Today’s page of Talmud deals with the topic of dusk or twilight, known in Hebrew as Bein Hashmashot. It is not quite night, and certainly still not day, so what may and may not be done during that period on Friday afternoon? Has Shabbat begun, in which case work is prohibited, or is it still technically Friday, in which case work is permitted?
שבת לד,ב
תָּנוּ רַבָּנַן: בֵּין הַשְּׁמָשׁוֹת סָפֵק מִן הַיּוֹם וּמִן הַלַּיְלָה, סָפֵק כּוּלּוֹ מִן הַיּוֹם, סָפֵק כּוּלּוֹ מִן הַלַּיְלָה — מְטִילִין אוֹתוֹ לְחוֹמֶר שְׁנֵי יָמִים.
The Sages taught a baraita which discusses the range of problems that arise with regard to the twilight period. Twilight is a period of uncertainty. It is uncertain whether it consists of both day and night, it is uncertain whether it is completely day, and it is uncertain whether it is completely night. Therefore, the Sages impose the stringencies of both days upon it. If there is a stringency that applies on either of the days, one is obligated to adhere to it during the twilight period.
So far so good. But when does this liminal period start and end? There are at lest three opinions:
מִשֶּׁתִּשְׁקַע הַחַמָּה כָּל זְמַן שֶׁפְּנֵי מִזְרָח מַאֲדִימִין. הִכְסִיף הַתַּחְתּוֹן וְלֹא הִכְסִיף הָעֶלְיוֹן — בֵּין הַשְּׁמָשׁוֹת. הִכְסִיף הָעֶלְיוֹן וְהִשְׁוָה לַתַּחְתּוֹן — זֶהוּ לַיְלָה, דִּבְרֵי רַבִּי יְהוּדָה.
רַבִּי נְחֶמְיָה אוֹמֵר: כְּדֵי שֶׁיְּהַלֵּךְ אָדָם מִשֶּׁתִּשְׁקַע הַחַמָּה חֲצִי מִיל
רַבִּי יוֹסִי אוֹמֵר: בֵּין הַשְּׁמָשׁוֹת כְּהֶרֶף עַיִן, זֶה נִכְנָס וְזֶה יוֹצֵא, וְאִי אֶפְשָׁר לַעֲמוֹד עָלָיו
From when the sun sets, as long as the eastern face of the sky is reddened by the light of the sun. If the lower segment of the sky has lost its color, and the upper segment has not yet lost its color, that is the twilight period. If the upper segment has lost its color, and its color equals that of the lower one, it is night; this is the statement of Rabbi Yehuda.
Rabbi Nechemya says: The duration of the twilight period is the time it takes for a person to walk half a mil after the sun sets.
Rabbi Yossi says: Twilight does not last for a quantifiable period of time; rather, it is like the blink of an eye: This, night, enters and that, day, leaves, and it is impossible to calculate it due to its brevity.
We have previously met the opinion of Rabbi Yossi on the very first page of the Talmud. It deals with the obligation to recite three passages from the Torah called the shemah. These must be recited “when you lie down in the evening and when you stand in the morning” (Deuteronomy 6:7). The rabbis debate when these times might be, and link them several events, including sunrise and sunset. These two solar events are relatively easy to agree upon. But what about that period right after sunset and before nightfall, which we call dusk? Or that period right before sunrise and after the night, which we call dawn? Defining these periods of time are much more subjective, since they depend on shades of light, rather than the position of the sun on the horizon.
As we have seen, there are various opinions as to when these liminal periods start and end, including that of Rabbi Yossi, whose underscored the subjectivity of the whole enterprise.
Civil, Nautical, and Astronomical dusk
Jews were not the only ones that had to define the parameters of dusk and dawn. So too did astronomers. Their most widely accepted definitions depend on the position of the center of the sun below the horizon as seen at sea level, as shown below.
Here are the definitions, according to the US National Weather Service.
