. See HaZemanim BaHalachah chapters 47-51. See also Peninei Halachah Shabbat I, p. 52 and in the footnotes. In Shabbat 35 the time of bein hashemashot (twilight) is explained according to Rabbi Yehudah to be the amount of time it takes to walk three-quarters of a mil, and according to Rabbi Yossi, to be a short amount of time (approximately half a minute). The majority of poskim maintain that Rabbi Yossi’s bein hashemashot is immediately after the time of Rabbi Yehudah’s bein hashemashot. The halachah sides with both of them, meaning that from sunset until a little more than the time it takes to walk three-quarters of a mil is the time of bein hashemashot. (Some maintain that the bein hashemashot of Rabbi Yossi is a few minutes after the bein hashemashot of Rabbi Yehudah, as brought by HaZemanim BaHalachah 40:8-11, and then the time of Ma’ariv is later by a few more minutes.) As we learned earlier in chapter 11, note 1, there are three opinions regarding the amount of time a mil represents: 1) 18 minutes, 2) 22.5 minutes, and 3) 24 minutes. Based on this, three-quarters of a mil is between 13.5 minutes and 18 minutes. In order to incorporate the time of Rabbi Yossi, a little more time must be supplemented, and the result according to this, is that the final time of bein hashemashot is between 14 and 19 minutes.
However, there are differences between the seasons of the year, for in Nisan (March 5th) and Tishrei (October 5th), the light fades faster after sunset and therefore it is possible to see three stars earlier. For instance, what can be seen in Nisan, 19 minutes after sunset, is visible at the peak of summer (June 22nd) almost 22 minutes after sunset, and at the height of winter, 21 minutes after sunset.
It is also important to understand that one who stands high above sea level sees the sunset later. For example, one who stands on a mountain, or in a tower at a height of approximately 800 meters above sea level, will see the sunset about five minutes after his friend who stands at sea level below. In other words, the sky will darken at the same time for both of them and they will see the stars simultaneously, but if each of them were to calculate the amount of time that passed from sunset to tzeit hakochavim, there would be a difference of approximately five minutes. Those who are higher see the sun for five more minutes. Based on this, if the Chachamim were talking about Jerusalem, whose height is approximately 800 meters above sea level (at places where the mountains do not block the horizon), in regard to this calculation, 14 or 19 minutes in Jerusalem is like 19 or 24 minutes at sea level. A few laws can be clarified after understanding this point.
In Israel, on a straight horizon at sea level during the days of Nisan (March 5th) and Tishrei (October 5th), 14 minutes after sunset, the sun descends only 3.75 degrees below the horizon. This approach poses difficulty, for at that time, few, save the most skilled, rarely succeed in seeing three stars. See HaZemanim BaHalachah 41:7 for explanations given by the Acharonim. Some say that perhaps the halachah is based on people with better vision, and others argue that maybe people living at that time had superior eyesight. As aforementioned, it is possible to conclude that the Chachamim were talking about Jerusalem, at a place where the hills do not block the horizon, and then the sunset is five minutes later, and it turns out that 14 minutes after sunset in Jerusalem is like 19 minutes after sunset in the Shefelah (the lowlands region of Israel).
Indeed, according to those who maintain that tzeit hakochavim is 19 minutes after sunset in the Shefelah, the sun descends at that time 4.8 degrees below the horizon, and then experts who know where the first stars should be seen in the sky can see three medium-sized stars in the days of Nisan. However, in the summer days, one must wait 22 minutes for that same situation to arise, and in the winter, 21 minutes.
Some say that three medium stars are actually only visible when the sun descends approximately 6.2 degrees below the horizon, which is 25.5 minutes after sunset during Nisan, 28 minutes at the height of winter (December 22nd), and 29.5 minutes at the height of summer (June 22nd). At that time, most people see three medium-sized stars. Perhaps it somewhat resembles the opinion which maintains that tzeit hakochavim is 19 minutes after sunset in Jerusalem, and at sea level it comes out to approximately 24 minutes after sunset (maybe another minute and a half were added since today eyesight has weakened, or because the abundance of electric lights make it more difficult to see the stars, although perhaps it is truly possible to see the stars slightly before that).
