Time When Shabbat Ends

How is the time that Shabbat ends calculated in Israel and why? For the most part, it seems like the time is calculated as 35 minutes after Shekiah but sometimes it is less (31 for example). Where is the source for using this amount of time?

Shalom, The end of Shabbat is when three small stars appear in the sky close together (Shulchan Aruch 293,2).It is correct to wait until there is no more red in the western sky (Mishna Brurah ibid, 4). According to the major opinion in the gemara and commentators (and followed here in Israel), night time is calculated as equal to the time it takes to walk 3/4 of a "mil" after sunset (see gemara Shabbat 34b). There exists an argument as to how long this is in minutes, as well as an argument as to how exactly to calculate sunset - whether based on what we see, or on sea-level etc. However, this time should be around about 20 minutes after sunset. We generally use this time (or close to it - 18 -25 minutes) to fix the weekday ma'ariv, the end of the fasts etc. When it comes to fixing the time for the end of Shabbat, where there is a danger of breaking a Torah prohibition if we do work too early, we are stricter. This is especially true because one can see with one's eyes that at 20 minutes after sunset there is still much light in the sky, and three small stars are not yet visible. (How to align this reality with the Gemara above is a longer topic we will leave aside for another time). So great Rabbis of the previous generations (especially Rav Tukachinkski zt"l) calculated that these three stars (and the absence of red in the western sky) occurs 32 minutes after sunset in winter, and up to 38 minutes in summer. This is when the sun is 8 degrees below the horizon. This calculation of 8 degrees below the horizon is used in many (most?) countries - and comes out to 50-60 minutes in Europe and America. It gets longer as one goes north. There are other opinions - such as the Chazon Ish who fixed it at 45-50 minutes after sunset. Or those who act according to Rabbenu Tam, who wait 72 minutes. For further sources see Piskay Tshuvot Shabbat (new volume) 293,4. Blessings, D. Sperling