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The hour (symbol: h) is a unit of time. It is not an SI unit but is accepted for use with the SI. A pocket watch, a device used to keep time There are two distinct views on the meaning of time. ...
Cover of brochure The International System of Units. ...
Definition
In modern usage, an hour is a unit of time 60 minutes, or 3,600 seconds in length. It is approximately 1/24 of a median Earth day. Measurement is the determination of the size or magnitude of something. ...
A pocket watch, a device used to keep time There are two distinct views on the meaning of time. ...
A minute is a unit of time equal to 1/60th of an hour and to 60 seconds. ...
This article is about the unit of time. ...
Earth (IPA: , often referred to as the Earth, Terra, the World or Planet Earth) is the third planet in the solar system in terms of distance from the Sun, and the fifth largest. ...
Water, Rabbit, and Deer: three of the 20 day symbols in the Aztec calendar, from the Aztec Sun Stone. ...
There is also the hour of right ascension, a unit of both time and angle. Equatorial Coordinates Right ascension (abbrev. ...
An angle is the figure formed by two rays sharing a common endpoint, called the vertex of the angle. ...
History The hour was originally defined in ancient civilizations (including those of Egypt, Sumer, India, and China) as 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, and also to the fact that humans have 12 finger bones (phalanges) on one hand (3 on each of 4 fingers). (It is possible to count to 12 with your thumb touching each finger bone in turn.) There is also a widespread tendency to make analogies among sets of data (12 months, 12 zodiacal signs, 12 hours, a dozen). The duodecimal (also known as base-12 or dozenal) system is a numeral system using twelve as its base. ...
The phalanges in a human hand The name Phalanges is commonly given to the bones that form fingers and toes. ...
The Ancient Egyptian civilization is usually credited with establishing the division of the night into 12 parts, although there were many variations over the centuries. Astronomers in the 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 10 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. In astrology, a decan is the subdivision of a sign. ...
The heliacal rising of a star (or other body such as the moon or a planet) occurs when it first becomes visible above the eastern horizon at dawn, after a period where it was hidden below the horizon or when it was just above the horizon but hidden by the...
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 and Greeks and Jews used this definition; so 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 mean sun 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.
Latitude, usually denoted symbolically by the Greek letter phi, , gives the location of a place on Earth north or south of the equator. ...
Layers of Atmosphere (NOAA) Air redirects here. ...
The straw seems to be broken, due to refraction of light as it emerges into the air. ...
Nickname: The Eternal City Motto: SPQR: Senatus PopulusQue Romanus Location of the city of Rome (yellow) within the Province of Rome (red) and region of Lazio (grey) Coordinates: Region Lazio Province Province of Rome Founded 21 April 753 BC Mayor Walter Veltroni Area - City 1,285 km² (580 sq mi...
A clock (from the Latin cloca, bell) is an instrument for measuring time. ...
The astronomical clock in the Old-Town Square of Prague The Prague Astronomical Clock or Prague Orloj (Czech: Pražský orloj, pron. ...
Solar time is based on the idea that when the sun reaches its highest point in the sky, it is noon. ...
Solar time is based on the idea that when the sun reaches its highest point in the sky, it is noon. ...
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Counting hours There are different ways of counting the hours: - In ancient and medieval cultures, in which the division between night and day mattered far more than in societies with widespread use of artificial light, the counting of hours started with sunrise. So sunrise was always exactly at the beginning of the first hour, noon at the end of the sixth hour and sunset exactly at the end of the twelfth hour. This meant that the length of hours varied with the season. This type of counting is sometimes referred to, on astrolabes and astronomical clocks, for example, as "Babylonian" hours. It is also the system used in Jewish religious law (Halakha) and texts.
- In so-called Italian time, or "Italian hours", the first hour started at sunset (or the end of dusk, i.e., half an hour after sunset, depending on local custom). 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. This manner of counting hours had the advantage that everyone could easily see 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, or in some regions until the mid-19th century. It was also used in Poland and Bohemia until the 17th century. The system of Italian hours can be seen on a number of clocks in Italy, where the dial is numbered from 1 to 24 in either Roman or Arabic numerals. The St Mark's Clock in Venice is a famous example.
- The medieval Islamic day began at sunset. The first prayer of the day (maghrib) was to be performed between sunset and the end of twilight.
- 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, differing according to the equation of time (by up to about 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.
- 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.
Sunrise and sunset are much more conspicuous points in the day than noon or midnight; starting to count at these times was, for most people in most societies, much easier than starting at noon or midnight. However, with modern astronomical equipment (and the telegraph or similar means to transfer a time signal in a split-second), this issue is much less relevant. A 16th century astrolabe. ...
