HALACHA OF THE WEEK:
When traveling, it is often necessary to daven before sunrise. While this is permitted under extenuating circumstances [Mishna Berura 58 note 12] - and even if someone must go to work early in the morning [Iggrot Moshe Orech Chaim 4:6] - reciting Shma and Shemona Esrei before the time of "alot ha'shachar" (approximately 72 minutes before sunrise) is prohibited. (For a more detailed discussion of "alot hashachar" see: http://www.vbm-torah.org/archive/halak58/08shachr.doc)
However, even when davening as early as "alot hashachar" is permitted, reciting a bracha on tallit and tefillin that early is questionable.
In the '70s during the oil crisis, Rav Moshe Feinstein permitted a person to make a bracha on tefillin as early as "alot hashachar but did not extend that same leniency to a bracha on a tallit [Iggrot Moshe ibid.].
Instead, a person must don his tallit without a bracha, and then once the time of mi'sheyakir [lit. recognizing] arrives, the person should adjust his tallit and recite the bracha [Orech Chaim 18:3, Mishna Berura 18 notes 6-11]. Practically, many early minyanim do this "adjusting" following the bracha of "yishtabach."
But what is the time of mi'sheyakir?
In Shulchan Aruch [Orech Chaim 18:3; 30:1; and 58:1] it is defined as the time when a person can see his friend (with whom he is somewhat accustomed to seeing)at a distance of four amot (approximately 6-8 feet], or can distinguish between the colors of blue and white. (Interestingly, the Eshel Avraham [Orech Chaim 58] questions if there is a single objective time, or if each person's "mi'sheyakir" is different. He even considered whether moonlight or the reflection of light off of snow could be a consideration in determining this time.)
While we have no definitive source for measuring this time in minutes or hours, the Kaf HaChayim [Orech Chaim 18:18] and Rav Y. Henkin estimated this time to be approximately one hour before sunrise. The Pri Megadim, ruled is was six minutes after "alot hashachar" and Rav Moshe Feinstein ruled it was 35-40 minutes before sunrise [ibid.]. Others measure the time of mi'shayakir using astronomical measures, between 10.2 and 11.5 degrees of the sun below the horizon.
DVAR TORAH
This Erev Shabbat will be Tu B'av (15th of Av), which was in times of the prophets and Chazal, a day of great celebration. Among the reasons for this celebration was because it was on this date that the remains of the Jews massacred at Beitar (the final battle of the Bar Kochba Revolt 135 CE) were finally buried.
In recognition of this, and of the miracle that accompanied it, our sages added the fourth blessing to Birkat Hamazon, the blessing of "HaTov V'HaMeitiv." "HaTov - for the miracle that the bodies did not decompose, and HaMeitiv - that they were finally permitted to be buried" [Brachot 48b].
But why add this blessing to the Birkat Hamazon? What is the connection between the tragedy of Beitar and "bentsching"?
According to Rabbi Meir Simcha of Dvinsk it is because Birkat Hamazon reminds us "why" we eat - to maintain the strength to fulfill G-d's Torah. But even more, the specifics of Birkat Hamazon deal with the creation of a Jewish nation - beginning with the forty years in the desert when G-d fed us the manna (the 1st blessing written by Moshe); the conquest of the land (the 2nd blessing composed by Yehoshua); and the building of Yerushalayim and the Temple (the 3rd blessing authored by King David).
But then, the Temple was destroyed, and the Jews faced a terrible exile, and they did not know what the future would bring.
It was at that point, explained Reb Meir Simcha that the miracle of Beitar occurred. As witnesses to this miracle, our sages saw evidence that through G-d's grace the Jewish people would survive.
Therefore, they added the 4th blessing to the Birkat Hamazon - thanking G-d not just for the building of a nation ("binyan ha'uma") but the survival of the nation ("kiyum ha'uma"). [Meshech Chochma, Deut. 8:10; and also, Rav Kook in Siddur Olat Re'eya I, pp. 361-3).