Why Clocks Show 12 Hours
The numbers on an ordinary clock face are a 3,500-year-old inheritance from Egyptian astronomers, Babylonian mathematicians and Greek map-makers.
Twelve hours of daylight
The story of the 12-hour clock begins in ancient Egypt. Even earlier, NIST notes, Egyptian obelisks built as early as 3500 BCE cast shadows that split the day into a morning and an afternoon. Most historians credit the Egyptians as the first to divide the day into parts, and by about 1500 BCE a T-shaped sundial bar was splitting the time between sunrise and sunset into 12 sections.
NIST describes a portable Egyptian shadow clock from around that time that divided daylight into 10 parts, plus two twilight hours, for a total of 12. The device was turned around at noon so it could measure the afternoon.
These were not equal hours. Because daylight lasts longer in summer than in winter, each of the 12 daytime hours stretched in summer and shrank in winter.
Twelve hours of night
Egyptian astronomers measured the night with stars instead of shadows. According to Scientific American, they used a set of 36 stars, 18 of which marked the night. Three were assigned to each twilight period, leaving 12 for full darkness.
During the New Kingdom, from 1550 to 1070 BCE, this system was simplified to 24 stars, 12 of which marked the night. Water clocks did the same job without needing a clear sky: a clepsydra from the Temple of Amun at Karnak, dating to about 1400 BCE, divided the night into 12 parts.
Put 12 hours of day together with 12 hours of night and you get the 24-hour day we still use.
But why twelve?
Nobody knows for certain. Scientific American offers two leading explanations. One links the number to the roughly 12 lunar cycles in a year.
The other is anatomical. Each of your four fingers has three joints, and by touching each joint with your thumb you can count to 12 on one hand. Both ideas are plausible, and neither can be proven.
For centuries afterward, ordinary people kept using seasonal hours. Scientific American notes that fixed-length hours did not appear until the Hellenistic period, when Greek astronomers used them mainly for calculations. Hipparchus, active between 147 and 127 BCE, proposed dividing the day into 24 equinoctial hours, based on the 12 light and 12 dark hours of an equinox day.
Everyday life caught up much later. Fixed-length hours became common only after mechanical clocks appeared in Europe in the 14th century.
Where 60 minutes and 60 seconds came from
The hour came from Egypt, but the minute and second came from Mesopotamia by way of Greek geography. The Sumerians developed a base-60 counting system around 2000 BCE, and the Babylonians inherited it for astronomy.
Why 60 is unknown, but it is convenient. Scientific American points out that 60 is the smallest number divisible by 1, 2, 3, 4, 5 and 6, and it also divides evenly by 10, 12, 15, 20 and 30.
Greek scholars applied base-60 to circles and maps. Eratosthenes, who lived from about 276 to 194 BCE, used it to divide a circle into 60 parts for a system of latitude, and Hipparchus later added lines of longitude spanning 360 degrees. Around 150 CE, Claudius Ptolemy divided each degree into 60 parts and each of those into 60 smaller parts. The first division, partes minutae primae, became our minute, and the second division became our second.
Clocks were slow to adopt them. For centuries, clock dials split the hour into halves, thirds, quarters and occasionally 12 parts, but never into 60, and mechanical clocks that displayed minutes did not appear until near the end of the 16th century. Today the second no longer depends on the sun at all: since 1967 it has been defined by 9,192,631,770 energy transitions of the cesium atom.
See also: a rundown of the units we use to measure time on OnlineClock.app.
Why the dial shows 12, not 24
Since the day has 24 hours, a clock hand could go around once a day. Some clocks do exactly that. The Shepherd Gate Clock at the Royal Observatory in Greenwich, installed in 1852, has a single 24-hour dial with midnight at the top and noon at the bottom.
Most clocks, though, divide the dial into 12, echoing the old Egyptian split between a day half and a night half, so the hour hand goes around twice a day. The 24-hour dial takes some getting used to: at the Shepherd Gate Clock, the hour hand points straight down at noon.
See also: how the 12-hour clock and AM and PM work on OnlineClock.app.
The mystery of IIII
Look closely at a clock with Roman numerals and you may notice something odd: four o'clock is often written IIII, not IV. The Seiko Museum in Tokyo notes that IIII was the usual way of writing four in medieval Europe until around the 17th century, and that turret clocks made before then show IIII. The museum points to the late 14th-century clock at Wells Cathedral in England as an early example.
How common is it today? In a widely shared FAQ compiled by the Electric Time Company from horology discussion lists, the contributor Alan Heldman reported that when he went through a book showing hundreds of tower clock dials, about 95 percent or more used IIII.
There are several explanations, and none is settled. The Seiko Museum calls visual balance the most popular: on a round dial, the four strokes of IIII mirror the heavy VIII opposite it. The museum also mentions a complaint that IV was too easy to confuse with VI. Another idea from the Electric Time FAQ is practical: if numerals were cast in sets, using IIII made the counts of I, V and X divide evenly into molds.
Historical stories are harder to pin down. Heldman notes that classicists found IIII very often used in Roman inscriptions. A popular tale says IV was avoided because it began the Latin name of the god Jupiter.
Royal legends are popular too. The Seiko Museum relates one in which France's Charles V, in the late 14th century, had IV changed because he thought it unlucky: it took one away from V, the numeral of his own title. A similar story is told about Louis XIV, but W. I. Milham, in his 1947 book Time and Timekeepers, as quoted in the FAQ, pointed out that IIII dials long predate that king.
Heldman offers one more idea: the first clocks to use IV may have been those with Roman striking, which used two different bells to sound 1 and 5. That system, he notes, had nearly vanished by the early 18th century.
Not every clock follows the custom. The Great Clock at Westminster, better known as Big Ben, uses IV, as UK Parliament points out.
See also: reading a Roman numeral clock face on OnlineClock.app.
Questions people ask
Why does a day have 24 hours?
Ancient Egyptians divided daylight into 12 parts and the night into 12 parts, giving 24. Greek astronomers later turned these into 24 equal hours.
Why are there 60 minutes in an hour?
The base-60 system came from the Sumerians and Babylonians. Ptolemy used it to divide degrees into 60 parts and then 60 smaller parts, which became the minute and the second.
Why do clocks use IIII instead of IV?
No one knows for sure. IIII was the usual medieval way of writing four, and common explanations include visual balance with the VIII, easier reading, and legends involving Jupiter, Charles V of France and Louis XIV. Some clocks, including Big Ben, use IV.
Are there clocks with 24 hours on the dial?
Yes. The Shepherd Gate Clock at the Royal Observatory in Greenwich has shown all 24 hours on one dial since 1852.
Keep exploring
Sources
- How did we come to divide the day into hours, minutes and seconds?, Scientific American
- A Walk Through Time: Early Clocks, NIST
- FAQ: Roman IIII vs. IV on Clock Dials, Electric Time Company
- Why do some clocks use IIII instead of IV?, Seiko Museum Ginza
- Great Clock facts, UK Parliament
- Shepherd Gate Clock, Royal Museums Greenwich