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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. 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 te be adjusted virtually never. However, as earth rotation slows down, this definition has been abandoned. See UTC.

1 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 PolandThe Republic of Poland a country in Central Europe, lies between Germany to the west, the Czech Republic and Slovakia to the south, Ukraine and Belarus to the east, and the Baltic Sea, Lithuania and Russia (in the form of the Kaliningrad Oblast exclave) t and BohemiaBohemia Cechy in Czech, Bohmen in German) is an historical region in central Europe, occupying the western and middle thirds of the Czech Republic. With an area of 52,750 sq. 25 million of the country's 10. 3 million inabitants, Bohemia is bounded by Germ until the 17th century16th century 17th century 18th century more centuries) As a means of recording the passage of time, the 17th century was that century which lasted from 1601- 1700. During this period, the power of England and the United Provinces increased; while that of.
• In the modern 12-hour clockThe 12-hour clock is a time-keeping convention in which the 24 hours of the day are divided into two periods called ante meridiem (am, Latin for "before noon") and post meridiem (pm, Latin for "after noon"). Each period consists of 12 hours numbered 12, 1, counting the hours starts at midnight and restarts at noon. Hours are numbered 12, 1, 2, ..., 11. NoonNoon is the time exactly halfway through the day, written 12:00 in the 24-hour clock and 12:00 pm in the 12-hour clock. Midday is also used as a synonym for noon, although this may also be a more general term to mean around noon, or very early afternoon ( is always close to 12 pm (differing according to the equation of time). At the equinoxIn astronomy, an equinox is defined as the moment when the sun reaches one of two intersections between the ecliptic and the celestial equator. The word "equinox" comes from the Latin for equal night . The equinoxes in March and September are the two timees, sunrise is around 6 am, and sunset around 6 pm.
• In the modern 24-hour clock24-hour clock 12-hour clock 00:0012:00 am 01:001:00 am 02:002:00 am 03:003:00 am 04:004:00 am 05:005:00 am 06:006:00 am 07:007:00 am 08:008:00 am 09:009:00 am 10:0010:00 am 11:0011:00 am 12:00 (noon)12:00 pm 13:001:00 pm 14:002:00 pm 15:003:00 pm 16:004:0, 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 astronomic equipment (and the telegraph or similar means to transfer a time sign in a split-second), this issue is no more relevant.

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.