Hour

From Free net encyclopedia

The hour (symbol: h) is a unit of time. It is not an SI unit but is accepted for use with the SI.

1 Definition
2 History
3 Counting hours
4 See also

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.

[edit]

History

The hour was originally defined in ancient civilisations (including those of Egypt, Sumeria, 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 (counting with each thumb the spaces between the joints of the other finger on the same hand, i.e. 3 x 4 = 12) and the equally widespread tendency to make analogies among sets of data (12 months, 12 zodiacal signs, 12 main compass points, 12 hours, a dozen).

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 or unequal hours. Romans and Greeks 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.
One twenty-fourth of the mean solar day. See mean sun for more information on the difference to the apparent solar day. When an exact 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.

[edit]

Counting hours

The definition of the hour arose with its own starting point for 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.
In so-called Italian time 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 0 to 23. So the Sun rose at, for instance, Lugano in December around 14:46 and noon was around 19:23; in June the Sun rose at 7:51 and noon was around 15:55. Sunset was always at 24:00. This manner of counting hours had the advantage that everyone could easily read the clock to see how much time they had to finish their day's work without artificial light. It was introduced in Italy during 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.
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.

Sunrise and sunset are much more conspicuous points in the day than noon or midnight; starting to count then 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.

Sundials often show the hour length and count according to some of the older definitions and countings.

[edit]

Hour

From Free net encyclopedia

Contents

Definition

History

Counting hours

See also

Views

Personal tools

Search

Partner sites