There are those who are more stringent and maintain that in actuality, most people do not see three stars before the sun descends 7.1 degrees below the horizon, which is around 30 minutes after sunset during Nisan and Tishrei at sea level, and in the summer, approximately 35 minutes. This is what has been said in the name of the Chazon Ish. However, it is difficult to explain the words of these stringent poskim according to the Gemara who allotted that the time it takes to walk three-quarters of a mil to bein hashemashot. Additionally, concerning calculations made in open areas in which there are no lights and electricity, people saw three stars before then. See HaZemanim BaHalachah 47:12; 50:6-7. The additional aspects of this issue require further examination. For example, in Tishrei, even though the duration of the darkening of the skies is similar to Nisan, we see the stars slightly later due to the position of the stars in the skies; and the question is, do we go by the stars or according to the darkness? See Hazemanim BaHalachah, chapter 48.
All the aforementioned refers to the time of three medium stars. However, so that people will not come to err, the Shulchan Aruch rules that they must wait until three small stars emerge, meaning, another few minutes. Some say that nowadays, since we use clocks, there is no need to wait. See HaZemanim BaHalachah 51, note 3. Nevertheless, even according to those who are stringent l’chatchilah, if one recited Shema after three medium-sized stars emerged, b’dieved he fulfilled his obligation.
In practice, the prevalent minhag is to start Ma’ariv 20 minutes after sunset, and this is good according to the majority of poskim, as written in many halachic works. At that time, in all places, experts can see three stars, which is when the sun is at 4.8 degrees below the horizon. (Even in the Shefelah in the summer, when the congregation reaches Shema, 22 minutes will have passed.)
Although it is possible to rely on the opinion of the majority of poskim and recite Shema 20 minutes after sunset, it is better to recite Keriat Shema 30 minutes after sunset. If one recites it before then, it is best that he repeats it after praying, although it is not unnecessary to say the Vayomer paragraph. Despite the fact that according to the Sha’agat Aryeh, remembering the Exodus must also be at the proper time for Keriat Shema, nonetheless, according to the Magen Avraham, it is permissible to mention it before then, at the proper time for Ma’ariv (Mishnah Berurah 235:11). Therefore, when a person recited Shema after 18 minutes following sunset, he need not repeat Vayomer. It is not even so crucial to repeat V’Hayah Im Shamo’a since, according to the majority of poskim, the obligation to recite it is rabbinic.
L’chatchilah, so as to take into consideration those poskim who are stringent, it is good to set the time of Ma’ariv 30 minutes after sunset. That way, people living in the hills also fulfill their obligation throughout the whole year even according to the stringent opinions (the sun is 7.1 degrees below the horizon), and those in the Shefelah fulfill their obligation the whole year round, according to most opinions (6.2 degrees below the horizon), and most of the year they fulfill their obligation even according to those who are stringent. In order to always fulfill one’s obligation in the Shefelah also according to the stringent opinions, it is necessary to delay the recital of Barchu during the summer by another three minutes so that when the congregation arrives at Keriat Shema, 35 minutes will already have passed.
The law concerning the emergence of three medium-sized stars is also significant with regard to establishing the time of a circumcision for a baby born at the beginning of Shabbat. If the baby is born after tzeit hakochavim, he must be circumcised on the following Shabbat; and if he is born at bein hashemashot, it is forbidden to circumcise him on Shabbat, and his circumcision is postponed until Sunday. The matter depends on the precise time of tzeit hakochavim is. The Yabia Omer, part 7, Orach Chaim 41:8 rules that 20 minutes after sunset is certainly nighttime, and therefore the baby is circumcised on Shabbat. Shemirat Shabbat Kehilchatah, part 2, 46:45 writes in the name of Rav Auerbach that night begins definitively 25 minutes after sunset. However, the fact that neither opinion makes a distinction between Jerusalem and the Shefelah, and between the different seasons of the year is problematic. The book Otzar HaBrit 9, 5, 7, mentions that there are those who maintain that nighttime officially starts at 24 minutes after sunset, and some say 28 minutes, but that the author himself was taught that it is no less than 32 minutes after sunset. Still, he too, neglects to distinguish between Jerusalem and the Shefelah and the different seasons of the year. It seems best to follow the approach that says that night begins when the sun is 6.2 degrees below the horizon. In Jerusalem that is approximately 20.5-24.5 minutes after sunset, and in the Shefelah, 25.5-29.5 minutes after sunset. It is possible to calculate the exact time with the help of the computer program Chazon Shamayim, based on the secular date, the place of birth of the child according to the lines of longitude and latitude, and the altitude of the location.