The observance of many Jewish rituals are time-based; that is, according to halakhah (Jewish law) they are to be performed at specific times. These times are known as zmanim ("times") and they are determined by calculation using various astronomical phenomena, specifically the position of the sun in the sky for the specific latitude and longitude. Thus, the times of sunrise, sunset, the time interval separating them, and the sun's angular position from the observer's location are all factors used in calculating the halakhic times and a special unit called the "proportional hour" (sha'ah zmanit), but simply referred to as "hour": this is an important concept that we will discuss below.
The time-determined mitzvot are listed in the tables* of Zmanim and include the times mandated for certain observances, rituals, and prayers such as the earliest time to don tallit and tzitzit, the latest time that one can say the Kriat Shema and the morning prayers, and the interval of time within which one must say the afternoon prayer, and related mitzvot. One also needs to know the time of sunset where one is located in order to determine the times of the beginning of Shabbat and holidays and the definition of "nightfall" in order to determine the time of havdalah.
Many of these times are dependent on the sha'ah zmanit ("proportional hour") mentioned above. This is a special unit of time that takes into account the seasonal changes in the length of the day. Thus this "hour" has special meaning in Jewish observance. When we say that a certain mitzvah may be performed three hours into the day, this doesn't mean at three o'clock in the morning or three 60-minute hours after sunrise. But even this term, the precise definition of the sha'ah zmanit and how these hours are used in zmanim calculations, is subject to differences of rabbinic opinion.
The major zmanim calculations are attributed to two rabbis (there are, of course, minority opinions as well, but for simplicity's sake, we'll only cover the opinion of these two giants of halakhah). Rabbi Avraham "Abaleh" Gombiner (1637–1683) of Kalisz, Poland, the "Magen Avraham," wrote his commentary on "Orach Chayim," the section of the Shulchan Aruch that deals with the "Laws of Daily Life and Holiday Behavior," and includes the "Laws Concerning Rising in the Morning," the various prayers, special prayers, and mitzvot of the holidays. According to Rabbi Avraham, the proportional hour is obtained by dividing by twelve the time interval between "alot hashachar," the pre-sunrise event that occurs when the rays of the sun first become evident and the sky begins to lighten, and "tzeit hakochavim," "starshine," which is defined as that time when three stars of medium brightness become visible in the sky. Using Rabbi Avraham's method, if the sky lightens at 5:00 a.m. and the stars appear at 7:30 p.m. (a duration of 14.5 clock hours or 870 minutes), then dividing by twelve produces the value for the sha'ah zmanit on this date: 72.5 minutes.
Rabbi Eliyahu ben Shlomo Zalman of Vilna (1720–1797), the "Vilna Gaon," used a different calculation. He held that the sha'ah zmanit is derived by dividing by twelve the interval between "hanetz hachamah," sunrise, the moment that the first sliver of the sun's disk becomes visible on the horizon, and "shkiat hachamah," sunset, the time that the disk of the sun disappears completely beneath the horizon. Using our example from above, sunrise would be about 6:30 a.m. and sunset at 6:40 p.m. and one sha'ah zmanit is almost exactly 60 minutes.
Obviously, the sha'ah zmanit as defined by the Magen Avraham is longer than the unit as defined by the Vilna Gaon. This is significant in determining certain zmanim.
In the daily zmanim tables,* the zmanim according to Magen Avraham are identified using M"A. The zmanim as defined by the Vilna Gaon are labeled with GR"A.
Below you will find the names of the zmanim, their meaning and associated mitzvot:
- Alot HaShachar:
- First-light dawn. Daytime (minor) fasts begin now. According to the Magen Avraham, the calculation of sha'ot zmaniot begins now.
- Earliest Tallit:
- The halakhic description of the earliest time to don tallit and tefillin is, according to the Talmud, when "one can distinguish his friend at a distance of four cubits" (BT Berakoth 9b). Such a determination, close to daily human experience, occurs when it is light enough for someone to first recognize an acquaintance, not a close friend who could be recognized from any number of subtle non-visual cues even in the absence of much light. This is the time, say chazal, that is the earliest that one may make a brakhah and recite the Shema. Since evaluating levels of light is a subjective experience, this time is approximated mathematically when the sun's disk is at at 11 degrees below the horizon.
