِلَّ لَڪُمۡ لَيۡلَةَ ٱلصِّيَامِ ٱلرَّفَثُ إِلَىٰ نِسَآٮِٕكُمۡ*ۚ هُنَّ لِبَاسٌ۬ لَّكُمۡ وَأَنتُمۡ لِبَاسٌ۬ لَّهُنَّ*ۗ عَلِمَ ٱللَّهُ أَنَّڪُمۡ كُنتُمۡ تَخۡتَانُونَ أَنفُسَڪُمۡ فَتَابَ عَلَيۡكُمۡ وَعَفَا عَنكُمۡ*ۖ فَٱلۡـَٔـٰنَ بَـٰشِرُوهُنَّ وَٱبۡتَغُواْ مَا ڪَتَبَ ٱللَّهُ لَكُمۡ*ۚ وَكُلُواْ وَٱشۡرَبُواْ حَتَّىٰ يَتَبَيَّنَ لَكُمُ ٱلۡخَيۡطُ ٱلۡأَبۡيَضُ مِنَ ٱلۡخَيۡطِ ٱلۡأَسۡوَدِ مِنَ ٱلۡفَجۡرِ*ۖ ثُمَّ أَتِمُّواْ ٱلصِّيَامَ إِلَى ٱلَّيۡلِ*ۚ وَلَا تُبَـٰشِرُوهُنَّ وَأَنتُمۡ عَـٰكِفُونَ فِى ٱلۡمَسَـٰجِدِ*ۗ تِلۡكَ حُدُودُ ٱللَّهِ فَلَا تَقۡرَبُوهَا*ۗ كَذَٲلِكَ يُبَيِّنُ ٱللَّهُ ءَايَـٰتِهِۦ لِلنَّاسِ لَعَلَّهُمۡ يَتَّقُونَ
It is made lawful for you to have sexual relations with your wives on the night of As-Saum (the fasts). They are body covering for you and you are the same for them. Allaah knows that you used to deceive yourselves, so He turned to you (accepted your repentance) and forgave you. So now have sexual relations with them and seek that which Allaah has ordained for you (offspring), and eat and drink until the white thread (light) of dawn appears to you distinct from the black thread (darkness of night), then complete your Saum (fast) till the nightfall. And do not have sexual relations with them (your wives) while you are in I'tikaaf in the mosques. These are the limits (set) by Allaah, so approach them not. Thus does Allaah make clear His Ayaat to mankind that they may become Al-Muttaqun. (Al-Baqarah, ayah 187)
Ibn 'Abbaas (radiallaahu 'anhumma) said that the Messenger of Allaah (salallaahu 'alayhi wa sallam) said: There are two Fajrs (dawns). As for the first, it does not prevent eating (for the fasting person) nor does it permit prayer (the Fajr prayer). As for the other, it prohibits eating (for the fasting person) and allows the prayer.[Ibn Khuzaimah and Haakim]
Shaikh Al-Albaanee (rahimahulaah) said: "And I have seen this myself many times from my home in Jabal Hamlaan in south-east 'Amman [Jordan]. It allowed me to confirm what had been claimed by some devout advisors concerned for the Muslims' worship, that the adhaan of Fajr in some of the Arab lands is called before the time of the true Fajr by a period of between 20 and 30 minutes, even before the false Fajr!
And I had often heard the Iqaamah of Fajr Prayer from some of the masjids being called along with the entrance of the true Fajr, meaning that they had called the adhaan a half hour before its time. This means that they had prayed the Sunnahs of Fajr before the entrance of Fajr's time, and they had occasionally hurried the actual Fajr prayer as well, in the month of Ramadaan, as I heard over the radio station in Damascus while I was eating my suhoor (pre-dawn meal before fasting) last Ramadaan (in the year 1406).
This makes things difficult on the people, forcing them to stop eating before they have to, and it subjects their Fajr prayer to the danger of being unacceptable. And the only reason for this is that the people have relied upon astronomical calculations, and turned away from the legislated times of the prayers. [Allaah said (what means):] And eat and drink until the white thread (light) of dawn appears to you distinct from the black thread (darkness of night)..." For more on this matter, click on The Legislated Times for the Five Daily Prayers.
So Fajr (Dawn) begins at the first light of the dawn of the sun, the first glow on the Eastern horizon where the sun will rise from. This is understood from Allaah's statement (which means): Until the white thread (light) of dawn appears to you distinct from the black thread (darkness of night)..." (Al-Baqarah, ayah 187)
Here is a photo of the lying fajr (al-fajr al-kadhib) and below that a picture of the true fajr (al-fajr as-saadiq):
The Permanent Committee for Scholarly Research and Ifta' said: "Al-Fajr-ul-Kadhib [the lying fajr] appears as a rectangle in the sky from the top to the bottom of the horizon, while Al-Fajr-ul-Sadiq [the true fajr] appears in the sky spreading along the horizon. The time for offering the Fajr (Dawn) Prayer starts with the appearance of Al-Fajr-ul-Sadiq [the true fajr]."
