Why do clocks usually show just a division into 12 hours, when there are 24 hours in a day?
The origins of our 24 hour day can be traced back at least 4000 years, to ancient Egypt and Babylon, and perhaps further back in time. The Egyptians and Babylonians divided the parade of stars that appeared in the sky each night into 12 sections, marked by the various stars that rose and set that night. For example, the star Procyon might rise shortly after sunset one evening, followed about an hour later by Sirius. This defined a kind of heavenly clock, although different groups of 12 stars were used to cope with the slow shift of the night sky during the year. The daylight hours were divided into 12, to match. Two sets of 12 give 24, hence the number of hours in a day.
The famous astronomical ceiling at Senmut shows a series of circles divided into 24 sections. It’s not clear what these circles signify – the 12 circles are labelled with month names.
Why 12? 12 is more or less the number of moon cycles in a year, so it’s a special number in most cultures.
The dial of a sundial is equivalent to a 24 hour clock face. Sundials track the motion of the sun, so noon and midnight appear directly opposite each other on the face, with 06:00 (VI) and 18:00 (VI) on either side.
Not many sundials show all 24 hour markers, though some do. This is probably for aesthetic reasons: it makes a symmetrical design, and gives the maker something to put all round the edge of a circle, even though half of them would never be used. The famous modern sundial near Tower Bridge, in London, shows the full set of 24 hour marks, but you can see them on some of the more decorative antique sundials too.
The equatorial dial on this polyhedral sundial is marked with all 24 hours (1..12, 1..12). It was made by Hans Koch in Munich in 1578. The original is in Munich.
Here’s an engraving from an 18th century treatise by Ferguson showing how a sundial would look if numbered all the way round the edge.
If you ever get to spend a summer’s day at the North Pole, why not use the pole itself to make a sundial? You’ll be able to construct a 24 hour dial, and you’ll see that the midnight and noon marks are opposite each other. During the winter, the sun isn’t visible, however.
What did the Romans ever do for us?
The Romans inherited the 24 hour day (in the double-12 form, two sets of 1 to 12 numbers) from the Egyptians, via the Greeks: 12 hours of daylight, followed by 12 hours of night, with hours of variable length depending on the time of year. They started counting from sunrise (hour 1 = Prima), so hour 3 (Tertia) was mid-morning, hour 6 (Sexta) was midday, and hour 9 (None) was mid afternoon.
Echoes of this system linger today – we call a midday break a siesta, and noon is derived from None (but may have crept forward due to hunger).
Arabs and astrolabes
The astrolabe was another forerunner of the clock. The Arab astronomers were adding gears to their astrolabes by the 8th century, producing prototype clocks. As the astrolabe is a model of the solar system, it obviously uses the 24 hour dial, rather than show two revolutions of 12 hours each per day. But given their generally better weather, they didn’t need to develop alternatives to the astrolabe and sundial, such as water and weight-driven clocks.
The early history of the clock (from about 1200 to 1350 AD) is not well known, and is still argued over by historians. None of the early clocks have survived, so we still don’t know who invented it, or where.
Monks and religious institutions wanted to run their services (called Prime, Tierce, Sext, None, etc – using the Roman divisions of 1, 3, 6, 9) at regular intervals throughout the day and night, and were looking for something better than sundials, astronomy, and water clocks, all unreliable in cloudy and cold Northern Europe. In particular, the timing of the midnight service proved very difficult. Hence the intense interest in making a mechanical clock.
We can guess that the first clocks didn’t even have dials – they were probably just devices for ringing bells at regular intervals. (The word ‘clock’ means bell.) Bells were used for relaying information around churches and monasteries, and in towns and cities. Bells signalled the times of services, the start and end of work shifts, the opening of markets, the start of meetings, court events, trials and executions, and so on, and would have used combinations and special sequences to indicate messages. So a device for automating the ringing of bells eventually led to the development and rapid introduction of a generalized time-measuring system, and machines for keeping it.
And behind all this practical need for machines and systems to keep religious and secular life punctual was a desire to look upwards and emulate something of the spectacular display of the sun, moon, planets, and stars, to inspire the religious spirits.
One of the earliest mentions of a clock-like machine was in 1271, when Robertus Anglicus wrote that:
clockmakers are trying to make a wheel that will move exactly as the motion of the equinoctial circle
So a wheel that revolved once every 24 hours was the obvious solution for a device that not only kept the time, day and night, but echoed the religious view of the universe, where the heavens revolve in an God-given orderly procession around the earth.
