Amud Hashachar vs. Netz

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:

1. Amud Hashachar: when the darkness begins to give way to light.
2. 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.

Struggling with Rabeinu Tam

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.

Bein Hashmashot

Bein Hashmashot is a short period after sunset about which the halacha is in doubt whether to consider night or day. In his שעורים לזכר אבא מרי ז"ל, Rav Soloveichik takes a closer look at the nature of this doubt.

Incidental vs. Inherent Doubt

The Rav presents two different understandings of the doubt over the status of bein hashmashot:

1. There is an exact moment that divides day from night, but we are unsure when it is. This is a conventional example of a ספק and may be either a doubt over the halacha or the metziut. Maybe there is a halachic dispute as to the exact time when day turns to night. On the other hand, perhaps there is simply a practical problem in measuring the moment precisely.
2. Bein Hashmashot is a time containing both aspects of day and of night simultaneously. This situation creates a halachic doubt of which aspect to follow.

שניות זו מחדשת שתי אפשריות: א) לאמר שזמן זה יהא מקצתו יום ומקצתו לילה, על פי חלוקה כמותית. ב) שכולו מורכב משתי בחינות, יממיות וליליות, והספק לא יתברר על ידי חלוקה כמותית.(שעורים לזכר אבא מרי ז"ל, יום ולילי, ג)

Nafka Mina 1: Rabeinu Tam vs. the Gra

The debate between Rabeinu Tam and the Vilna Gaon over the timing of bein hashmashot is fairly well known.

• The Gra says Bein Hashmashot begins with the astronomical sunset and ends 18 minutes(זמן הליכת 3/4 מיל) later with 3 stars
• Rabeinu Tam says Bein Hashmashot begins 54 minutes(זמן הליכת 3 ו3/4 מיל) after astronomical sunset and ends 18 minutes(זמן הליכת 3/4 מיל) later with complete darkness

Rav Soloveichik uses this debate to demonstrate the two views about the doubt over Bein Hashmashot.

1. Rabeinu Tam says that darkness defines the point at which night time ends, but that we are in doubt as to when precisely it is dark enough to be considered night. It is a typical incidental doubt.
2. The Gra says that we have some psukim that say that darkness/light determines the change-over, while others imply that the visibility of the sun is the determining factor. Therefore, Bein Hashmashot, when the sun has set but it is still light out, has both aspects of day and night, creating an inherent doubt.

 Nafka Mina 2: Rashi vs. Rabeinu Tam

For another example of this debate, the Rav brings Rashi and Rabeinu Tam's debate on שבת לד:ב.

בסוגיא הנ"ל, בראשיתה(לד,ב), ביאר רש"י כי דברי המשנה בזבים, "ראה שני ימים בין השמשות ספק לטומאה ולקרבן", מתייחסים לזב שראה במשך כל בין השמשות בשני הימים, והספק הוא שמא ביה"ש הראשון כולו שייך ליום שלפניו, והבין השמשות השני שייך ללילה שלאחוריו.

Rabeinu Tam asks the obvious question: it should be consistent and therefore either way it should be considered consecutive days. If Bein Hashmashot is considered daytime then it should be for both, and if it's considered night, then it should be for both.

The Rav explains this difficult Rashi as holding that Bein Hashmashot is an inherent doubt. Since the the Zav's first ראיה happened during a time that simultaneously belonged to day 1 and day 2, and his second ראיה happened during a time simultaneously belonged to day 2 and day 3, so there is a ספק whether the ראיות have the status of having happened on consecutive days.

According to Rabeinu Tam, however, the doubt is incidental and therefore the Mishna must be talking about momentary ראיות belonging either to day or to night, but not to both simultaneously.