The hour (common symbol: h or hr; also known as a stound) is a unit of measurement of time. In modern usage, an hour comprises 60 minutes, or 3,600 seconds. It is 1/24th 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.
Although it is not a standard defined by the International System of Units, the hour is a unit accepted for use with SI, represented by the symbol h.
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, from Germanic *stundō ("time, interval, while").
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, India, and China 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.
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.
Earlier definitions of the hour varied within these parameters:
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.
Astrolabes, sundials, and astronomical clocks sometimes show the hour length and count using some of the older definitions and counting methods.
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 ba:Сәғәт be:Гадзінa be-x-old:Гадзіна bs:Sat (mjerna jedinica) bg:Час ca:Hora cv:Сехет (вăхăт) 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) hy:Ժամ hi:घंटा hr:Sat io:Horo ia:Hora is:Klukkustund it:Ora he:שעה jv:Jam ka:საათი (ერთეული) 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 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
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