Nearly 100 years later, the clockmakers had succeeded, since the great Italian clockmaker De Dondi could quickly gloss over the mechanics of the 24 hour clock when he writes about his astronomical clock:
but the method of making this clock will not be discussed in such detail as the rest because its construction is well known, and there are many varieties of them, and however it is made the diversity of methods does not come within the scope of this work since we desire nothing more from it [the clock at the center of his planetarium] than the uniform and equal motion of a wheel which shall complete its course in the space of a natural day, and such a wheel is called the horary sphere.
From about 1300 on, clocks developed quickly along two main paths. In towns, the public or church clock became a common sight (and sound), ordering and marking the passing of the hours of day and night. When dials were eventually introduced, they would be 24 hour ones, although few have survived in their original form, so we can’t be sure.
Other clocks developed into sophisticated planetaria: Richard of Wallingford’s famous astronomical clock, which he built over a course of 30 years at St Albans starting in 1330, and Giovanni de Dondi’s clock, completed in 1364, were both mechanical marvels, calculating moon and planetary orbits, and even eclipses. These had 24 hour dials, showing the sun and earth’s relative motions during the day and night in graphic form.
The clock at Wells Cathedral dates from the end of the 14 century, although the dial is probably later.
Timekeeping methods varied from country to country. In Italy, they counted their hours from sunset, so by sunrise in the morning the hour was already about 12 or 13. The 1 to 24 numbering system used in Italy was known in France, Germany, and England as Italian Hours. The northern Europeans often divided the 24 hours up into two sets of 1 to 12, probably arranged so that 12 noon is at the top of the dial, and 12 midnight at the bottom. Notice how the Wells dial, above, doesn’t actually use the Roman numeral XII for either.
The famous clock of the Beata Vergine (later San Gottardo) in Milan, built around 1330, was one of the earliest to strike a bell a number of times to tell the time (not just striking once on the hour). In 1335, Galvano Fiamma writes:
There is there a wonderful clock, because there is a very large clapper which strikes a bell 24 times according to the 24 hours of the day and night, and thus at the first hour of the night gives one sound, at the second two strokes, and so distinguishes one hour from another, which is of greatest use to men of every degree.
By contrast, the French and Northern Europeans were using the 24 hour double-XII dial, and were also introducing the single 12 hour dial. The 12 hour system was known as German hours in Bohemia, and French hours in Italy. Clocks imported into Italy were often converted from French-style numbering to use the 24 hour Italian system. In Southern Germany (Augsburg), which became one of the main centres of clockmaking by the 15th century, clockmakers were making clocks with both 12 hour and 24 hour dials, sometimes with both on the same clock. They used the terms ‘great’ and ‘small’ time systems to refer to the 24 and 12 hour time systems. This German alarm clock from the 1500s has three rings, each showing Arabic rather than Roman numerals:
When did the roman-style counting (starting from dawn with 1) give way to our modern style counting, starting at midnight (or midday) with 0? Perhaps when clocks were regularly synchronized with the sun. Noon/midday is the easiest and most accurate way of synchronizing clocks with solar time, the basis for time until clocks were accurate enough to track the Earth’s irregularities.
Why the move to 12 hour time?
Both the 24 hour time system (1 to 24, as used in Italy) and the double-XII system (1 to 12 then 1 to 12 again, as used in England and Northern Europe) can be displayed on a 24 hour dial. So why did the 12 hour dial, with the added ambiguity of AM and PM, become popular and eventually dominate, for general-purpose public clocks, at least, between 1400 and 1600?
