Somebody had to divide up the day, but why the number 24?

006

24, the title of a popular television series, marks the time in which the hero needs to resolve a crisis. “24/7” is a phrase we use to refer to the schedule of an extraordinarily hard worker. Twenty-four hours, the length of a day, is so deeply engrained in our consciousness of time that we tend to think of it as part of the fabric of nature.

But it’s not. Although the day is determined by nature, the time it takes for the earth to make one rotation on its axis, the division of that fixed period into 24 units is just a man-made convention—though one that is now standard throughout the world. Where did 24, or two groups of 12, come from? And how did we ever get 24 hours of uniform length?

The modern 24-hour day, with each hour consisting of exactly 60 minutes, came into being with the rise of mechanical clocks in the Middle Ages, toward the end of the 13th century. Before mechanical clocks, people’s lives were normally guided by the daily course of the sun and the yearly succession of the seasons. Systematic timekeeping was mainly limited to monastic and ecclesiastical life.^{a}

So where did medieval clockmakers get the idea of a 24-hour day? Why didn’t they, for example, divide the day into two sets of 10 hours—a much more convenient system for us base-10-using moderns? (Until the 19th century, in fact, the Japanese divided the daytime into six hours.) Where did that 24 come from?

For most scholars, the primary candidates are Babylonia and Egypt. It seems clear, however, that the Babylonians did not employ a 24-hour division; at least there is no evidence for it in the cuneiform sources. Instead, they used a sexagesimal (base-60) system of counting, and their astronomers divided the day into 360 parts, corresponding to the division of the circle into 360 degrees. Their basic time-unit, then, was the length of the day divided by 360. There are 1,440 modern minutes in a day, and 1,440 divided by 360 equals 4. So the basic time-unit for the Babylonians, called the usÿ (pronounced “oosh”), was 4 minutes long.

The earliest attestations of a 24-part day divided into two divisions of 12 are all from ancient Egypt.^{b} By 2100–2000 B.C., the night had begun to be divided into 12 parts, as we know from the so-called star clocks painted on the lids of some coffins. The division of the day into 12 parts is evident in a text describing a shadow-clock (a kind of sundial) from the reign of Pharaoh Seti I (1290–1279 B.C.).

This brings us to another distinction, between variable seasonal hours and fixed equinoctial hours. Daytime and nighttime 007change in length depending on the season; for example, in the summer, the days become longer and the nights shorter. Therefore, if daytime and nighttime are each divided into 12 equal parts, these parts—or “hours”—will also vary in length; in the summer, a daytime hour will be longer than a nighttime hour. Hours of varying length are called seasonal hours.

At the equinoxes (in March and September), however, day and night have the same length. The hours of night and day are then all 60 minutes long, like our modern hours. Such fixed hours of uniform length are called equinoctial hours. Whereas a sundial measures seasonal daytime hours, a mechanical clock measures equinoctial hours. Since the 13th-century B.C. clocks from the reign of Pharaoh Seti were shadow-clocks (sundials), we know that they were measuring seasonal hours.

The earliest-known evidence of the equinoctial hour is found in the 12th-century B.C. Papyrus Cairo 86637 (in the Cairo Museum). This document records the changing lengths of day and night over the course of a year. The lengths are expressed in parts—or “hours”—that differ in number as the season changes; in the summer, for example, the day has more hours than does the night. This hour therefore must be equinoctial.

If the ancient Egyptians almost certainly invented the 24-hour day, or the 12-hour day/night, where did they get the number 12? No one knows. There are about 12 lunar months in a year, so perhaps the lunar year inspired astronomers to divide the night and day into 12 portions. Whatever the case, 12 is indeed a kind of iconic number: 12 prophets, 12 tribes, 12 apostles.

In the Greek world, the division of the day into 24 seasonal hours seems to have taken hold after Alexander’s conquests of the 330s and 320s B.C. Since Alexander’s conquests included Egypt, the Greeks may have borrowed the 24-hour day from the Egyptians. One of the earliest, and most puzzling, references to the hours in Greek sources comes from Herodotus (c. 485–425 B.C.), who mentions the “twelve parts of the day that the Greeks adopted from the Babylonians.” Although the Babylonians did sometimes divide the day into 12 parts, corresponding to the 12 parts of the zodiac, we do not know whether Herodotus had this system in mind.

After Alexander, 24 hours seems to have spread throughout the Mediterranean. One of the parables in the Gospel of Matthew, for example, speaks of laborers in the vineyard who are hired at the eleventh hour but paid a full day’s wage (Matthew 20:6–12). The 24-hour day is also well attested in Latin literature.

Our day of 24 60-minute equinoctial hours became the norm with the advent of mechanical clocks. The first town clocks began appearing in Italian cities in the early 14th century. By 1308 Orvieto had a public clock. Milan had a clock by 1355, Bologna by 1356, and Siena by 1360. In the 15th and 16th centuries, Italy fell behind northwest Europe, where, by 1500, public clocks had become common even in villages, loudly ringing out each of the 24 hours and further impressing this ultimately arbitrary division of the day into our souls.

, the title of a popular television series, marks the time in which the hero needs to resolve a crisis. “24/7” is a phrase we use to refer to the schedule of an extraordinarily hard worker. Twenty-four hours, the length of a day, is so deeply engrained in our consciousness of time that we tend to think of it as part of the fabric of nature. But it’s not. Although the day is determined by nature, the time it takes for the earth to make one rotation on its axis, the division of that fixed period into 24 units […]

You have already read your free article for this month. Please join the BAS Library or become an All Access member of BAS to gain full access to this article and so much more.

Gerhard Dohrn-van Rossumtells the clock’s history in greatdetail in his History of the Hour (University of Chicago Press, 1996). Another useful survey is Carlo Cipolla’s Clocks and Culture, 1300-1700 (W.W. Norton, 1977).

2.

See Richard A. Parker’s and Otto Neugebauer’s Egyptian Astronomical Texts, 3 volumes (Providence, 1960-1969) and Marshall Clagett’s Ancient Egyptian Science, II: Calendars, Clocks, and Astronomy (Philadelphia, 1995).