In the Torah, night precedes day for all matters. This is derived from the description of the world’s creation, about which the Torah states: “And there was evening and there was morning, day one” (Bereishit 1:5). This tells us that each 24-hour day begins at night, and thus Shabbat, the seventh day, begins at night. An important concept is enfolded within this Jewish outlook – night and darkness precede day and light. First questions and dilemmas arise and one is mired in darkness and uncertainty, but from this answers emerge and light shines upon him. This is also true of our history. At first we were enslaved to Pharaoh in Egypt, but from there were freed, received the Torah, and entered Eretz Yisrael. This is how it always is for the Jewish people – darkness and troubles are followed by light and redemption. First we must deal with our problems, but from within them we are elevated and refined.
In contrast, when it comes to the nations of the world, day precedes night. Nation after nation ascends the stage of history, makes a vast noise, and shakes up the world. But when difficulties arise and problems begin, the night draws nearer, and finally the nation declines and disappears. It happened to the Babylonians, the Persians, the Greeks, and the Romans. The secret of Jewish immortality is connected to the fact that for us, the night precedes the day.
Since the night precedes the day, the seventh day begins at the beginning of the night. But the Sages were uncertain as to exactly when night begins. Is it when the sun sets and is no longer visible to us, or is it when it becomes dark and three medium-sized stars can be seen in the sky? In other words, are day and night defined by sun or by light? In Eretz Yisrael, about twenty minutes separate sunset and the emergence of the stars, though this interval fluctuates based on the time of year and the particular locale’s elevation above sea level, as explained in n. 1 below.
Another unique feature of Judaism is that not every question has an absolute answer. Doubt and uncertainty occasionally play a role, and the present halakha is an example of this. The time between sunset (shki’at ha-ĥama, or “shki’a”) and the emergence of stars (tzeit ha-kokhavim, or “tzeit”) is classified as a time when it is uncertain whether it is day or night and is called “bein ha-shmashot.”
In practice, for all Torah laws, including Shabbat, we follow the well-known principle: “Safek de-Oraita le-ĥumra,” that is, we rule strictly when in doubt about Torah law. Therefore, Shabbat begins at shki’a and ends at tzeit.
Shki’a refers to the time when the sun disappears from sight entirely (Responsa Maharam al-Ashkar §96). It would seem that nearby hills that hide the sun are not taken into consideration; rather the horizon is determinant. However, it is possible one should take into account distant mountains that span the entire horizon. See Peninei Halakha: Prayer ch. 11 n. 7 regarding hanetz ha-ĥama (sunrise). In terms of tzeit, several uncertainties arise, for different times have been suggested for the appearance of three middle-sized stars. The uncertainty is rooted in the question of who must be able to see these three stars. Is it those with perfect or impeccable vision who know about star charts and know where the first stars appear in each season, or is it determined by the average person on the street? It would seem that people with impeccable sight can see three stars once the sun sinks more than 4.8 degrees below the horizon. Regular people can see three stars when the sun is 6.2 degrees below the horizon. One should also know that there are three factors that impact the interval between shki’a and tzeit: 1) The season; in the spring and fall, the sun sets along a line that is perpendicular to the horizon, and therefore the interval is shorter, while in the winter and summer it sets on a line that intersects the horizon diagonally, so the interval is longer. 2) Elevation; the higher the elevation of a place, the longer the sun is visible (and thus the later shki’a is). Tzeit, however, will be at the same time regardless of elevation. Therefore, in higher places, the time separating shki’a and tzeit is shorter. 3) Latitude; the closer one is to the equator, the shorter bein ha-shmashot is. As one approaches the poles, it lengthens. There are places where it can last for hours and even days.
In practice, in Eretz Yisrael at sea level the time of bein ha-shmashot based on those with good vision (4.8 degrees) in the middle of spring and fall is about 19 minutes, and at the height of the summer it is about 21 minutes. Based on people with regular vision (6.2 degrees), in the middle of spring and fall bein ha-shmashot is 25 minutes, and at the height of the summer it is nearly 30 minutes. In Jerusalem, whose highest hills are about 830m above sea level, shki’a is about five minutes later, so bein ha-shmashot is about five minutes shorter. Thus, in Jerusalem, bein ha-shmashot based on those with good vision in the middle of the spring and fall is about 14 minutes, and at the height of summer it is 16:22 minutes. Basing it on people with normal vision, in spring and fall bein ha-shmashot is about twenty and a half minutes, and at the height of summer is about 24 minutes. (One who explores this issue will come to understand how these calculations work out with the traditional time for bein ha-shmashot as equal to the time it takes to walk three quarters of a mil. All this is explained at length in Harĥavot and briefly in Peninei Halakha: Prayer ch. 25 n. 3 in the context of the time for the nighttime recitation of Shema.)
Everything written here so far follows the approach of the Ge’onim. However, Rabbeinu Tam has a different approach. He maintains that after shki’a, the day continues for the time it takes to walk 3.25 mil, or approximately 58.5 minutes. Bein ha-shmashot begins only after that, and lasts for the length of time it takes to walk three quarters of a mil, or approximately 13.5 minutes. This is followed by night. Although many follow this approach, including SA 261:2, it is still very puzzling, because in fact we see three medium-sized stars much earlier than R. Tam’s reckoning of tzeit (see BHL ad loc.). In fact, in Eretz Yisrael and its environs, the custom has always followed the Ge’onim. Some explain that Rabbeinu Tam’s approach was based on his location. He lived in France, which is further north, and there bein ha-shmashot lasts longer. In any event, common practice is not to take R. Tam’s opinion into account (Tzitz Eliezer 17:62; Shemesh U-magen 1:5; Ha-zemanim Ba-halakha chs. 43-45, 48, 50). Some write that ideally one should be stringent like Rabbeinu Tam and assume that Shabbat ends 72 minutes after shki’a (Yabi’a Omer 2:21). See Harĥavot.