Why does the day have 24 hours?
How the need to divide the day came about, why it is precisely 24 hours, and how people learnt to measure each of those hours.
This article is part of a trilogy on time. You can read all the parts here:
St Mark's Square in Venice is quite an experience. I would be lying if I said that any of the Basilica of San Marcos is not the most striking. The first time you come across this gem, it is impossible not to be captivated. But in the same square you can also contemplate the exterior of the Museo Correr1, St Mark's Campanile or the St Mark's Clocktower.
This clock tower, in fact, is crowned with a very unusual clock.
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I am not referring to the curious choice of Roman numerals, with the use of IIII instead of IV2, but to the 24 hours that the clock covers.
Today's article will be a historical journey to explain how we came to divide the day into 24 hours instead of 10, 12 or 100.
The need to divide the day
The only natural division of the day, as given by nature, is day and night. We can assume that this breakdown, which affects the dynamics of nature so much, is something that humans have internalised since they first set foot on Earth. The Sun's journey across the sky reaches its highest point consistently halfway through the day. This characteristic point marked the second great division of the day, dividing it into four fundamental parts: morning, when the Sun was rising; noon, when the Sun was at its highest point; afternoon, when the Sun was setting; and night, when the Sun was absent.
This division possibly set certain patterns of cooperation in the first human groups, but they were definitely key when the first agricultural societies emerged. It was much better to get up with the sunrise, when it was not so hot, to work the land, and consequently be able to eat and rest when conditions were more difficult.
With the emergence of the first settlements and the first civilisations, social complexity increased. Beliefs became sophisticated religions, which required a certain structure and regular rituals. The need to mark the time of day when a ceremony was to begin led to the division of the day into smaller chunks of time.
Although we have no archaeological records to confirm it, it is most likely that the first measurings of time throughout the day were based solely on the position of the Sun in the sky. Possibly, together with certain environmental references or even the shadows that different elements cast on the ground, which allowed for a certain consistency in the measurement. In China, Mesopotamia, and Egypt, it is likely that the gnomon3, a primitive sundial, began to be used around 5,500 years ago, although there is no record of this due to the perishable materials used4.
The first artefacts we have records of date back 3,500 years and were found in Egypt. There were the sundials carved in stone, whose operation only needs a gnomon to mark the shadow, some lines carved in the stone and the Sun to do its work every day. For times when the sun was not available, either on cloudy days or at night, water clocks were also developed.
Water clocks had a much more sophisticated, although tremendously intuitive, way of working. The oldest one that we are aware of is described in an Egyptian tomb from the 16th century BC. It was depicted as a large stone vessel with a small hole near the bottom, and a second container at the bottom with twelve columns with regularly spaced markings on each one. Once the vessel was filled with water, it would drip steadily to fill the lower container. The water rising in the lower container, compared to the markings on each column, allowed to measure the time that had passed since the moment the stone vessel was filled, regardless of the time of day.
Why did it have 12 columns with markings? Because the nights throughout the year did not last the same, so they had a column that was used each month. That way, they could divide the night into the same consistent number of hours. The hours did not always last the same, but the nights always had the same number of hours, which, again, could be used to perform rites and sacrifices at specific times across the night.
Why 24 hours?
Although the decimal system has become the norm all over the world, this was not always the case. Throughout history, different civilisations have used different systems with different numerical bases, which were usually used for different purposes. In Babylonia, the base-60 system was very common, while the Kharosthi (India) and the Chumash (California) got by with a base-4 system. The Mayans and Aztecs are known for their particular base-20 system, and the Romans flirted with base-5, 10 and 12 number systems.
But we only need to look at the first regions that began to measure the hours of the day to find an answer5. Both Mesopotamia and Egypt used the base-12 number system. Among the hypotheses that could have contributed to the choice of this number are the 12 lunar cycles that, more or less, constitute a calendar year. Also, the twelve phalanxes that can be counted on the thumb of each hand.
The importance of this number led to the division of the daytime into twelve hours of equal duration, as shown in the twelve divisions of the sundials that have been preserved. To equate night as the second fundamental part of the day, it was also divided into twelve equal parts, although one hour was reserved for sunset, another for sunrise and only the remaining ten for the dark night.
