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Pamphlet 4 - On Twilight

Astronomy and Twilight

The determination of the exact beginning of night is a complicated matter. Simple observation tells us that the setting of the sun is a long process. First, as the sun begins to set, much of its light falls below the horizon and the sky begins to darken. As it gets darker, the sun descends further until it falls below the horizon. Even then, it is still not totally dark. The indirect rays of the sun still brighten the sky. Eventually, the sun falls so far below the horizon that even the indirect light does not reach the sky. At what point in this long process of twilight does day end and night begin? While the setting of the sun seems like a natural break, there is still sunlight visible for a long time after sunset. If we were to select the time when sunlight is no longer visible, we would need to precisely identify this time. Let us see how this has been addressed in halacha.

As we have already discussed in our essay on the shape of the earth, there is a debate in Pesachim 94b whether the sun at night goes above the firmament (sky), as the sages of Israel claimed, or below the earth, as the wise men of the nations claimed. According to the sages of Israel, the world is flat and the sun travels back and forth over it. During the day, the sun travel from east to west below the firmament and is therefore visible. At night, the sun travels back from west to east above the firmament and is therefore hidden from the world. According to the wise men of the nations, the sun travels around the world. As astronomy has clearly demonstrated, the wise men of the nations were correct. This is reflected in the above Gemara where Rebbe, the compiler of the Mishna, said that the wise men of the nations were correct. In his humility and intellectual honesty, he felt that the wise men of the nations had a better proof and accepted their view.

However, when R' Yehuda tried to determine the length of time between day and night, he assumed the astronomy of the sages of Israel. The process of sunset we described above, in which the sun gradually disappears and then the light as well, was seen by R' Yehuda as the travel of the sun through the firmament. The sun, and then the light, gradually disappear as the sun travels from below to above the firmament. This process, the time between sunset and sufficient darkness for stars to be seen, takes the time equivalent of four mil, R' Yehuda tells us in Pesachim 94a. Clearly, R' Yehuda was basing his calculations according to an understanding that Rebbe attested is incorrect. Yet, R' Yehuda's calculations are the basis for modern halacha! If Rebbe was willing to reject a false understanding, how can we follow this same incorrect view as normative halacha?

The answer, as given by the Minchat Cohen (1:10), is very simple. R' Yehuda observed certain natural phenomena that we can all see on a daily basis. He saw the sun set, the indirect sunlight dissipate, and darkness ascend until the stars became visible. While he interpreted these phenomena based on his pre-Ptolemaic astronomy, his observations are still valid. Stars come out at the same time regardless of one's astronomical views. We do not follow R' Yehuda's astronomy; after all, Rebbe rejected it. However, we do follow R' Yehuda's observations. In fact, a similar understanding of R' Yehuda's approach was offered by the Tosefot Rid on Shabbat 34b in the early 13th century. He follows the same understanding of R' Yehuda as given above - which is the view of Rabbeinu Tam - but explains the times as reflecting different stages of the sun's setting.

Three Views on the Beginning of Night

There is another statement of R' Yehuda that is very important for the determination of night. In Shabbat 34b, R' Yehuda is quoted as saying that bein hashmashot, the period of twilight between night and day, is the time equivalent of three quarters of a mil. Let us quote the exact language because it will be important later on.

When is bein hashmashot? After sunset, when the eastern face [of the sky] is red. If the bottom has darkened but not the top - it is bein hashmashot. If the top has matched the bottom - it is night. These are the words of R' Yehuda... It was said: How long is bein hashmashot?... Three fourths of a mil.

Above, R' Yehuda implied that the time between day and night is four mil. Here, he implies that it is three quarters of a mil. This apparent contradiction regarding the doutbful period of bein hashmashot between day and night is the subject of dispute. (See R' Yehuda (Leo) Levi's Jewish Chrononomy for an extensive treatment of this topic)

The Geonim explained that three quarters of a mil after sunset is when night begins. At that point, the sun is far enough below the horizonand enough of its indirect light has dissipated for it to be considered definite night. Maharam Alshaker (Responsa, 96) says that this was R' Yehuda's final opinion and that he had retracted his earlier statement about four mil. The Maharal (Chidushei Gur Aryeh, Shabbat 34b) and the Vilna Gaon (Orach Chaim 261:12) said that the three quarters of a mil after sunset mark the time when three stars can be seen and definite night begins. The four mil after sunset mark the onset of total darkness when a full sky of stars can be seen.