Here are some suggestions:
|power needed to ring bell||Ringing a bell up to 24 times uses up a lot of power (up to twice as much as ringing the bell 12 times!), and reduces the period between windings. The 12 hour dial reduced the amount of power required to ring the bell on the hour. Also, portable spring-driven clocks – and later watches – were the latest thing, and the power savings gained by the 12 hour dial were eagerly seized by the clockmakers. Italy eventually followed other European countries, which had started switching to the 12 hour clock by 1550.||A double-XII dial could have had matching bell ringing sequences (1 to 12, then 1 to 12) but still displayed on a 24 hour dial. Why change a familiar dial?|
|lose count of 24 strokes||It’s been suggested is that people lost count while listening to a long run of up to 24 bells. So the switch to 12 made it easier to tell the time||Clock makers would have introduced some form of code (one bell rings before 12, another bell rings from 12 to 24, for example) if they had had the power to spare. Besides, counting bells isn’t that difficult, really. 4 year old children can do it. Although it’s easy to lose count if you’re in a noisy environment.|
|too difficult to read||A 24 hour dial squeezes in 6 hours between what the 12 hour dial shows as 0900 and 1200 – so it’s difficult to tell the difference between, say, 10 and 11, because they’re closer together. This is particularly true before minute hands were widely used (before about 1650?) People are looking at a tower clock from some distance too, and upwards – it would be much easier for a 12 hour dial (or even a 6 hour dial).||It would depend on the design, partly, and the way the dial was numbered. And ‘difficult’ is very subjective: we now learn the 12 hour dial intensively when young, so how do we know whether 24 hour dials are really harder to read? Do we find minutes hard to read – they’ve got 60 divisions?|
|shows up inaccurate clocks too quickly||A 24 hour dial increases the required precision for the hour hand, because the hour hand moves less during an hour: if it’s not accurate, it will soon start to be slower or faster. On a 12 hour dial with the same works, it takes twice as long to look like it’s an hour adrift.||An ingenious but not entirely convincing argument!|
|not enough resolution for accurate time keeping||With more hours squeezed into a smaller arc, it was harder to read the exact time from a single hand on the 24 hour dial than on the 12 hour dial. This was before the widespread introduction of the minute hand, of course, so the difference between half past 10 and a quarter past ten would be harder to discern on a 24 hour dial, unless the radius of the dial was very big.||This is a more convincing version of the above argument. But I’m not sure if the dates for the introduction of the minute hand and the growing popularity of the 12 hour dial coincide, though. Anyway, no clocks were really accurate (to a minute or so) before the introduction of the pendulum in about 1650.|
|dominance of one country forced others to adapt||The clockmakers of France and, to some extent, Germany tended to use the 12 hour dial more than the 24 hour dial, and the technical superiority of their clocks forced out the less efficient makers of clocks with 24 hour dials, such as Italy. The gradual drift northward of the power centres of Europe during the renaissance is well documented.||This is plausible. But is it true? And what about English clockmakers?|
|avoids duplication so quicker to make||The urge to simplify complexity and thereby reduce work and hopefully improve accuracy led clockmakers to eliminate the duplicate dial numbers and rely on context to resolve the ambiguity. There are other examples of similar attempts: the six hour dial, Benjamin Franklin’s 8 hour dial, and so on.||This theory suggests that clockmakers are the type of people who would just hate to have unnecessary duplication, even if it was merely the numbering on the dial. Making life easier for developers and harder for users has a very modern ring to it.|
|24 hour time became Italian-only||Italians were counting from sunset with their 1-24 system, and the Northern Europeans were counting from midnight with their double-XII systems. Perhaps the 24 hour dial became associated with sunset-starting clocks, so the 12 hour dial became the standard for midnight-starting clocks (which eventually became the standard).||This links in with the Northern-drift theory of the renaissance above, which may be true.|
I don’t suppose we’ll ever know for sure!
17th Century: Tompion, Harrison, and Mudge
Gradually the 12 hour dial became used for most standard clocks, and the 24 hour dial was reserved for the more esoteric, technical, or complex clocks. The famous clockmakers were happy to design both 12 and 24 hour dials according to need or preference. The big astronomical clocks, such as those to be found at Prague or Strasbourg, usually used the 24 hour dial, with Arabic numerals from 1 to 24 for the Italian hours and Roman numerals (double-XII) for the North European numbering.
Thomas Tompion built some longcase clocks with 24 hour dials, which you can see in museums today. Tompion has been called the father of English clockmaking, and is sometimes reckoned to be one of the greatest craftsmen of all time. This ‘Equation Clock’ was built in 1695 for King William III. As well as the double-XII hour dial, it has a double-60 minute dial – the minute hand makes one revolution every two minutes. There’s also an outer minute ring which rotates back and forward to show apparent solar time. So you could read off the difference between mean time and local solar time by comparing the minute rings.
Tompion also built this Astrolabe clock in 1677:
You can see the sun and moon hands, and the moon phase in the central disk, next to the astrological graphics.
Tompion and George Graham collaborated to make this orrery (a working model of the solar system), which naturally used the 24 hour dial on the side.
It’s interesting to compare this idea with the older church clocks, which are often described as illustrating the Ptolemaic – earth-centred – view of the solar system. The orrery shows the Copernican view – but even in an Einsteinian view of the universe, a terrestrial rotation simulator would probably have a 24 hour dial. The original is in the Museum of History of Science in Oxford.
John Harrison (famous clockmaker and star of the best-selling book Longitude) used the 24 hour dial for his first chronometer H1, built in about 1730.
Harrison’s H1 chronometer. Read more about Harrison’s clocks at The Royal Observatory Greenwich site.
