The hour (common symbol: h or hr also known as a stound is a unit of time It is not an International System of Units unit but is Units accepted for use with SI with the symbol h http://www.bipm.org/en/si/si_brochure/chapter4/table6.html

Definition

In modern usage, an hour is a unit of measurement of time of the duration of 60 minute , or 3600 second . It is 1/24 of a median Earth day An hour in the Universal Coordinated Time (UTC) time standard can include a negative or positive leap second and may therefore have a duration of or for adjustment purposes.

Etymology

Middle English urefirst appears in the 13th century, as a loanword from Old French ure, ore from Latin hora ultimately from Greek "season, time of day, hour". Middle English ure Anglo-French houre replaced Old English tīd(which survives as Modern English [[tide]] and stund(which survives as [[Wikt:stound|stound]]— compare Old High German stunta from a Germanic *stundō"time, interval, while"). Greek is cognate to English [[year]] both from a PIE {{PIE|*i̯ēro-}}"year, summer".

History

The hour was originally defined in ancient civilizations (including those of Egypt lt;ref>lt;/ref>, 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 (number) has been attributed to the number of lunar cycles in a year. Astronomers in the Middle Kingdom of Egypt (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. 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 Earth's atmosphere s index of refraction . For this reason, these hours are sometimes called temporal seasonal or unequal hours Rome Greece and Jews of the ancient world used this definition; 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 solar time (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 equalor equinoctialhours. * 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 Earths rotation slows down, this definition has been abandoned. See UTC

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 (the zerohour), noon at the end of the sixth hour and sunset exactly at the end of the twelfth hour. This meant that the duration of hours varied with the season. This type of counting is sometimes referred to, on astrolabe and astronomical clock , for example, as "Babylonian" or temporalhours. It is also the system used in Jewish religious law (Halakha and frequently called Talmudic hour("Shaa Zemanit") in a variety of texts. The talmudic hour is one twelfth of time elapsed from sunrise to sunset, therefore being longer during summer than in winter. * In so-called [[Italy|Italian]] time "Italian hours", or "Old Czech Time", the first hour started with the Angelus at sunset (or 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 days 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 centuryThere is a "trace" of that system, for instance, in Verdi s opera where in [[Rigoletto]]or in [[Un ballo in maschera]]midnight is announced by the bell striking 6 times, not 12 as we are accustomed to it today! But in his last opera, Falstaff (opera) 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.. 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 time is always close to 12 noon, differing according to the equation of time by as much as fifteen minutes either way. At the equinox s 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 doesnt change during a single nights 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. Astrolabe , sundial , and astronomical clock sometimes show the hour length and count using some of the older definitions and counting methods.

Further reading

* *"Astronomy before the telescope". Ed. Christopher Walker. London: British Museum 1996.

See also

*Canonical hours

External links

References

Category:Orders of magnitude (time) Category:Units of time als:Stunde ar:ساعة (وحدة) arc:ܫܥܬܐ ast:Hora bn:ঘণ্টা (সময়) zh-min-nan:Tiám-cheng be:Гадзінa be-x-old:Гадзіна bs:Sat (mjerna jedinica) bg:Час ca:Hora cs:Hodina da:Time (enhed) de:Stunde et:Tund el:Ώρα es:Hora eo:Horo eu:Ordu fa:ساعت (زمان) fr:Heure fy:Oere fur:Ore gv:Oor gd:Uair gl:Hora hak:Sṳ̀ ko:시간 (단위) hi:घंटा hr:Sat io:Horo is:Klukkustund it:Ora he:שעה jv:Jam sw:Saa ht:Lè (tan) la:Hora lv:Stunda lb:Stonn lt:Valanda ln:Ngonga hu:Óra (időegység) mr:तास arz:ساعه (وحده) ms:Jam nl:Uur ja:時間 (単位) no:Time (tidsenhet) nn:Time nrm:Heure oc:Ora (temps) pa:ਘੰਟਾ pnb:کینٹہ nds:Stünn pl:Godzina pt:Hora ksh:Shtůndt ro:Oră qu:Ura ru:Час sah:Чаас se:Diibmu sco:Oor (time) simple:Hour sk:Hodina sl:Ura sr:Сат sh:Sat su:Jam fi:Tunti sv:Timme ta:மணித்தியாலம் th:ชั่วโมง tr:Saat (zaman) uk:Година ur:گھنٹہ vi:Giờ wa:Eure wo:Waxtu yo:Wákàtí zh-yue:鐘頭 zh:小时