# Tzeit HaKokhavim According To The Gr"a & Geonim

I have been investigating the various shitoth regarding sheqiah ("sunset") and sseth ha-kokhavim ("tzeit ha-kokhavim" or, when three stars are visible) and cannot find certain info regarding the shitah of the Gr"a (and the Geonim) regarding the time it takes to walk 3/4 mil (i.e. how long it takes after Sheqi`ath HaHamah to become laylah/ sseth ha-kokhavim).

What I have found is that according to the Gr"a - assuming an 18 minute mil - 13.5 minutes after sunset is definite night. Others use the same shitah of calculation, but have a different definition for halikhath mil ("[the time it takes to] walk a talmudic mile"), ranging from 16 minutes to 24 minutes after sunset. However, unless it is just less possible due to street lights, etc. I have a hard time believing that 14 minutes after sunset in Wisconsin would truly be night with three stars visible. I have also heard that this amount of time was only a reality in Bavel and Eress Yisrael.

So, my question is: What would the shitah of the Gra/the Geonim be in various places in the world? What would the calculation be? When would sseth ha-kokhavim be - according to the Gr"a - in Wisconsin for instance?

• Related: "Rambam, Shkiah, and Tzet HaKokhavim". May 26, 2020 at 11:10
• You're right that there's no way to see three stars in Israel after 13.5 minutes. Even the best experts under the best viewing conditions with computers and such to tell them exactly where to look can't spot it in less than 15, and it's highly implausible that that's what chazal meant when they gave the rule of three stars for general consumption. Mar 11, 2021 at 20:13

When the sun sets, the light decreases in accordance with the sun moving lower and lower than the horizon (of course not the sun is moving but rather the earth is moving). The decreasing of the light is what gradually enables us to see more and more stars, until we reach complete darkness (no light from the sun) and we can see all the visible stars.

Therefore, when we want to calculate tzet hakochavim, we are really calculating a certain level of darkness, which is measured today in the degrees the sun is below the horizon. This tells us the level of darkness and how many stars we can see.

However, the time it takes to reach that level of darkness flactuates based on two major factors: