Kinds Of Hours

Hour: usually means 1/24th of a mean solar day, unless otherwise stated. Scientifically, it is defined as 3600 standard seconds.

Second: the fundamental unit of time. The accepted scientific definition of the second is now 9,192,631,770 periods of radiation corresponding to the transition between two hyperfine levels in the ground state of cesium 133.  This definition was adopted on 1967 and replaced the earlier (since 1955) ephemeris second which was defined in terms of a fraction of the mean tropical year. The above frequency was chosen because it gives a close approximation of the number of seconds in a day (86,400). Fluctuations in the Earth's rotational rate since about 1969 have been such that the day is between 1 to 2 milliseconds longer than this number of seconds. The word second derives from the Latin "secunda minuta" or second minute.

Mean Solar Day: the time between successive transits of the fictitious mean Sun (i.e. an imaginary sun which appears to circle around the celestial equator at a constant rate equal to the average rate of the Earth's real rotation). The basis of civil time keeping.

The definition given above for the "hour" did not always apply. The sun came up, the sun went down. Who knew about the fictitious mean Sun? When Thor got up to hunt he probably had a pretty good idea how long he had before he had to bring home supper. And he was probably quite aware of the fact that he had less time to do this in winter than in the summer. If he didn't, he probably was the supper.

The first kind of hours used were to define the periods of day and night.

Unequal Hours: an hour system where the duration of an hour depends on the date and is different from day time to night (except at the equinoxes). The number of hours during day time is usually 12, but may be 8 and just possibly 10 (e.g. on some mass dials). Counting of the day time hours begins at sunrise.

Assuming the number of day time hours is 12 and night time hours is also 12, each duration of 12 hours is divided into 12 equal intervals for any given day. The only days these hours will be equal over the 24 hour period is the equinoxes when day and night are of equal duration. The only place where these hours will be equal every day of the year is at the Equator. The length of day time and night time hours will vary throughout the year. In the summer, the day is longer and so will be each of the 12 day time hours. In the winter, the day is shorter and so will be each of the 12 day time hours. It is the opposite for the night time hours.

The following are other definitions for unequal hours that may be encountered.

Antique Hours: same as unequal hours or seasonal hours.

Biblical Hours: same as unequal hours or seasonal hours.

Great Hours: a term used for any of the unequal hour systems. Often labelled "gross uhr" on Nuremberg dials.

Seasonal Hours: a form of unequal hours, usually with 12 day time and 12 night time hours. Named from the fact that the hour length of an hour varies with the season.

Temporal or Temporary Hours: an unequal hour system with 12 hours from sunrise to sunset, and 12 hours (of a different duration) from sunrise to sunset.

In the early 1300s, with the advent of mechanical clocks, the unequal hours began to be displaced by equal hours. This is the kind of hour we are most familiar with today.

Equal Hours: any hour system where the length of the an hour is independent of the date, and the same during day time and night time.

The following are other definitions for unequal hours that may be encountered.

Common Hours: the standard 2 x 12 hour system, also called German or French hours. In Latin "horae communers", they are often labelled "kleine uhr" (small hours) on Nuremberg dials.

French Hours: an early name for the equal hour system with 2 x 12 hours per day, beginning at midday and midnight.

French Revolution Hours: the equal hours according to French Revolution time.

Modern Hours: the equal hours as used in modern time systems. they may occasionally be referred to as common, European, French, German or vulgar hours.

The following definitions are for hours that can be found on older sundials and are still used on present day sundials. they should be used with care as the sundial can easily become confusing and difficult to read.

Babylonian Hours: the number of hours elapsed since sunrise, with 24 equal hours per day. The origin of the term is unclear, but may be related to the fact that the ancient Babylonians originated the base-60 counting system for angles, etc. They are sometimes written as "horae ab ortu solis" or "H. AB ORT." on dials.

Italian or Italic Hours: the number of hours elapsed since the most recent sunset (hour 0), with 24 equal hours per day. They were used in many European countries during the period 1200 to 1800. They are sometimes written "horae ab occasu solis" or "H.AB OCC." on dials. The two terms (Italian and Italic" are used synonymously in modern works but there is some evidence in older works that Italian hours were counted from 30 minutes after sunset.

The following are other definitions for the previous two hours that may be encountered.

Bohemian Hours: same as Italian hours.

Greek Hours: the same as Babylonian hours.

Welsch Hours: i.e. foreign. Same as Italian hours.

A few additional definitions follow.

Canonical Hours: the seven times of the day (as opposed to time periods) used to define the services or divine offices in the medieval church. These offices were base on the sixth century rule of St. Benedict.

Decimal Hours: an hour system with ten equal hours per day (as sometimes used by the Chinese and and ancient Egyptians, and during the French revolution.

Nuremberg Hours: a hybrid equal hours system. The daylight hours were measured using the Babylonian hours system, staring with 1 at sunrise, while the night hours started with 1 at sunset and used the Italian hours system. In Latin, "horae norimbergenses".

Octaval Hours: a time system with the period of daylight divided into eight hours. Probably introduced by the Romans, circa 250 AD.

Planetary Hours: a planetary hour is the time needed for 15° of the celestial equator to rise above the horizon, counting from sunrise. As there are always 180°of celestial equator above the horizon, there are 12 planetary hours from sunrise to sunset but they are unequal not only from day to day but also from hour to hour.

The majority of sundials designed and constructed today show equal hours. There are those that include such hours as Babylonian, Italian or Planetary. A simple modification to a Italian hours sundial allows it to show the number of hours remaining until sunset. Such a sundial could be quite useful in the right application.