[Avodah] Defining Tzeis

[Micha Berger via Avodah]

I wrote the following to FB. If anyone can add or make corrections before
the thread there dies, I would appreciate it.

I think I summed up the highlights of the sugya as I understood it
from previous conversations here.

Tir'u baTov!
-Micha

[Someone asked what the sources in question were, so I replied:]

Shabbos 34b-35a, 35b; Pesachim 94a. As Tosafos on Pesachim note, in
Shabbos (34b) R' Yehudah is quoted as saying bein hashemashot (sunset to
the stars coming out) is 3/4 mil (or 2/3 mil, depending on which amora's
version of R' Yehudah you're using, but we hold 3/4), but in Pesachim
he says a person can walk 4 mil after sunset before the stars emerge.

Rabbeinu Tam notes a slight difference in language. It takes 4 mil
"misheqi'as hachamah," in Pesachim, but 3/4 mil from "mishetishqeh
hachamah." RT therefore resolves the two gemaros by saying sunset takes
3-1/4 mil. So, 4 mil from the beginning of sunset will take you to 3/4
mil after sunset, which is R' Yehudah's tzeis.

The Vilna Gaon's resolution of the confliciting gemaros (OC 235 and 261)
is not to split sunset into a beginning an end, but to split tzeis. The
halachic tzeis is 3/4 mil after sunset. The gemara in Pesachim is using
the term colloquially, not halachically, and simply refers to "all the
stars" rather the standard 3.

The Re'eim (R' Eliezer of Metz, Seifer Yerei'im, written in the 1170s)
splits both -- the two gemaras are discussing different sunsets AND
different tzeis. To him, the gemara in Shabbos is saying sheqi'ah
(which we usually translate "sunset") is 3/4 mil BEFORE the sun hits
the horizon, and tzeis is when the sun is at the horizon. Whereas the
gemara in Pesachim then measures how much after the sun is below the
horizon that the sky is fully dark.

Note that both the Vilna Gaon and the Re'eim assume the gemara in
Pesachim is using at least one of the idioms differently than the gemara
in Shabbos, and therefore it has no bearing on halakhah. ... [Material
in response to what someone else wrote, ellided.]

BUT... The Vilna Gaon understands the defining feature to be the emergence
of stars, not the fixed time R' Yehudah gives. And therefore inflates
the 3/4 mil to adjust from Usha (R' Yehudah's home town) to Vilna.

Which brings us to the third topic, on the other side of the page on
Shabbos (35b). Again it's R' Yehudah, quoting Shemu'el's prescription
of which stars to watch for: [If you see] 1 star, it is day; 2 -- bein
hashemashos; 3 -- night. Rabbi Yosi said, "Not large stars that are
visible in the day, nor small stars that are only visible at night,
but medium stars..."

35b cannot describe 3/4 mil after sunset. Even in Usha or Nehardaa
(Shemuel's home town, today's Anbar, central Iraq, on the Euphrates),
you will not see 3 middle sized stars 18 min after sunset. R Michl
Tukaczinsky and R' Dovid Spitzer did the observations in Jerusalem, and
found it takes 31-43 min or 28-40 min, respectively. (Use the smaller end
of the range for rabbinic fasts, like the OP, and the larger for Torah
laws like Shabbos. Jerusalem is closer to the equator than either city,
not that I think it's by enough to matter much.)

Rav Moshe Feinstein similarly gave 50 min for Shabbos, 31 for rabbinic
fasts -- but then, NY's days vary more in length, so a "never later than"
will be more minutes after sunset.

With a computer, we could translate all these numbers into degrees
below the horizon, as that would tell us how dark it would be outside,
and therefore how many stars could be seen, if we weren't bathed in
artificial lighting.

And even then you get into questions about which stars. The western
horizon (the side with the glow of sunset) will show stars well after the
east. Three stars seen at once (near each-other) or total when scanning
the whole sky? Etc...

Rather than decide whether we follow Shabbos 34a or 35a, even those who
do not use Pesachim to shed light on 34a pad the 3/4 mil to plausibly
allow for people to see three medium sized stars. Given the fact that
it's this measure which actually gives the landmark on our clock its name,
assuming it is less definitional than a fixed time delay is difficult.