R' Eliezer of Metz in his Yereim (274) wrote that night begins at sunset and that the three quarters of a mil represent the beginning of sunset. At sunset, when the sun falls below the horizon and there is no longer any direct sunlight, it becomes theoretically possible to see stars. Four mil later the stars become visible to everyone.

Rabbeinu Tam (Tosafot, Pesachim 94a sv R' Yehuda, Shabbat 35a sv trei) suggested that night only begins four mil after sunset when stars become visible. The three quarters of a mil represent the time immediately before the end of the four mil and are a subset of it (according to Ramban's explanation; see Jewish Chrononomy, Hebrew section p. 25).

Thus, according to the Yereim definite night beings at sunset. According to the Geonim definite night begins three quarters of a mil after sunset. And according to Rabbeinu Tam definite night begins four mil after sunset.

While above we have used lengths of time after sunset to refer to the beginning of night, the Minchat Cohen (2:2-3) argued at length that the proper reference should be to the stage in the process of sunset. Since the length of sunset varies based on place and time of year, reference to the position of the sun above or below the horizon gives us a truer perspective. The Talmud only speaks of a day in Tishrei in Israel and the lengths of the three quarters of a mil and four mil are specific to that location and season. However, the Talmud was really referring to the onset of darkness that corresponds to the astronomical phenomenon of sunset. For example, R' Yehuda in Shabbat 34b (quoted above) speaks of the eastern face of the sky turning red and the darkening of the top and then bottom of the sky. R' Yehuda (Leo) Levi wrote (p. 11):

We see that according to most (and maybe all) poskim the times of four mil and three quarters of a mil are not constant and therefore cannot define bein hashmashot according to R' Yehuda... We must therefore say that, according to R' Yehuda, bein hashmashot is determined by the darkness of the sky...

Based on this understanding, R' Levi calculated that definite night according to the Geonim begins when the sun is 4.9 degrees below the horizon (p. 19). According to Rabbeinu Tam, due to a difficulty we plan to address elsewhere, one must take the amount of time between when the sun in the morning is 16 or 20 degrees below the horizon until sunrise and add that to the time of sunset (p. 22). These final times represent the beginning of definite night according to Rabbeinu Tam.

Astronomy in Jewish History

Some have claimed that the Levush (R' Mordechai Yaffe) and the Vilna Gaon created a new approach when they recognized the astronomical problems with Rabbeinu Tam's view. However, that is definitely incorrect. The Geonim, the earliest post-talmudic authorities from whom we have records, essentially followed along the same path that the Levush and Vilna Gaon later took. Maharam Alshaker (Responsa, 96) documents this extensively. Additionally, Professor Shlomo Sternberg of Harvard has even suggested that the view of the Geonim was the standard practice in medieval Europe (Bar Ilan's BDD Journal 6, Winter 1998, p. 77):

[I] do not believe that this novel theory of Rabbenu Tam was ever practiced by anyone during the lifetime of Rabbenu Tam or for the next several centuries. On Friday night, it is inconceivable that Rabbenu Tam could have lit candles long after sunset... Rabbenu Tam's position, at least as far as Friday night is concerned, did become standard practice in Eastern Europe around the end of the 16th and beginning of the 17th century, for reasons that I hope to explain elsewhere.

While we look forward to Professor Sternberg's discussing his assertion at length, we can at least say that Rabbeinu Tam's view was not the universal view prior to the Levush and the Vilna Gaon. Certainly, the Geonim and R' Eliezer of Metz (a student of Rabbeinu Tam's!) disagreed with Rabbeinu Tam's view centuries before the Levush and the advent of modern astronomy.

There is a voice of maximalists who argue that almost every rishon followed Rabbeinu Tam's view. A prime example of this approach is R' Ovadia Yosef. In his Yabia Omer vol. 2 no. 21, Rav Yosef argues this way. However, many scholars are unimpressed by his arguments. Therefore, the "Alter Rebbe", the author of Tanya, wrote in his siddur that most rishonim follow the view of the Geonim. Rav Yosef argues that even Rav Hai Gaon does not follow the view that we attribute to the Geonim. The Minchat Yitzchak (4:23) and R' Yosef Dov HaLevy Soloveitchik (Shiurim Lezecher Aba Mori vol. 1 ch. 5), however, disagreed with this revisionist view of the Geonim. See also Encyclopedia Talmudit sv bein hashmashot However, even Rav Yosef concedes that Rav Nissim Gaon and R' Avraham ben HaRambam held the view that the Vilna Gaon later championed. In other words, even according to Rabbeinu Tam maximalists, the Vilna Gaon did not create his own approach. He used an approach that had been already suggested by rishonim.

