Unequal hours are the division of the light day and the night into 12 sections each, whatever the season. They are also called temporal hours, seasonal hours, biblical or Jewish hours, as well as ancient or Roman hours (). They are ''unequal duration'' periods of time because days are longer and nights shorter in summer than in winter. Their use in everyday life was replaced in the late Middle Ages by the now common ones of equal duration. The first temporal hour of daylight begins at Sunrise, the first of night at

Sunset Sunset, also known as sundown, is the daily disappearance of the Sun below the horizon due to Earth's rotation. As viewed from everywhere on Earth (except the North and South poles), the equinox Sun sets due west at the moment of both the spring ...

. For example, if daylight and night are each divided into twelve temporal hours, Midday and

Midnight Midnight is the transition time from one day to the next – the moment when the date changes, on the local official clock time for any particular jurisdiction. By clock time, midnight is the opposite of noon, differing from it by 12 hours. ...

are each the beginning of the seventh hour. A clock that displays the temporal hours is called a temporal clock.

Astronomical basics

To the concept of ''light day'' corresponds the astronomical concept '' Day arc of the Sun''. With the exception of the

equator The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can als ...

, the duration of daylight depends on the latitude and the season. At 49° north/south latitude (e.g., in Karlsruhe), it varies between 16

equinoctal hour An equinoctial hour is one of the 24 parts of the full day consisting of light day and the night. Its length, unlike the temporal hour, ''does not'' vary with the season, but is constant. The measurement of the full day with equinoctial hours ...

s in summer and 8 equinoctial hours in winter. Due to the continuous change of the duration of daylight over the course of the year, the duration of the day division, i.e. the temporal day hours and the temporal night hours, also changes over the year. The temporal hours of day and night are equal only at the spring and autumn equinoxes. From 66.5° north/south latitude ( Polar circles) the sun no longer sets (the

horizon The horizon is the apparent line that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This line divides all viewing directions based on whether i ...

) every day in summer and rises every day in winter. Day does not occur.

History

Temporal hours were common in many cultures. A division of day and night into twelve hours each was first recorded in Ancient Egypt. A similar division of day and night was later made in the

Mediterranean Basin In biogeography, the Mediterranean Basin (; also known as the Mediterranean Region or sometimes Mediterranea) is the region of lands around the Mediterranean Sea that have mostly a Mediterranean climate, with mild to cool, rainy winters and w ...

from about Classical Greek Antiquity into twelve temporal hours each (). In Western culture they were adopted from the Roman calendar and were adopted in the European Medieval era. They had particular relevance in the fixed daily schedule of the monastic

orders Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of d ...

. This division of time allowed the work of the day -such as eating, praying, or working -to always be performed at the same (temporal) hour, regardless of season ( Prayer of the Hours). This chronology is used by Jewish religious law ( Halacha), hence '' Jewish'' or ''

Biblical The Bible (from Koine Greek , , 'the books') is a collection of religious texts or scriptures that are held to be sacred in Christianity, Judaism, Samaritanism, and many other religions. The Bible is an anthologya compilation of texts of a ...

Division of hours''. Mechanical clocks encouraged the adoption of equinoctial hours.

Temporal time

For the display of temporal hoursKarlheinz Deußer: ''Temporaluhren: Die Suche nach mechanischen Uhren, die mit Temporalstunden liefen.'' 2012. almost exclusively the

Sundial A sundial is a horological device that tells the time of day (referred to as civil time in modern usage) when direct sunlight shines by the apparent position of the Sun in the sky. In the narrowest sense of the word, it consists of a flat ...

with as hand was once used. The Sun position, which varied throughout the year, served as a parameter on which the varying duration of the temporal hours during the year depended. Many astronomical clocks created during the transition to the equal-duration equinoctial hours display temporal hours in addition to the new equal-duration hours. Even where temporal hours continued to be used (especially in monasteries), the mechanical clock was used. This required two different settings for the day and for the night, or one clock each for the day and the night. For the latter, the speed of the Waag was changed, for example, in 26 steps (i.e., half the numerical value of 52 weeks). In the weeks of the Equinox, both clocks could be operated with the middle weight position on the balance.

Bibliography

* Karlheinz Deußer: ''Temporaluhren: Die Suche nach mechanischen Uhren, die mit Temporalstunden liefen.'' In: ''Jahresschrift der deutschen Gesellschaft für Chronometrie.'' Band 51, 2012, S. 143–160. * Jürgen Osing: ''Hieratische Papyri aus Tebtunis I (Carsten Niebuhr Institute of Ancient Eastern Studies Copenhagen)''. Museum Tusculanum Press, Copenhagen 1998, ISBN 8-7728-9280-3. * Rudolf Wendorff: ''Zeit und Kultur. Geschichte des Zeitbewusstseins in Europa''. Westdeutscher Vlg, Wiesbaden 1980, ISBN 3-531-11515-4.

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