Because there are 24 hours in a day!
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.
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