hour

The hour (symbol: h, or occasionally hr; via Latin hora from Greek ὥρα "season, time span", ultimately cognate to English year) is a unit of time. It is not an SI unit but is accepted for use with the SI.

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.

There is also the hour of right ascension, a unit of both time and 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 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.

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.

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.

Astrolabes, sundials, and astronomical clocks sometimes show the hour length and count using some of the older definitions and counting methods.

See also

• Canonical hours
• Times from 1 kilosecond to 10 kiloseconds
• 24-hour clock (use of this system is sometimes indicated with the word "hours" being placed after the time)

References

• "Astronomy before the telescope". Ed. Christopher Walker. London: British Museum Press, 1996.

Further reading

• Gerhard Dohrn-van Rossum (1996). History of the hour: clocks and modern temporal orders. University of Chicago Press. ISBN 0226155102. 

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