I found the second half of Rav Soloveichik’s lecture on יום ולילה to be more challenging than the first half on Bein Hashmashot. After changing my mind several times as to what the Rav meant, I'm cautiously confident of my final understanding, so, for what it’s worth, here goes...
In this second half of the lecture, The Rav moves on to discuss the beginning of the halachik day. Astronomically it’s similar to the end of the day in that there are two distinct transitions:
- Amud Hashachar: when the darkness begins to give way to light.
- Netz Hachama: when the sun rises above the horizon.
The Mishna(מגילה כ ב) says that, in general, halachot that apply during the day should be performed from Netz, but bediavad can be performed even from Amud Hashachar.
זה הכלל דבר שמצותו ביום כשר כל היום דבר שמצותו בלילה כשר כל הלילה
Halachik Significance of the Two Times
Rashi(מגילה כ א) understands the Minsha's ruling as meaning that the Halachik day begins unequivocally at Amud Hashachar, but that chazal decided to push back the time to the more easily recognizable Netz so that people don’t make a mistake and perform daytime mitzvot too early.
Rabeinu Tam(יומא לז ב ד"ה אמר), on the other hand, understands that the Mishna's rule is a din torah. The Rav explains that the transition from night to day has two distinct chaluyot. The chalut at Amud Hashachar is based on light/darkness while the chalut at Netz is based on meorot. Ideally we use the later time, but the earlier time can also be used.
Rabeinu Tam's shita comes out strange, according to the Rav. The Rav sets up a chakira and which side does Rabeinu Tam come out on? Both! The day begins with this two-step transition from day to night, yet ends in a moment(albeit one that can’t be identified precisely).
Why should the day start and end so differently? Similarly, why doesn't the same safek of "how dark is dark" that applies to bein hashmashot apply to Amud Hashachar, according to Rabeinu Tam? The Rav doesn't really address these questions...
Exceptions to the Rule:
After explaining how Rabeinu Tam understands Amud Hashachar/Netz, the Rav goes on to explain cases that don't conform to the general rule.
Raveinu Tam says that the morning Shema can only be said from netz, even bediavad. The reason is that it is dependent on זמן קימה, not daytime.
The Rambam(הלכות קרבן פסח ה:ט) says that the ptur of derech rechoka is measured from netz. On the other end of the day, the Raavad says that a Yoledet's days end at shkiyah, without the usual safek applied to bein hashmashot. Similarly, Rabeinu Tam says blood becomes nifsal at shkiah rishona. The Rav explains all these cases by saying that Amud Hashachar is used as the cutoff for questions of day/night, while Netz is used when we need to measure the amount of a day:
כי בכל מקום שאנו עושקים בחלות שם יום או לילה, אז עמוד השחר קובע את היום, אבל במקום שאנו מעוניינים ביום בתור יחידה כמותית, בחלות שיעורין, אז הזריחה היא התחלת שיורי יום.
This is quite a nice little distinction in and of itself, and I found it to be quite persuasive.