Since we are on the topic of modern astronomy, it might be worthwhile to discuss some famous rishonim and early achronim who were also trained in the most modern astronomy of their times.

In the early middle ages, the primary source of information on astronomy was from Arabic. One of the first Hebrew translations of Ptolemy's Almagest was by Yaakov Anatoli in approximately 1235 and was called Chibbur Hagadol Hanikra Almagesti. Yaakov Anatoli also translated Averroes' summary of Almagest as Kitzur Almagesti.

But even before that, R' Shlomo Ibn Gabirol (11th century) had discussed Aristotelian and Ptolemaic astronomy in his Keter Malchut.

R' Avraham bar Chiya HaNasi (early 12th century) compiled astronomical tables called Luchot HaNasi based on the writings of the Arabic astronomer Al Battani. Al Battani also influenced Rambam and is quoted by R' Yehuda HaLevy, R' Avraham Ibn Ezra, and R' Yitzchak Yisraeli. R' Avraham HaNasi wrote three important astronomical works that influenced generations of scholars - Tzurat HaAretz, Sefer HaIbbur, and Cheshbon Mahlachat HaKochavim.

R' Avraham Ibn Ezra (12th century) compiled astronomical tables on the movements of the planets and translated the work Reason for the Al Khwarizmi Tables by Ahmed Ibn Elmenthi. He also wrote Sefer HaIbbur on the subject of cycles throughout the year, Shalosh Sheelot in response to three questions on the intercalation of the year, and Klei Nechoshet on instruments used for astronomical observation.

Ralbag (13th century) was perhaps the greatest Jewish astronomer of all time. Maximillian Curtze, a historian of astronomy, has listed Ralbag among the forerunners of Copernicus. Ralbag compiled his own astronomical tables and the first section of the fifth book of his philosophical work Milchemot Hashem is rooted in astronomy. He invented a device for measuring angles and spherical distances that was called Jacob's staff. There has even been a crater on the moon named "Rabbi Levi's Crater" in honor of Ralbag's contributions to astronomy.

R' Yitzchak Yisraeli (early 14th century) wrote Yesod Olam which is an extremely important work on astronomy that has been used for halachic rulings. Many commentaries were written to this work, including one by R' Eliyahu Mizrachi.

R' Eliyahu Mizrachi (early 16th century), whose responsa and commentary on Rashi are still frequently quoted, also wrote a commentary on Ptolemy's Almagest.

R' Avraham Zacuto (15th-16th century), author of Sefer HaYuchasin, was an influential astronomer. His main work was written in Hebrew but was translated into Spanish and Latin as Almanach Perpetuum. His astronomical tables proved particularly important to the Spanish and Portuguese explorers of that time.

R' Moshe Almosnino (early 16th century), the famous rabbi of Salonica, translated Georg Peuerbach's Theorica Planetaruum into Hebrew.

The Maharal (16th century) was also reputed to be an accomplished astronomer and his student R' David Gans was a colleague of Johann Kepler and Tycho Brahe. R' David Gans, who also wrote the important work on Jewish history Tzemach David, wrote an astronomical work titled Nechmad VeNaim.

R' Moshe Isserles (16th century), the Rama, demonstrated his knowledge of astronomy in his Torat HaOlah and commentary on Theorica Planetaruum.

R' Mordechai Yaffe (16th century), the author of the many Levush books, included a lesson on astronomy in his commentary on Rambam's Hilchot Kiddush HaChodesh and wrote a commentary on R' Avraham HaNasi's Tzurat HaAretz.


What we have seen is that none of the three main views in understanding twilight and determining when definite night begins contradict astronomy. They can all be understood within the framework of modern astronomy, even if originally based on a pre-Ptolemaic understanding. Furthermore, the view of the Levush and the Vilna Gaon is not only not a modern innovation, it is arguably older than Rabbeinu Tam's view. Additionally, our brief trip through history has shown that there were many famous rabbis who were intimately involved with the astronomy of their times.

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Contributor(s): Gil Student
Last revised: 12/12/01
© Aishdas 2001