Hour
The hour was originally defined in Egypt as 1/24 of a day, based on their duo-decimal numbering system (which counted finger
joints on each hand).
In modern usage, an hour was redefined as a unit of time 60 minutes, or 3600 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.
Earlier definitions of the hour:
- One twelfth of the time from sunrise to sunset. As a consequence, hours on summer days are longer than on winter days, their
length varies 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 unequal hours. Romans and Greeks used this definition and divided the night into three or
four night watches. Later, the night (i.e., the time between sunset and sunrise) was also divided into twelve hours. When a
clock showed these hours, its speed had to be changed every morning and evening (for
example by changing the length of its pendulum), or it had to keep position of the Sun on 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 vary 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 had to be adjusted virtually never.
However, as Earth rotation slows down, this definition has been abandoned. See UTC.
Counting hours
Every definition of the hour came with its own starting point for counting the hours.
- In antiquity, the counting of hours started with sunrise. So, sunrise is always exactly at the beginning of the 1st
hour, noon at the end of the 6th hour, and sunset exactly at the end of the 12th hour.
- In the so-called Italian time, the first hour starts at sunset (or the end of dusk, i. e., 1/2 hour after sunset,
depending on local custom). The hours are numbered from 0 to 23. So, the sun rises at Lugano in December around 14:46 and noon is
around 19:23; in June, the sun rises already at 7:51 and noon is around 15:55. Sunset is always at 24:00. This manner of counting
hours has the advantage that everyone can easily read the clock to see how much time he will have to finish his daywork 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. Noon is always close to 12
PM (post meridiem, after noon), differing according to the equation of time. At the equinoxes, sunrise
is around 6 AM (ante meridiem, before noon), and sunset around 6 PM.
- In the modern 24-hour clock, counting the hours starts at midnight,
and hours are numbered from 0 to 23. Noon is always close to 12:00 (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 day than noon or midnight; starting to count then is much easier than
starting at noon or midnight. With modern astronomical equipment (and the telegraph or similar means to transfer a time signal in
a split-second), this issue is no more relevant.
Sundials often show the hour length and count according to some of the older
definitions and countings.
The division of the day into 12 hours dates back to the Sumerians. There are
probably 12 hours because there are approximately 12 lunar months in a solar year. Symmetries of this sort are common in ancient
units of measurement.
See also
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