Black vs. red thread marking sunset

[brianmelendez]

I vaguely recall hearing once that, in Jewish tradition, a sunset ended at the point when you couldn’t distinguish a black thread from a red thread held up against the sky. Can anyone confirm my recollection, or supply any more detail?

[DPRK]

The Muslims and Jews talking about black, white, and red threads for determining the time of twilight/dawn are not talking about textiles. They mean the colour of the sky.

E.g. (from Allameh Tabatabaei’s Tafsir al-Mizan:)

Qur’an: and eat and drink until the white thread becomes distinct unto you from the black thread (of night) at dawn‑break:

There are two dawns: the first is called the ‘false’ dawn because it vanishes in a short time. It is also called the tail of the wolf because it looks as if a tail is raised. This false dawn is a beam of light like‑a vertical column; it appears at the end of the night on the eastern horizon when the sun reaches an angle of 18 degrees below the horizon. Then it gives way to a horizontal line of light which looks as if a white thread has been stretched on the horizon. This is the second dawn. It is called ‘true’ dawn because it truthfully announces the arrival of day‑time and is connected with sunrise.

Obviously, the white thread in this verse means the “true dawn”; and “at”(min) , in “at dawn‑break”(mina’l‑fafri) , is explanatory so as to clarify this phrase. This sentence is metaphorical and it likens the streak of light, stretched across the horizon, to a white thread and the darkness of night adjoining that light to a black thread.

On the red thread:

Prayer times in Judaism and Islam | Judaism and Islam – comparing the...

[Isha’a / Shema]:

This can be said from twilight, according to the Hanafi school, Isha begins when complete darkness has arrived and the white twilight in the sky has disappeared. According to the Maliki and Shafi’i schools, the time begins when the red thread has disappeared from the sky. It can be said until the beginning of dawn when the time for Fajr prayer begins. It should ideally be said within the first third of the night but in extenuating circumstances it can be recited up until dawn.

[brianmelendez]

Thank you, DPRK, for an erudite and helpful answer!

[LSLGuy]

You can sure tell these religions were invented in a place with very few clouds.

[Chronos]

So if the threads aren’t a physical test, and are instead just vague poetic language that means “not until it’s the real sunset”, then what physical test do you use to determine the real sunset?

[DPRK]

The “thread” test determines the time of astronomical twilight, i.e., when the sky is fully dark. Sunset is the instant when the sun disappears below the horizon.

In today’s practice (see @LSLGuy 's comment about clouds!) the times can be pre-calculated. Sunset is more or less straightforward, and the instant of twilight can be approximated by taking it to be when the geometric centre of the sun is a certain angle below the horizon. Note that different traditions take different values for this angle (I asked about this in a previous thread!) and it is not necessarily the same angle for dawn and dusk. E.g., the table here gives, for the Isha angle in question, variously 18 degrees, 15, degrees, 17, degrees, and 90 minutes after sunset, according to different authorities. The page further explains that calculation is OK and you don’t need to watch the threads with your own eyes.

[LSLGuy]

Continuing @DPRK’s comments but into the secular modern realm, “twilight” is actually a more complicated topic than many folks know.

In addition to sunrise, there are 3 precisely defined dawns that happen sooner. Astronomical dawn, nautical dawn and civil dawn in that order. Likewise at the end of the day comes sunset, civil dusk, nautical dusk, and finally astronomical dusk. So 8 distinct events/point of time per complete daily cycle.

The Army cares a bunch about these various times of day. And, inevitably, has an alphabet soup of acronyms initialisms for them. All sorts of bad guys doing bad stuff becomes much more doable just as there’s enough natural light to attack by. So you need to know when to have already rousted your men to sit pre-dawn alert against those potential attacks. Night vision equipment existed back in my day, but was far from universal. I don’t know how much attention they pay the details of twilight in combat zones nowadays.

See here for more

Twilight

Which, tying back to my earlier comments, also points out that although the dawns/dusks are defined astronometrically, weather and atmospheric pollution makes a practical impact; significant high altitude particulates can extend functional twilight deeper into the night at both ends, and heavy clouds can extend it farther into the day at both ends.