- HaNetz HaChamah:
- Sunrise. The calculation of sha'ot zmaniot begins now according to the Vilna Gaon and most opinions. Those who wish to daven vatikin time their prayers so that the amidah is said at sunrise.
- Kriat Shema:
- The latest time to fulfill the biblical daily requirement to recite the Shema is three sha'ot zmaniot into the day. B'di'eved (post facto) one is permitted to recite Shema with its blessings until four sha'ot zmaniot into the day.
- Four sha'ot zmaniot into the day is the latest time to say Shacharit, the morning prayer. It is permitted to recite Shacharit until chatzot (midday).
- Midday. This is six hours into the day, the halfway mark. Half-day fasts end at chatzot. B'di'eved (post facto) one may recite Shacharit until this time.
- Minchah Gedolah:
- Half a sha'ah zmanit or roughly 30 minutes after chatzot, whichever is greater (in the summer the time may be closer to 45 minutes). This is the earliest time one may say Mincha, the afternoon prayer.
- Minchah K'tanah:
- Nine-and-a-half proportional hours after sunrise. Some poskim maintain that this is the earliest time to pray Mincha.
- Plag HaMinchah:
- The last eighth of the day. Technically this is one-and-a-quarter sha'ot zmaniot before shkiat (sunset). This is the earliest one may bring in the Shabbat. According to Rabbi Judah, if one recites Mincha before plag haminchah, Ma'ariv (the evening prayer) may be recited any time afterward.
- Shkiat HaChamah:
- Sunset. This is the latest time to say Mincha. The time following shkiat and before tzeit hakochavim is called bein hashmashot, "twilight." Many laws relate to this period and it can be categorized as either the previous or the next day. The halakhah concerning this period is extremely complex as is its history. Details may be found here and here. The accepted custom is to light Shabbat and festival candles 18 minutes before shkiat. Some communities may have adopted an earlier time as their unique custom as to the time to light candles. If one missed saying Mincha, b'di'eved one can do so (and fulfill all other daytime mitzvot) until tzeit, but no berekhot are said over performing missed mitzvot.
- Tzeit HaKochavim:
- Nightfall ("starshine"). This is the earliest time to recite Ma'ariv (the evening service), according to Rabbi Judah's school. Full-day fasts end at this time. This is the earliest time to say the evening Shema and for counting the omer. This is also the earliest time that a woman who has completed seven pure days after her cycle may go to the mikvah. This time is determined when the top of the sun's disk is located at 5.95 degrees below the horizon. There are several definitions used to determine the time of nightfall; this is the earliest.
- 7.08° below horizon:
- Some communities observe tzeit when the sun's disk is at 7.08 degrees below the horizon.
- 3 Medium Stars:
- Since nightfall has traditionally been defined as the appearance of three stars in the sky, one might wonder why nightfall can't simply be confirmed by seeing the presence of three stars. There are several reasons; first, there is no standard for determining how bright the three stars must be: "bright," "medium," or "dim/small." Second, there is the question of discerning the difference in the stars' brightness. Third, the presence of clouds in the sky will also tend to make any determination difficult. Thus the rabbis had to come up with guidelines that eliminated obvious bright stars, and thus only medium and small stars were considered. We now use an objective measure for the calculation of the time of appearance of three medium stars; this is when the sun's disk is at 7.5 degrees below the horizon.
- 3 Small Stars:
- This is the time set for the ending of Shabbat and festivals; after this time the weekday begins. This constitutes a stricter calculation of tzait hakochavim and is used since one is reluctant to see the holy days end while ensuring that the sanctity of the holy day is not violated. The time called "three small stars" is defined as when the sun's disk reaches 8.75 degrees below the horizon.
- Rabbeinu Tam:
Determining the time of "nightfall" that will agree with halakhah is highly complex and controversial. Marking the exact time that day becomes night, in reality a gradual process, is highly subjective and there are many celestial markers that can be used as landmarks to assist in setting a specific time to mark the shift from day to night. This uncertainty has produced many opinions about how to set the time of nightfall. An additional complexity arises because different areas of halakhah may require different levels of stringency, such as, for example, setting the ending time of the Yom Kippur fast. (Also note that the very observant may use a different standard; see Tosafot, Zevachim 56a and Chiddushei Chatam Sofer, Sukkah 38a).
