Relative hour

Relative hour (Hebrew singular: / ; plural: / ), 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 radical, but changes with the length of 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 inevitably makes for a longer hour or a shorter hour. 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, by most exponents of Jewish law, and with sunrise by the Vilna Gaon and Rabbi Hai Gaon. Midnight (12:00am) was also the "sixth hour" of the night, whereas the "first hour" of the night began when the first three stars appeared in the night sky.
Jewish tradition.
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. 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.
The conventional Jewish way of calibrating the time of day is to reckon the "first hour" of the day with the rise of dawn (), that is to say, approximately 72 minutes before sunrise, and the end of the day commencing shortly after sunset when the first three medium-size stars have appeared in the night sky. From the moment of sunset when the sun is no longer visible until the appearance of the first three medium-size stars is a unit of time called evening twilight (). In the Talmud, twilight is estimated at being the time that it takes a person to walk three quarters of a biblical mile (i.e. 1,500 cubits, insofar that a biblical mile is equal to 2,000 cubits). According to Maran's "Shulhan Arukh", a man traverses a biblical mile in 18 minutes, meaning, one is able to walk three quarters of a mile in 13½ minutes. According to Maimonides, a man walks a biblical mile in 24 minutes, meaning, three quarters of a mile is done in 18 minutes. In Jewish law, the short period of dusk or twilight (from the moment the sun has disappeared over the horizon until the appearance of the first three stars) is a space of time whose designation is doubtful, partly considered day and partly considered night. When the first medium-size star appears in the night sky, it is still considered day; when the second star appears, it is an ambiguous case. When the third star appears, it is the beginning of the "first hour" of the night. Between the break of dawn and the first three medium-size stars that appear in the night sky there are always 12 hours.
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 the actual day begins approximately 72 minutes before sunrise, and ends 13½ minutes after the sun has already set and can no longer be seen over the horizon (according to Maran), or 18 minutes (according to Maimonides), 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 as much as 77 minutes or more, whereas the length of each hour during nighttime can be less than 42 minutes.
Practical bearing.
In Jewish Halacha, the practical bearing of this teaching is reflected in many "halachic" practices. For example, whenever observant Jews refer to the appointed time for reciting the verses of "Kriyat Shema", ideally, this recital must be made from the time of sunrise until the end of the "third hour" of the day, a time that actually fluctuates on the standard 12-hour clock, depending on summer and winter. Its application is also used in determining the time of the Morning Prayer, traditionally said, as a first resort, from sunrise until the end of the "fourth hour", but as a last resort 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 Summer months and Winter months.
On the eve of Passover, Jews are only permitted to eat chametz until the "fourth-hour" of the day.
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".