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Hours and hours
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?
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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