This is the hour dial of H1.
Following Harrison, Thomas Mudge designed three marine chronometers, this one with a 24 hour dial.
©Trustees of the British Museum
A feature of this is that the hour dial has two scales: one for the hours (1-12/1-12), then, inside, degrees in 15 degree steps from 0 to 360. This presumably allows some kind of conversion from hours (time) to degrees (longitude). (The fourth dial indicates how much the clock is wound up.) This example is in the British Museum.
Often, it was the professional time-users – astronomers, navigators, for example – who used the 24 hour dial, and, sometimes the 24 hour time system as well. For example, the log book for Captain Cook’s second voyage – during which he tested various clocks that would help find the longitude – shows handwritten entries that use the 24 hour time system, so switching between 24 and 12 hour dials wouldn’t have been a problem for them. In fact, anyone working with time needed to have their wits about them.
In general, there was much less standardization of time and clocks than today, and – probably – much confusion, not just with time zones, but with time systems too.
For astronomers, the day started at noon (easy to measure the sun’s position), so they counted their hours from noon. This was still the case up to 1925, although the civil day continued to end at midnight. For navigators at sea, however, the day ended at noon. At 6 o’clock on Monday morning civil time, it was 18:00 on Sunday for astronomers. 12 hours later, at 6 o’clock in the evening on Monday, civil time, it was 6pm on Tuesday for the navigator, but 06.00 on Monday for the astronomer. When a ship entered harbour, the navigator switched from nautical time to civil time. Captain Cook and his astronomer William Wales recorded the same time in different ways, and switched when they entered harbour.
A gallery of old clocks and watches
In the 18th and 19th centuries, makers continued to build a wonderful variety of timepieces, and a substantial minority of them had 24 hour dials. Some of these clocks were built as ‘exhibition’ clocks, to be taken on tour round the country and displayed to the wondering public for a small entry fee. Here is a small selection of 24 hour analog clocks and watches.
This is one side of a two-sided watch – the other side is a conventional 12 hour dial. Perhaps the owner was intending to travel across Europe, switching between 12 hour and 24 hour dials as needed.
Around the edge of the 24 hour dial are the signs of the zodiac and a month hand – the other side shows the day of the month (1 to 31). It was made by David Pons, England, in 1770.
This is one of the monumental showpiece clocks. It’s an astronomical organ clock by Henry Jenkins (1770):
While the curious observer sits at the hinged bureau flap thoughtfully provided to support his notebook, the organ beguiles him with a selection of twelve tunes.
The clock shows details of the moon, high tide times, lengths of the day and night, and includes a map of the world and the stars.
The map probably rotates around the centre, thus showing the time in any location on the earth. The counterclockwise numbering reminds us that the earth does in fact rotate in that direction, rather than clockwise.
This extravagant table clock was made by Peter Klein in Dresden about 1738:
The globe in the centre rotates once a day, like the earth, and moves the pointer around the clock, with the North Pole facing outwards. There’s also a glass shade that darkens the areas of the earth that are currently experiencing darkness.
Here is an overgrown watch doubling as a ship’s chronometer:
It was made by George Margetts in about 1780. At the center is a 12 hour dial with the names of 8 English ports; the 24 hour dial shows the times of low and high tide. This watch also manages to show the moon’s position in the zodiac, its declination, its latitude, its ecliptic nodes, its age, as well as eclipses of the sun and moon, the date, sun’s declination, twilight period, sun’s position in the zodiac. It apparently has only 16 gears. It might be in the National Maritime Museum at Greenwich.
At the other extreme, here’s a Swiss watch, made in 1880, recently on sale at http://www.antique-watch.com/:
In this design midnight is marked as ’24’, rather than ‘0’. Also, the minutes start with 0 at the bottom of the dial, rather than at the top.
This watch was made by John Johnson of Preston, England, in 1868:
It has Harrison’s Maintaining Power (which means that it keeps going while you’re winding it) and Bosley regulation. It was recently on sale at Robert Young’s PocketWatch shop. From midnight to noon is numbered in Roman numerals, and from noon to midnight in Arabic. Notice that this watch puts midnight on the top, whereas the previous one put noon at the top.
This unusual 10 inch high spherical skeleton clock was made in about 1760. It just makes this web site because it has a spherical band with the 24 hour numbers:
During the 19th century, the 24 hour dial was being used for more earthly reasons, rather than with the astronomical and religious intentions of the early clock makers. The ability of the 24 hour dial to display a wide range of times up to a day apart made it useful for showing the time in different parts of the world, and this use started to become important.