In order to determine when sunset occurs and when it becomes "night," the rabbis had to make a number of seemingly arbitrary choices. Some of the choices to define the time of "sunset" included when the disk of the sun touched the horizon or had sunk completely below the horizon, or even when the sky became dark except for the western horizon's red glow. There are even more choices for when "night" occurs. Several overlap the options given for sunset; in addition, other possibilities include a totally dark sky (absence of a red western glow) or the appearance of a certain number of stars. Rabbeinu Tam's opinion (he was a grandson of Rashi) was his attempt to strictly define whan nightfall occurs, but his argument (and the counter-arguments of other rabbis) are quite complex. If you don't want to wade through the details, you can skip all the arguments below and just read the last sentence in this section.
Identifying the exact moment of "nightfall" sets a boundary. First, it determines when the old day is over and the next has begun. Second, it defines the time of "nightfall" for the purposes of halakhah. Shabbat 34b offers three possibilities for how sunset/twilight should be regarded: first, it's still daytime; second, it's part of night; and third, it's a combination of day and night. The result is thus indeterminant: whether or not it is night, and whether or not the prior day has ended. This is called a safek (indeterminate situation), and when a safek occurs pertaining to a Torah law, a strict interpretation is made, but in the case of a rabbinic law, the interpretation may be lenient. The discussion may be found in Shabbat 34b–35a. It is this discussion that gave rise to the need for the rabbis to define nightfall with a fair degree of stringency.
But before we can discuss the calculation methods for determining the time of nightfall, we must first define the term "mil." A "mil" is a talmudic unit of distance, approximately equal to the Roman mile, and is used in many places as a period of time; the amount of time it takes a average person to walk a distance of a mil, according to most poskim, is 18 minutes. The word mil is Aramaic, borrowed from the Latin.
Rabbeinu Tam's time is related to a debate based on the resolution of two statements attributed by the Talmud to R. Yehudah. First, R. Yehudah identified the period of bein hashmashot (twilight, in between shkiat and tzait hakochavim) as three-quarters of a mil, according to Shabbat 34b. Since the time for a typical person to walk one mil is 18 minutes, then it emerges that tzait hakochavim occurs 13½ minutes after shkiat. However, a different impression comes from Pesachim 94a. There, in a discussion concerning the dimensions of the earth, R. Yehudah stated that in between shkiat and tzait hakochavim there are four milin. Again assuming an 18-minute mil, that would result in a bein hashmashot of 72 minutes; quite different from his first statement.
This contradiction gave rise to a major disagreement between the opinions of the geonim and the Vilna Gaon (GR"A) on one side and on the other side with Rabbeinu Tam. According to the geonim (see responsa Maharam Alashkar, 96, citing R. Sherira Gaon and R. Hai Gaon) and the GR"A (see Biur to Shulchan Arukh, 261:2) cite different reasons stating that the statement in Pesachim is not applicable to the halakhah for these cases. Thus, ¾ of a mil after shkiat is tzait hakochavim and the time in between is bein hashmashot.
Rabbeinu Tam, however, has a different view of the issue (see Tosafot, Berakhot 2b; Shabbat 35a; Pesachim 94a, s.v. R. Yehudah). In his analysis there are actually two points called shkiat. The first shkiat point begins when the sun's disk touches the western horizon. The second shkiat point occurs when the sun's disk has dropped below the horizon. Rabbeinu Tam's four-mil period is the period starting with the first shkiat and ending with tzait, while the ¾-mil period begins with the second shkiat and ends with tzait. (In addition, a third opinion exists and may be found in Sefer Yereim 274.)
Both opinions have garnered influential support. The GR"A, affirming the position of the geonim, stated that Rabbeinu Tam cannot be right about nightfall being delayed so long because hachush makhchish, "one's senses contradict it": simply glancing outside confirms that it's pitch black long before four mil have passed from the time of shkiat. R. Herschel Schachter and R. Mordechai Willig (cf. Am Mordechai to Berachot, 2) endorse the viewpoint of the GR"A and the geonim, which is the practice of much of modern Judaism.
