The hour (common symbol: h or hr, h being the international form of the symbol) is a unit of measurement of time. In modern usage, an hour comprises 60 minutes, or 3,600 seconds. It is approximately 1⁄24 of a mean solar day.
The Middle English word ure first appears in the 13th century, as a loanword from Old French ure, ore, from Latin hōra. Hora, in turn, derives from Greek ὥρα ("season, time of day, hour"). In terms of the Proto-Indo-European language, ὥρα is a cognate of English year and is derived from the Proto-Indo-European word *i̯ēro- ("year, summer").
The ure of Middle English and the Anglo-French houre gradually supplanted the Old English nouns tīd (which survives in Modern English as tide) and stund. Stund is the progenitor of stound, which remains an archaic synonym for hour. Stund is related to the Old High German stunta (whence German Stunde, "hour, lesson"), from Germanic *stundō ("time, interval, while").
Ancient Egyptians used sundials that "divided a sunlit day into 10 parts plus two "twilight hours" in the morning and evening." The Greek astronomer, Andronicus of Cyrrhus, oversaw the construction of a horologion called the Tower of the Winds in Athens during the first century BCE. This structure tracked a 24-hour day using both sundials and mechanical hour indicators.
Ancient Sumer and India also divided days into either one twelfth of the time between sunrise and sunset or one twenty-fourth of a full day. In either case the division reflected the widespread use of a duodecimal numbering system. The importance of 12 has been attributed to the number of lunar cycles in a year. In China, the whole day was divided into twelve parts. 
Astronomers in Egypt's Middle Kingdom (9th and 10th dynasties) observed a set of 36 decan stars throughout the year. These star tables have been found on the lids of coffins of the period. The heliacal rising of the next decan star marked the start of a new civil week, which was then ten days. The period from sunset to sunrise was marked by 18 decan stars. Three of these were assigned to each of the two twilight periods, so the period of total darkness was marked by the remaining 12 decan stars, resulting in the 12 divisions of the night. The time between the appearance of each of these decan stars over the horizon during the night would have been about 40 modern minutes. During the New Kingdom, the system was simplified, using a set of 24 stars, 12 of which marked the passage of the night.
Ancient Sinhalese in Sri Lanka divided a solar day into 60 peya (now called Sinhala peya). One peya was divided into 24 vinadi. Since 60 (peya) × 24 (vinadi) = 24 (hours) × 60 (minutes), one vinadi is equal to one minute.
Earlier definitions of the hour varied within these parameters:
- One twelfth of the time from sunrise to sunset. As a consequence, hours on summer days were longer than on winter days, their length varying with latitude and even, to a small extent, with the local weather (since it affects the atmosphere's index of refraction). For this reason, these hours are sometimes called temporal, seasonal, or unequal hours. Romans, Greeks and Jews of the ancient world used this definition (although Jews of the Old Testament period did not have a word for hour and did not keep time this way); as did the ancient Chinese and Japanese. The Romans and Greeks also divided the night into three or four night watches, but later the night (the time between sunset and sunrise) was also divided into twelve hours. When, in post-classical times, a clock showed these hours, its period had to be changed every morning and evening (for example by changing the length of its pendulum), or it had to keep to the position of the Sun on the ecliptic (see Prague Astronomical Clock).
- One twenty-fourth of the apparent solar day (between one noon and the next, or between one sunset and the next). As a consequence hours varied a little, as the length of an apparent solar day varies throughout the year. When a clock showed these hours it had to be adjusted a few times in a month. These hours were sometimes referred to as equal or equinoctial hours.
- One twenty-fourth of the mean solar day. See solar time for more information on the difference to the apparent solar day. When an accurate clock showed these hours it virtually never had to be adjusted. However, as the Earth's rotation slows down, this definition has been abandoned. See UTC.
Many different ways of counting the hours have been used. Because sunrise, sunset, and, to a lesser extent, noon, are the conspicuous points in the day, starting to count at these times was, for most people in most early societies, much easier than starting at midnight. However, with accurate clocks and modern astronomical equipment (and the telegraph or similar means to transfer a time signal in a split-second), this issue is much less relevant.
Counting from dawn
In ancient and medieval cultures, the counting of hours generally started with sunrise. Before the widespread use of artificial light, societies were more concerned with the division between night and day, and daily routines often began when light was sufficient.
Sunrise marked the beginning of the first hour (the zero hour), the middle of the day was at the end of the sixth hour and sunset at the end of the twelfth hour. This meant that the duration of hours varied with the season. In the Northern hemisphere, particularly in the more northerly latitudes, summer daytime hours were longer than winter daytime hours, each being one twelfth of the time between sunrise and sunset. These variable-length hours were variously known as temporal, unequal, or seasonal hours and were in use until the appearance of the mechanical clock, which furthered the adoption of equal length hours.