Prague astronomical clock Astronomical clock in Lund Cathedral An astronomical clock is a clock with special mechanisms and dials to display the relative positions of the sun, moon, zodiacal constellations, and sometimes major planets. ...
Halakha (Hebrew: הלכה; also transliterated as Halakhah, Halacha, Halakhot and Halachah) is the collective corpus of Jewish religious law, including biblical law (the 613 mitzvot) and later talmudic and rabbinic law as well as customs and traditions. ...
This 14th-century statue from south India depicts the gods Shiva (on the left) and Uma (on the right). ...
(17th century - 18th century - 19th century - more centuries) As a means of recording the passage of time, the 18th century refers to the century that lasted from 1701 through 1800. ...
Alternative meaning: Nineteenth Century (periodical) (18th century — 19th century — 20th century — more centuries) As a means of recording the passage of time, the 19th century was that century which lasted from 1801-1900 in the sense of the Gregorian calendar. ...
Flag of Bohemia Bohemia (Czech: ; German: ) is a historical region in central Europe, occupying the western and middle thirds of the Czech Republic. ...
(16th century - 17th century - 18th century - more centuries) As a means of recording the passage of time, the 17th century was that century which lasted from 1601-1700. ...
St Marks Clocktower undergoing restoration in 2004 St Marks Clock is the clock housed in the St Marks Clocktower, on St Marks Square in Venice, adjoining the Procuratie Vecchie. ...
Islam (Arabic: ; ( ▶ (help· info)), the submission to God) is a monotheistic faith, one of the Abrahamic religions and the worlds second-largest religion. ...
Maghrib is an Arabic term for of the setting (sun); from the root ghuroob (to set; to be hidden). It is also used in a manner similar to the metaphorical use of to be eclipsed, which is used in the English language. ...
The 12-hour clock is a timekeeping convention in which the 24 hours of the day are divided into two periods called ante meridiem (a. ...
Solar time is based on the idea that when the sun reaches its highest point in the sky, it is noon. ...
During the course of the year, the time as read from a sundial can run ahead of clock time by as much as 16 min 33 s (around October 31–November 1) or fall behind by as much as 14 min 6 s (around February 11–12). ...
Illumination of the Earth by the Sun on the day of equinox, (ignoring twilight). ...
The 12-hour clock is a timekeeping convention in which the 24 hours of the day are divided into two periods called ante meridiem (AM, Latin for before noon) and post meridiem (PM, Latin for after noon). Each period consists of 12 hours numbered 12, 1, 2, 3, 4, 5...
The 12-hour clock is a timekeeping convention in which the 24 hours of the day are divided into two periods called ante meridiem (AM, Latin for before noon) and post meridiem (PM, Latin for after noon). Each period consists of 12 hours numbered 12, 1, 2, 3, 4, 5...
The 24-hour clock is a convention of time-keeping in which the day runs from midnight to midnight and is divided into 24 hours, numbered from 0 to 23. ...
1925 (MCMXXV) was a common year starting on Thursday (link will take you to calendar). ...
The Julian day or Julian day number (JDN) is the number of days that have elapsed since 12 noon Greenwich Mean Time (UT or TT) on Monday, January 1, 4713 BC (in the proleptic Julian calendar; or November 24, 4714 BC in the proleptic Gregorian calendar). ...
Astrolabes, sundials, and astronomical clocks sometimes show the hour length and count using some of the older definitions and counting methods. A 16th century astrolabe. ...
Wall sundial Wall sundial in Warsaws Old Town A sundial measures time by the position of the sun. ...
Prague astronomical clock Astronomical clock in Lund Cathedral An astronomical clock is a clock with special mechanisms and dials to display the relative positions of the sun, moon, zodiacal constellations, and sometimes major planets. ...
See also The astronomical clock in the Old-Town Square of Prague The Prague Astronomical Clock or Prague Orloj (Czech: Pražský orloj, pron. ...
Canonical hours are ancient divisions of time, developed by the Christian Church, serving as increments between the prescribed prayers of the daily round. ...
To help compare orders of magnitude of different times this page lists times between 103 seconds (a kilosecond) and 104 seconds (16. ...
The 24-hour clock is a convention of time-keeping in which the day runs from midnight to midnight and is divided into 24 hours, numbered from 0 to 23. ...
References - "Astronomy before the telescope". Ed. Christopher Walker. London: British Museum Press, 1996.
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