This extravagant French clock made in 1856 has 13 dials, the lower 8 of which are 24 hour dials, showing the time in London, New York, St Petersburg, Canton, Tahiti, Alexandria, Algiers, and St Helena:
The bottom half of each 24 hour dial has a dark border, to indicate night time. The other dials show the equation of time (for use with sundials), month/days of the week, and phases of the moon (and the time in 12 hour system). It makes you wonder: why would a clock owner in 1856 want to know the exact time in Jakarta or Johannesburg in 1850 – before the introduction of telephones or radio?
This is a late 19th century sidereal clock, which tells the time according to the stars:
A sidereal day is about 4 minutes shorter than the solar day. They were of most use in observatories. This one can be found in the Norman Lockyer Observatory in Devon.
In 1852, this 24 hour clock was installed outside the Royal Observatory at Greenwich, London:
It was the first clock to make Greenwich time available to the public. The master clock (installed inside the observatory) controlled this one by remote control and electric wires. It says about 16:19 (or, in 12-hour speak, 4 o’clock Post Meridiem plus 19 minutes), and midnight is at the top. The Roman numerals aren’t very readable to a modern eye.
The Modern era
In the 20th century, new groups encouraged the manufacture of 24 hour clocks. Radio enthusiasts needed to know the time around the world, and often adopted the 24 hour dial. The military wanted unambiguous times and adopted the 24 hour time system already widely used in Europe, encouraging the construction of 24 hour analog clocks and watches.
Pilots – already confident in reading a wide assortment of unusual dials – adopted the 24 hour dial for clocks in cockpits, and for their watches too. Here are a few mechanical cockpit clocks (collected by tgarn on the 24 hour watches forum).
This is the Willis World clock from the 1930s, seen in the London Science Museum.
New and not so new 24 hour clocks and watches from the 20th century still turn up today. Here are some of the clocks I’ve seen on sale on the net recently. eBay is always a good place to look. All these examples put midnight at the top, by the way – it’s become the standard.
This clock includes various cities of the world in the middle:
The map disk on this MasterCrafters clock rotates manually. You typically set the clock to read GMT and then adjust the disk to show the correct time in your time zone:
Bakelite clocks are very collectable. This US Military clock with its functional design reminds me of modern central heating controllers:
Here’s another US military clock:
Today, although the 12 hour dial is standard; the 24 hour analog dial is used by the specialist and the connoisseur, and sought after by the individual and the collector. Kids struggle for years to learn how to tell the time using the 12 hour dial, and would probably find the 24 hour analog dial and 24 hour time system much more logical.
24 hour analog clocks and watches are still being made, and are highly sought after.
I’ve seen them used just once in computer user interfaces. It provides an elegant and simple graphical way of selecting a time, which is not so easy to do on a 12 hour dial, because of the AM/PM issue. About ten years ago, there was an application called WebArranger. This was a new kind of application that organized and integrated your web browsing and personal information. WebArranger was made by CE Software, who discontinued it in about 1996, I think. The widget for selecting a time used a double-12 analog dial, with the evening half coloured black:
The Long Now foundation are thinking hard about the future. Will their clock have a 12 hour or 24 hour dial?
There’s no shortage of other ideas for radical changes in the way we keep, measure, and display time, but I believe that the logical and elegant 24 hour dial will be with us for many years to come. So why not make your next clock or watch a real 24 hour one?
Here are some books on the development of clocks that you might be interested in.
History of the Hour: Clocks and modern temporal orders: Gerhard Dohen van Rossum This is a study of the development of clocks and the development of modern time-keeping practices from Roman times to today. While a bit dry and scholarly at times (it’s translated from the German), it’s a great introduction to a fascinating subject.
Revolution in Time: Clocks and the making of the modern world David Landes
This is a very readable account of the history of timekeeping written by an economic historian. At times he’s more interested in the economic aspects of clock and watch manufacturing than I felt I wanted to be, but it’s a good read.
A history of clocks and watches Eric Bruton
An extremely well-illustrated general survey of clocks and watches, back in print. Buy it for the pictures, then read the text!
Greenwich time, and the discovery of the longitude Derek Howse
This tells the history of confusing time systems, and the gradual standardization in the 18th and 19th centuries. There’s a lot of information about the introduction of Greenwich time, and amusing descriptions of the bureaucratic proceedings as every faction tried hard not to give anything away. (“Legal time in France is Paris Mean Time, retarded by 9 minutes 21 seconds” – the French in 1898, trying hard to avoid mentioning Greenwich.)