Shaikh al-'Uthaymeen (rahimahullaah) said in his explanation to Bulugh al-Maram (the seventh tape):
"The 'ulama said there are two fajrs. One true and one lying, and there are three differences between them.
The first difference (is) in how they look. So, the true fajr extends from north to south, and the lying fajr it extends from east to west - oblong like the tail of the wolf.
The second difference (is) that the true fajr doesn't have darkness between it and the horizon, rather, its light is connected with the horizon. As for the lying fajr, there is darkness between it and the horizon, meaning the light does not extend until the end of the horizon.
The third difference (is) the lying fajr, its oblong light becomes dark and its light goes away. As for the true fajr, its light persists and becomes stronger until the sun rises. These are the three differences between the true fajr and the lying fajr."
The adhaan for the SubH salat (Fajr) should be called when the white thread appears - picture 2. What you see in the first picture is called "lying fajr," which disappears shortly after it appears. The true fajr only spreads after its entrance.
What Shaikh Al-Albanee (rahimahullaah) alluded to is that many are deceived by the lying fajr (al-Fajr al-Kaadhib) - falsely believing that to be the entrance of Fajr - and therefore offer their naafila (two superogatory rak'ah) before the entrance of the true Fajr, and may even fall into offering their obligatory SubH salat before the entrance of the fajr, due to relying on the astronomical calculations. And astronomical calculations are merely estimates and are prone to human error.
The Permanent Committee of Scholars for Research and Fatwaa stated: "Schedules are a kind of ijtihaad (reasoned deductions); those who produce them are human beings who are erroneous sometimes and correct sometimes. Those who produce them are human beings who are erroneous sometimes and correct sometimes. Therefore, it is not befitting that we assign the exact times of the beginnings and endings of our prayers and fasts based on them, because their beginning and ending times have come in the Book and the Sunnah, and we must rely on what the Legislative evidences have indicated." [Fataawaa al-Lajnah ad-Daa'imah (6/141)]
Here, again, is the above hadeeth of the Messenger of Allaah (salallaahu 'alayhi wa sallam): There are two Fajr (dawns). As for the first, it does not prevent eating (for the fasting person) nor does it permit prayer (the Fajr prayer). As for the other, it prohibits eating (for the fasting person) and allows the prayer.
So notice that he (salallaahu 'alayhi wa sallam) said regarding the first fajr (the lying fajr) ----> ...nor does it permit prayer (the Fajr prayer)... And notice that he (salallaahu 'alayhi wa sallam) said regarding the second fajr (the true fajr) ----> ...and allows the prayer.
And of course, the entrance of the time for the salat is among the conditions of the salat, thus explaining why Shaikh Al-Albanee said, "it subjects their [obligatory] Fajr prayer to the danger of being unacceptable." Allaahul Musta'aan.
And by scrolling toward the middle of this link here, you will - in sha` Allaah - come across a beneficial scientific explanation of the different definitions of fajr by the varying calculation methods, some erroneously defining fajr when the sun is 18 degrees below the horizon, some 12 degrees, and others 6 degrees.
So by examining the matter, you will find that the timetables that many masaajid use to calculate fajr is set at the default method, which is a whopping 18 degrees, thereby falsely setting "fajr" 25-40 minutes before the true entrance of fajr as defined by Allaah in His Noble Book.
Many are not even aware of these differences, much less aware that these timetables actually offer other calculation methods. And the gist of what appears in that beneficial link is:
According to these definitions, fajr should start sometime between the start of nautical twilight and the start of civil twilight, when the sun is between 6 and 12 degrees below the horizon. Therefore, all the other methods of calculating prayer times are incorrect, calculating fajr too early, using angles which are more in keeping with astronomical twilight.
And a summary of all that scientific data is:
1- Prayer calculation methods vary significantly between themselves.
2- Apparently, the prayer calculation methods consistently estimate fajr too early. This would mean that if someone rigidly followed these timetables, they would be likely to offer fajr too early, risking the validity of his salat.
The method most near the true fajr is the method between 6-12 degrees. And since the timetable that most out West use actually offers choices - such as 19.5 degrees, or 18.5 degrees or 18 degrees, or 10 degrees - then the one nearest to accurate is the one that falls between 6-12 degrees.
So if you insist, choose the 10-degree method, but remember these are mere estimates and ijtihaad of men, who are erroneous sometimes and correct sometimes.
Using them as estimates or as backups on days during which clouds obscure the sun is one thing. But to blindly rely on them is an error that can nullify your salat and the salat of the congregation. Instead, learn how to visually spot The Legislated Entrance of the Five Daily Prayers via proofs from the infallible Qur`an and infallible authentic Sunnah.
As Allaah, Ta'aala, said:
وَلَوۡ كَانَ مِنۡ عِندِ غَيۡرِ ٱللَّهِ لَوَجَدُواْ فِيهِ ٱخۡتِلَـٰفً۬ا ڪَثِيرً۬ا
Had it been from other than Allah, they would surely have found therein much contradictions. (An-Nisa`, ayah 82)
And Allaah knows best.
Notes: May Allaah reward those students who helped, those who advised, and those who translated.