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Relative hour (Hebrew singular: shaʿah zǝmanit / שעה זמנית; plural: shaʿot - zǝmaniyot / שעות זמניות), sometimes called halachic hour, seasonal hour and variable hour, is a term used in rabbinic Jewish law that assigns 12 hours to each day and 12 hours to each night, all throughout the year. A relative hour has no fixed length in absolute time, but changes with the length of daylight each day - depending on summer (when the days are long and the nights are short), and in winter (when the days are short and the nights are long). Even so, in all seasons a day is always divided into 12 hours, and a night is always divided into 12 hours, which invariably makes for a longer hour or a shorter hour. At Mediterranean latitude, one hour can be about 45 minutes at the winter solstice, and 75 minutes at summer solstice. All of the hours mentioned by the Sages in either the Mishnah or Talmud, or in other rabbinic writings, refer strictly to relative hours.
Another feature of this ancient practice is that, unlike the standard modern 12-hour clock that assigns 12 o'clock pm for noon time, in the ancient Jewish tradition noon time was always the sixth hour of the day, whereas the first hour began with the break of dawn according to many Halachic authorities, and with sunrise according to others. Midnight (12:00 am local official clock time) was also the sixth hour of the night, which, depending on summer or winter, can come before or after 12:00 am local official clock time, whereas the first hour of the night always begins at sunset or when the first three stars appeared in the night sky.
During the Spring (באחד בתקופת ניסן) and Autumnal (באחד בתקופת תשרי) equinox (around 20 March and 23 September), the length of a day and night are equal. However, even during the summer solstice and winter solstice when the length of the day and the length of the night are at their greatest disparity, both day and night are always divided into 12 hours.
Rabbi Pinchas said in the name of Rabbi Abba bar Pappa: One star is certainly day; two [stars] is a doubtful case; three [stars] is certainly night.
In old times, the hour was detected by observation of the position of the sun, or when the first three stars appeared in the night sky. During the first six hours of the day, the sun is seen in the eastern sky. At the sixth hour, the sun is always at its zenith in the sky, meaning, it is either directly overhead, or parallel (depending on the hemisphere). Those persons living in the Northern Hemisphere, the sun at noon time will appear overhead slightly towards the south, whereas for those living in the Southern Hemisphere, the sun at noon time will appear overhead slightly towards the north (an exception being in the tropics, the sun can sometimes be directly overhead). From the 6th and a half hour to the 12th hour, the sun inclines towards the west, until it sets. The conclusion of a day at the end of twilight may slightly vary in minutes from place to place, depending on the elevation and the terrain. Typically, nightfall ushers in more quickly in the low-lying valleys, than it does on a high mountaintop.
There are two major opinions how to calculate these times:
In the Modern Age of astral science and of precise astronomical calculations, it is now possible to determine the length of the ever-changing hour by simple mathematics. To determine the length of each relative hour, one needs but simply know two variables: (a) the precise time of sunrise, and (b) the precise time of sunset. Since according to the first opinion, the day begins approximately 72 minutes before sunrise and ends approximately 72 minutes after sunset (and according to the variant understanding of this opinion, ends approximately 13½ or 18 minutes after sunset), or begins at sunrise and ends at sunrise according to the second opinion, by collecting the total number of minutes in any given day and dividing the total number of minutes by 12, the quotient that one is left with is the number of minutes to each hour. In summer months, when the days are long, the length of each hour during daytime can be quite long depending on one's latitude, whereas the length of each hour during nighttime can be quite short again depending on one's latitude. It should also be noted that according to those opinions that the 72 minutes are computed according to 16.1 degrees, the further one goes from the equator, the longer it will get, such that in northern latitudes it could become 2 hours or longer.
In Jewish Halacha, the practical bearing of this teaching is reflected in many halachic practices. For example, according to Jewish law, the morning recitation of Kriyat Shema must be made between slightly before sunrise and the end of the third hour of the day, a time that actually fluctuates on the standard 12-hour clock, depending on the time of year. Its application is also used in determining the time of the Morning Prayer, which must be recited between sunrise until the end of the fourth hour, but post facto can be said until noon time, and which times will vary if one were to rely solely on the dials of the standard 12-hour clock, depending on the seasons.
On the eve of Passover, chametz can only be eaten until the end of the fourth-hour of the day, and must be disposed of by the end of the fifth hour.
In Jewish tradition, prayers were usually offered at the time of the daily whole-burnt offerings. The historian, Josephus, writing about the daily whole-burnt offering, says that it was offered twice each day, in the morning and about the ninth hour. The Mishnah, a compendium of Jewish oral laws compiled in the late 2nd-century CE, says of the morning daily offering that it was offered in the fourth hour, but says of the late afternoon offering: "The daily whole-burnt offering was slaughtered at a half after the eighth hour, and offered up at a half after the ninth hour." Elsewhere, when describing the slaughter of the Passover offerings on the eve of Passover (the 14th day of the lunar month Nisan), Josephus writes: "...their feast which is called the Passover, when they slay their sacrifices, from the ninth hour to the eleventh, etc." (roughly corresponding to 3 o'clock pm to 5 o'clock pm). Conversely, the Mishnah states that on the eve of Passover the daily whole-burnt offering was slaughtered at a half past the seventh hour, and offered up at a half past the eighth hour.
The opinion of our Rabbi, the Vilna Gaon, as well as of many other poskim, is that in all the halachic matters touching on the hours [of the day], we reckon [the day] from sunset until sunset, such as the latest time [one is required to recite] Kiryat Shema, [being] a quarter of the day (a third of the hours), and the latest time of the [morning] prayer, [being] a third of the day (four hours), and the latest time [permitted in] eating leaven on the eve of Passover, [being] a third of the day, and the burning of leaven in another relative hour – all [hours being reckoned] from sunrise until sunset. ...The practice is to be stringent in accordance with the Vilna Gaon's method when it comes to the latest time [in which a person recites] Kiryat Shema, and [says] the [morning] prayer, and when it comes to eating and burning the leaven [on the eve of Passover].
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