This is also the system used in Jewish law and frequently called "Talmudic hour" (Sha'a Zemanit) in a variety of texts. The talmudic hour is one twelfth of time elapsed from sunrise to sunset, day hours therefore being longer than night hours in the summer; in winter they reverse.
The Indic day began at sunrise. The term hora was used to indicate an hour. The time was measured based on the length of the shadow at day time. A hora translated to 2.5 pe. There are 60 pe per day, 60 minutes per pe and 60 kshana (snap of a finger or instant) per minute. Pe was measured with a bowl with a hole placed in still water. Time taken for this graduated bowl was one pe. Kings usually had an officer in charge of this clock.
Counting from sunset
In so-called "Italian time", "Italian hours", or "old Czech time", the first hour started with the sunset Angelus bell (or at the end of dusk, i.e., half an hour after sunset, depending on local custom and geographical latitude). The hours were numbered from 1 to 24. For example, in Lugano, the sun rose in December during the 14th hour and noon was during the 19th hour; in June the Sun rose during the 7th hour and noon was in the 15th hour. Sunset was always at the end of the 24th hour. The clocks in church towers struck only from 1 to 12, thus only during night or early morning hours.
This manner of counting hours had the advantage that everyone could easily know how much time they had to finish their day's work without artificial light. It was already widely used in Italy by the 14th century and lasted until the mid-18th century; it was officially abolished in 1755, or in some regions, customary, until the mid-19th century.[a]
The system of Italian hours can be seen on a number of clocks in Europe, where the dial is numbered from 1 to 24 in either Roman or Arabic numerals. The St Mark's Clock in Venice, and the Orloj in Prague are famous examples. It was also used in Poland and Bohemia until the 17th century.
Counting from noon
For many centuries, up to 1925, astronomers counted the hours and days from noon, because it was the easiest solar event to measure accurately. An advantage of this method (used in the Julian Date system, in which a new Julian Day begins at noon) is that the date doesn't change during a single night's observing.
Counting from midnight
In the modern 12-hour clock, counting the hours starts at midnight and restarts at noon. Hours are numbered 12, 1, 2, ..., 11. Solar noon is always close to 12 noon (ignoring artificial adjustments due to time zones and daylight saving time), differing according to the equation of time by as much as fifteen minutes either way. At the equinoxes sunrise is around 6 A.M. (ante meridiem, before noon), and sunset around 6 P.M. (post meridiem, after noon).
In the modern 24-hour clock, counting the hours starts at midnight and hours are numbered from 0 to 23. Solar noon is always close to 12:00, again differing according to the equation of time. At the equinoxes sunrise is around 06:00 and sunset around 18:00.
Derived measures and applications
Although the SI unit for speed is metres per second, in everyday usage kilometres per hour or, in the UK and US, miles per hour are more practical. The knot, defined as one nautical mile per hour, is a unit of speed commonly used in maritime and air navigation. Occasionally the metre per hour is used for slow-moving objects like snails.
Worker compensation is commonly based on working time in terms of number of hours worked, referred to as an hourly wage. Worker schedules are categorized by number of work hours per day or number of work hours per week; these are regulated and distinguish part-time from full-time jobs. The man-hour describes the amount of work that a person can complete in one hour, or alternately the length of time required to complete a job or time worked since a lost-time accident. Many professionals such as lawyers and therapists charge a fee per hour.
The credit hour measures the time commitment of an academic course (typically university level) in terms of "contact hours" between students and staff per week. For example, a class that meets for one hour three times a week is said to be a 3-hour class.
The kilowatt-hour, the energy expended by a 1000-watt device in one hour, is commonly used as a billing unit for energy delivered to consumers by electric utilities. Conversely, the Btu-hour is a unit of power used in the power industry and heating/cooling applications. In the railroad industry, when sharing locomotives, the horsepower-hour may be used as a measure of energy.
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- There is a trace of that system, for instance, in Verdi's operas where in Rigoletto or in Un ballo in maschera midnight is announced by the bell striking 6 times, not 12. But in his last opera, Falstaff, strangely, he abandoned that style, perhaps under influence of contemporary trends at end of 19th century when he composed it, and the midnight bell strikes 12 times.
- "Non-SI units accepted for use with the SI, and units based on fundamental constants". Bureau International des Poids et Mesures. 2007-04-11.
- "Hour". Oxford English Dictionary (3rd ed.). Oxford University Press. September 2005. (Subscription or UK public library membership required.)
- "A Walk Through Time". National Institute of Standards and Technology. Retrieved 2 April 2014.
- John L. McKenzie, “Dictionary of the Bible”, Touchstone, 1965, p 375
- Landes, David S. 'Revolution in Time. Harvard University Press, 1983, p. 76.
- Holford-Strevens, Leofranc (2005). The History of Time: A Very Short Introduction. Very Short Introductions. 133. Oxford: Oxford University Press. ISBN 9780192804990.
- "Different Classification of Hours".