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Hours and hours

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This is one of a series of articles written for "Clocks" magazine by the late Noel Ta'bois, and reproduced with permission here as a memorial to him.

First an ambiguity to be clarified! The word 'day' can mean the period from sunrise to sunset, daytime as opposed to night-time; or it can mean the time taken for the earth to revolve once on its axis, the period of daytime and night-time together. I shall meticulously refer to the former as daytime or daylight and the latter as day.

Nowadays everywhere in the civilised world the day is divided into 24 hours. At all times in all parts of the world all hours have the same length and they are known as equal hours. They are mean solar hours and each is one 1/24th of a mean solar day. Because the earth's orbit is elliptical and its axis inclines from the plane of the orbit, solar days vary in length. A mean value of all the days in a year is taken in order to give precise values to the lengths of the day and the hour which are units of time measurement.

In primitive times man lived and worked in daylight. It was therefore natural that only the daytime was divided into hours. Later, as artificial light came into use, the night was also subdivided.

At the equator all through the year, and all over the world at the equinoxes, all hours are mean solar hours. A little north or south of the equator, after the spring equinox, the daylight gets longer and the night shorter. If either the daylight or the night is divided into 12 equal parts, the hours so produced are a little longer in daytime and a little shorter at night than a mean solar hour and the difference in length between day hours and night hours increases to a maximum at the summer solstics.

Also, as one moves further from the equator the difference in length between daylight and darkness becomes greater, and so therefore does the difference in length between day and night hours. The difference is greatest at the Arctic and Antarctic circles beyond which there are six months daylight and six months darkness.

After the summer solstice the difference in length between day and night hours decreases until the autumn equinox when the difference is zero again. From the autumn equinox, through the winter solstice to the spring equinox the process is repeated but now the daytime hours are shorter than those at night, and again the effect is greater the further one is from the equator.

These hours which vary in length with the seasons and with the distance from the equator are called unequal hours. Note that the inequality is from day to day or between daytime and night-time hours. On a given day all day hours are equal to each other and all night hours are equal to each other.

These hours which vary in length with the seasons and with the distance from the equator are called unequal hours.

Unequal hours are also known as planetary, temporal, temporary, or seasonal hours.

It must have seemed strange to have lived in times when the length of the hour changed abruptly at sunrise and sunset. This change would have been most noticeable at the solstices ad the further one was from the equator.

Having explained the differences between equal and unequal hours I will now list and define all the types of hours that readers may come across in their sundial and associated literature. I do hope those who spot any omissions will let me know?

Babylonian hours

Equal hours obtained by dividing the day into 24 equal parts starting at sunrise. Compare Italian hours.
Bohemian hours
The same as Italian hours.
Canonical hours
Specific times of the day appointed by the canons for church offices, the most important being matins, lauds, prime, terce or tierce, sext, nones, vespers, and compline. The times within which marriage may legally be performed.
Day hours
Unequal hours produced by dividing the period from sunrise to sunset into equal parts.
Decimal hours
Hours obtained by dividing a time period usually half a day, into ten equal parts. Latin decem ten. Can be equal or unequal.
Quodecimal hours
Hours obtained by dividing a time period, usually half a day, into 12 equal parts. Latin duo and decem ten. Can be euqal or unequal.
Egyptian hours
Equal and unequal systems were used and in both of them the day was divided into 24 parts.
Equal hours
Hours which are always the same length and are units of time measurement.
Equatorial or Equinoctial hours
Mean solar hours.
Greek hours
The same as Jewish hours.
Italian hours
Equal hours obtained by dividing the say into 24 equal parts starting at sunset, or half an hour after sunset at Ave Maria. Compare Babylonian hours.
Japanese hours
Unequal hours obtained by dividing daytime and night-time into six hours each. This system was in use until 1873 and ingenious methods were adopted to produce clocks which would show unequal hours. These included dial charts which were changed every two weeks, adjustable hour plaques, and twin foliots which changed automatically every six hours one being wet for daytime hours the other for night-time hours.
Jewish hours
Unequal hours obtained by dividing the period from sunrise to sunset into 12 equal parts.
Little hours
A term occasionally used to denote canonical hours of lesser importance.
Mean solar hours
The hours to which we are accustomed, obtained by dividing a mean solar day into 24 equal parts.
Monastic hours
Times set aside in a monastery for devotions. Similar to canonical hours.
Night hours
Unequal hours produced by dividing the period from sunset to sunrise into equal parts.
Nurnberg (Nuremburg) hours
Equal hours. A system used in the Nurnberg area of Germany in medieval times in which the day was divided into 24 hours which were counted from both sunrise to sunset. Thus at the summer solstice there were 16 daytime hours and eight night hours, and at the winter solstice eight daytime hours and 16 night hours. At the equinoxes there were two periods of 12 hours like our present hours.
Planetary hours
Unequal hours so called from the belief that the hours were in turn dominated by one of the planets.
Seasonal hours
Unequal hours so called because the length of the hour varies with the seasons.
Sidereal hours
Equal hours obtained by dividing the sidereal day into 24 equal parts. A sidereal hour is about 10 seconds shorter than a mean solar hours.
Small-hours
Hours immediately after midnight.
Temporal or Temporary hours
Unequal hours.
Unequal hours
Hours produced by division of the periods from sunrise to sunset or from sunset to sunrise into equal parts. An important point to note is that when the daytime is divided into 12 parts to produce unequal hours, the sun is on the meridian at the 6th hour whatever the season. With Babylonian hours, which are equal hours counting from sunrise, the sun is on the meridian at the sixth hour only at the equinoxes. The same argument applies to the 18th hour of Italian hours, which are equal hours starting from sunset. The time of midday varies with the seasons.

A diagram of Babylonian and Italian hours on a sundial appeared in Clocks for September 1985, page 47, and shows the 6th and 18th hour crossing the meridian on the equinoctial lines.

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