Despite this, Rabbeinu Tam's position also carries strong support and is the viewpoint of many rishonim, including the Rama (Magen Avraham, 331:2; responsa Chatam Sofer) and R. Yosef Karo, author of the Shulchan Arukh (Orach Chaim 261:2). Because of these opinions, many are reluctant to end Shabbat before "Rabbeinu Tam's z'man." R. Moshe Feinstein endorsed this practice (responsa Iggerot Moshe, Orach Chaim IV, 64) and it is observed in communities such as Satmar (responsa Divrei Yoel, 18).
Rabbeinu Tam's position may seem to be extreme and difficult to defend but consider this thought. If one assumes the period of bein hashmashot to be ¾ of a mil (or 13½ minutes), it 's possible that darkness has not yet completely fallen since shkiat. The GR"A did state that this measurement isn't intended to apply to every part of the world, but only to Israel and Babylonia (which lie on the same latitude). However, R. Yechiel Michel Tukachinsky, in his Bein HaSh'mashot, noted that even in Jersualem, stars are not visible until about 22 minutes after shkiat. R. Yehudah Levi observed that the trained eye and the untrained eye have markedly different powers of observation (Z'manei HaYom B'Halakhah). In Jerusalem, during the month of Nissan, person with good eyesight can discern three stars after about 15 minutes, a time fairly close to ¾ of a mil.
In a related opinion, since the custom of many European Jews was to work until almost sunset on Fridays, the Radbaz resolved an apparent contradiction in the Rambam's writings. The Rambam wrote in Hilkhot Shabbat 5:4 that the indeterminate nature of bein hashmashot requires that one may not work on Friday evening after shkiat. However, in Hilkhot Kiddush HaChodesh 2:9 he ruled that, while sanctifying the new month must only be done during the day, it is still acceptable, within limits, to do so after shkiat. The ruling of the Radbaz stated that the manner of how "night" is defined may be flexible (Responsa, 1442). In this way he allows for the Rambam's opinion that Friday can (possibly) become Shabbat while it still maintains the character of "day." (This contradiction was also addressed in Chavatzelet HaSharon al HaTorah, Bereishit, p. 8).
R. Moshe Shternbuch (Moadim U'Zmanim, II, 155, note 1) considers the interesting question of those parts of the world where darkness never actually falls—the high latitudes. He suggests that the day changes to night at whichever point the distance between the sun and horizon is the smallest, but that halakhic "night" does not take effect at all. Thus, one living in those regions would be unable to fulfill any mitzvot dependent on night. He does consider, without a conclusion, that kriat shema might be an exception, as the controlling terminology is not "day" and "night" but rather "lying down" and "rising" (Deut. 11:19).
A major prevailing opinion about nightfall is based on the view of the geonim R. Sherira, R. Hai and R. Nissim. This viewpoint assumes that the period of bein hashmashot, which is actually an astronomical event used to define when "night" begins, starts as soon as the entire disk of the sun moves below the horizon. This observable event also accounts for variations in astronomical phenomena as seen from different parts of the earth and different altitudes at the same latitude. Thus, according to this view, the time interval between shkiat, sunset, and tzait hakochavim, night/starshine, will vary at a given location from time to time, and at different locations around the world.
Finally, the second opinion is that of Rabbeinu Tam. According to this, the period of bein hashmashot begins later, when light has disappeared from all of the sky except for a glow on the western horizon. To handle variations in the astronomical phenomena as seen from different parts of the earth, as well as the reasons stated by the Talmud—observers' subjectivity concerning the brightness of stars and the possibility of cloudy skies—Rabbeinu Tam falls back on the estimate provided by the Talmud for the amount of time between sunset and night/starshine as the time estimated for an average person to walk four milin, a time estimated to be seventy-two minutes, and this time is to be used throughout the year at a given location and at all points on the earth. Thus, according to R. Tam, night occurs 72 minutes after sunset.
- Sha'ah Zmanit:
- The "proportional hour" of this date as described above.
* If you arrived on this page via a search engine, you can access the Zmanim tables using this link.