1. Civil Twilight:
This period begins in the morning, or ends in the evening, when the geometric center of the sun is 6 degrees below the horizon. Under these conditions (and absent absent fog or other restrictions,) the brightest stars and planets can be seen, the horizon and terrestrial objects can be discerned, and in many cases, artificial lighting is not needed.
2. Nautical Twilight:
This begins in the morning, or ends in the evening, when the geometric center of the sun is 12 degrees below the horizon. In general, the term nautical twilight refers to sailors being able to take reliable readings via well known stars because the horizon is still visible, even under moonless conditions. Absent fog or other restrictions, outlines of terrestrial objects may still be discernible, but detailed outdoor activities are likely curtailed without artificial illumination.
3. Astronomical Twilight:
This begins in the morning, or ends in the evening, when the geometric center of the sun is 18 degrees below the horizon. During astronomical twilight, sky illumination is so faint that most casual observers would regard the sky as fully dark, especially under urban or suburban light pollution. Under astronomical twilight, the horizon is not discernible and moderately faint stars or planets can be observed with the naked eye under a non light polluted sky. Point light sources such as stars and planets can be readily studied by astronomers under astronomical twilight.
What can you see at dusk?
What you could see during Bein Hashmashot will depend on where you happen to live. Back in 1952 a very determined group of astronomers demonstrated this with a series of illumination measurements at dusk. One was taken on Sacramento Peak New Mexico (altitude 2,800m) and the second “in the country in Maryland near sea level” (altitude 30m). “It was the impression of the observers that owing to the clearness of the mountain air the overhead and eastern portions of the sky during evening twilight were much darker relative to the western sky at Sacramento Peak than in Maryland.”
Zenith sky brightness values at Sacramento Peak New Mexico. From Koomen M.J. et al. Measurements of the Brightness of the Twilight Sky. Journal of the Optical Society of America 1952: 42 (5); 353-356.
Another way of looking at this is by the change in brightness as we move from civil, through nautical and then astronomical twilight until we finally arrive at night.
Smoothed illuminance E (in lux) on a horizontal surface as a function of the zenith distance of the sun. From Grzegorz V. Rozenberg. Twilight: A Study in Atmospheric Optics. Springer Science, New York 1966. 18.
Here is a description of what is happening, by the Russian physicist Georgii Vladimirovich Rosenberg, who was the Deputy Director of the Institute of Physics of the Atmosphere, (a part of the “Academy of Sciences of the USSR”). It explains why the sky is darker at higher altitudes and cleaner air:
So yes, the more pollution, the more light scattering and the more you can see at twilight as the sun’s setting rays are refracted through the atmosphere. It’s counterintuitive. But hey, it’s science.
the long jewish tradition of being late for Shabbat
בבא קמא דף לב עמוד א
אמר מר ומודה איסי בע"ש בין השמשות שהוא פטור מפני שרץ ברשות
The Master said above: And Isi concedes with regard to one who runs and causes damage at twilight on the eve of Shabbat that he is exempt, because he is running with permission.
According to the talmudic sage Isi, if a person is running and as a result of his haste causes damage to others, he is exempt from damages if this happens at twilight on a Friday afternoon. Being late for Shabbat was clearly so common an occurrence that the rabbis had to make legal allowances for the late comers. But why were these people running? Not to compete a mundane activity, but to welcome the Shabbat Bride herself, as the Talmud continues:
בע"ש מאי ברשות איכא כדר' חנינא דאמר ר' חנינא בואו ונצא לקראת כלה מלכתא ואמרי לה לקראת שבת כלה מלכתא רבי ינאי מתעטף וקאי ואמר בואי כלה בואי כלה
The Gemara asks: What is the reason that running at twilight on the eve of Shabbat is considered to be with permission? The Gemara answers: It is like that which Rabbi Chanina would say, as Rabbi Chaninah would say at twilight on the eve of Shabbat: “Come and let us go out to greet the bride, the queen. And some say that this is what he would say: Come and let us go out to greet Shabbat, the bride, the queen. Rabbi Yannai would wrap himself in his tallit and stand at the eve of Shabbat at twilight, saying: Come, bride; come, bride.
