Hour

Midnight to 1 a.m. on a 24-hour clock with a digital face
Midnight (or noon) to 1 on a 12-hour clock with an analog face

An hour (symbol: h;[1] also abbreviated hr.) is a unit of time conventionally reckoned as ​124 of a day and scientifically reckoned as 3,599–3,601 seconds, depending on conditions.

The hour was initially established in the ancient Near East as a variable measure of ​112 of the night or daytime. Such seasonal, temporal, or unequal hours varied by season and latitude. The hour was subsequently divided into 60 minutes, each of 60 seconds. Equal or equinoctial hours were taken as ​124 of the day as measured from noon to noon; the minor seasonal variations of this unit were eventually smoothed by making it ​124 of the mean solar day. Since this unit was not constant due to long term variations in the Earth's rotation, the hour was finally separated from the Earth's rotation and defined in terms of the atomic or physical second.

In the modern metric system, hours are an accepted unit of time defined as 3,600 atomic seconds. However, on rare occasions an hour may incorporate a positive or negative leap second,[a] making it last 3,599 or 3,601 seconds, in order to keep it within 0.9 seconds of UT1, which is based on measurements of the mean solar day.

Name

Hour is a development of the Anglo-Norman houre and Middle English ure, first attested in the 13th century.[2][b]

It displaced tide tīd, "time"[4] and stound stund, span of time.[5] The Anglo-Norman term was a borrowing of Old French ure, a variant of ore, which derived from Latin hōra and Greek hṓrā (ὥρα).

Like Old English tīd and stund, hṓrā was originally a vaguer word for any span of time, including seasons and years. Its Proto-Indo-European root has been reconstructed as *yeh₁- ("year, summer"), making hour distantly cognate with year.

The time of day is typically expressed in English in terms of hours. Whole hours on a 12-hour clock are expressed using the contracted phrase o'clock, from the older of clock.[6] (10 am and 10 pm are both read as "ten o'clock".)

Hours on a 24-hour clock ("military time") are expressed as "hundred" or "hundred hours".[7] (1000 is read "ten hundred" or "ten hundred hours"; 10 pm would be "twenty-two hundred".)

Fifteen and thirty minutes past the hour is expressed as "a quarter past" or "after"[8] and "half past", respectively, from their fraction of the hour. Fifteen minutes before the hour may be expressed as "a quarter to", "of", "till", or "before" the hour.[8] (9:45 may be read "nine forty-five" or "a quarter till ten".)