We know this thanks to writings that detailed the operation of the merkhet, a clock that allowed the night hours to be measured by the transit of ten reference stars6. An Egyptian example from the 6th century BC is preserved in the Science Museum in London. It consists of a wooden pole with a plumb line which, together with observation points in the temples, was used to follow the passage of ten stars along a pre-established meridian. According to the texts, two individuals and two merkhets were required to take the measurings.
This division of the day into 24 hours, which originated in Egypt, was inherited by Ancient Greece and the Hellenistic world, where measuring tools underwent substantial technological improvements, especially regarding water clocks, called clepsydras by the Greeks. Not only did they have the capacity to adapt to different measuring stocks for each month, but they also allowed the volume of water to be changed daily, achieving a more efficient division for every day of the year.
The standardisation of time
In Ancient Greece, as a legacy of Egypt, the hours had a variable duration throughout the year. The daylight hours were longer in summer and shorter in winter, but always 12 consistently. The same applied to the nights, which had longer hours in winter and shorter hours in summer, but 12 on any day of the year.
Hipparchus was the first to propose that the 24 hours of the day should last the same amount of time, either daylight or night, and the same every day of the year. To accomplish this, he suggested that the length of the hours at the equinoxes, when day and night are of equal length, should be taken as a reference.
The theoretical model was complemented by Greek advances in the field of geography, thanks to which each of the 360 degrees into which the Earth was divided were further divided into 60 partes minutae primae and 60 partes minutae secundae. These first minutes and second minutes came to be called minutes and seconds, which were also incorporated as subdivisions of the hours of the day.
Despite having a defined way of measuring time, available tools were much more limited. Measurement systems developed by the Greeks allowed for much greater accuracy than those used by the Egyptians, but they were still far from being cheap and practical. For this reason, daylight hours continued to be measured with sundials, mechanisms that could be easily built on a large scale and that, moreover, each person could have at his or her disposal at a low cost.
With the appearance of the first mechanical clocks in Europe in the 14th century, it was possible to revisit the entire theoretical development of Hipparchus and the rest of the Greeks, and put it into practice. Initially, this mechanical clocks combined part of the knowledge acquired with water clocks along with other mechanisms such as pendulums, weights, and cogwheels. As a result, it was possible to build astronomical clocks in the towers of many Gothic churches, like this one in Wells Cathedral in England, from 1392.
This clock has the same peculiarity as the first clock with which I began today's article, the one in St Mark's Clocktower. Both clocks measure 24 hours a day, and not 12 like modern clocks. With these clocks, which a few centuries later would fall into disuse, one last pending question can be better understood. If historically the night has always been divided into 12 hours and the day into twelve hours, why does the 24-hour day begin in the middle of the night and not at dawn or dusk?
Just as Hipparchus took the hours of the equinoxes as a reference to define the duration of an hour, the new mechanical clocks needed to reference an hour that was consistent throughout all the days of the year. That moment, unmatched by any other moment of the day, was the moment when the Sun reached its highest point. Always, day after day, that moment marked the middle of the daytime part of the day, so it was fitting to assign it also half of the 24 hours of the day: noon.
As if this were not enough, on 24-hour clocks that moment was also located at the bottom of the clock, which can also be interpreted with the south marked by the sun at midday in the Northern Hemisphere.
What more could you ask for?
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This is present on many medieval clocks in Europe and, in some way, it became a tradition which can still be seen on many clocks today.
The gnomon refers to the part of the sundial that casts the shadow. In other words, the stick.
The oldest one on record was found in Taosi, China, and dates from 2300 BC.
In China, people began to measure time at around the same time as in Mesopotamia and Egypt, but the latter two had a direct influence on Greece and, as a consequence, on the whole of the Western world.
In reality, this is much more complex. Originally, the Egyptians divided the firmament into 36 decans or groups of stars that circulated across the sky throughout a complete day, taking a reference star for each one. 18 of those 36 reference stars were used for the night, although later the set of reference stars was reduced to 24 during the New Kingdom (approximately in 1500 BC).
I really enjoyed that essay and I ended up learning something too! Excellent work 👏 Best regards David M