Babylon and the Mystery of the Seven-Day Week

Months follow the Moon and years follow the Sun, but the week follows nothing in nature at all. There is no seven-day cycle in the sky. Yet almost every calendar printed today is organized into rows of seven days. To understand why, we need to travel to ancient Mesopotamia, the land between the Tigris and Euphrates rivers, where some of the world's first professional astronomers worked.

Astronomers of the clay tablet

The Babylonians recorded the sky on clay tablets for more than a thousand years. They wrote in cuneiform, wedge-shaped marks pressed into wet clay, and they kept careful notes about the Moon, the planets and eclipses. A huge collection of sky omens called Enuma Anu Enlil filled about seventy tablets, and the cover of this post shows a commentary on part of that series. Another work, known as MUL.APIN, listed stars and gave rules for keeping the calendar in step with the seasons.

These were not idle stargazers. Royal astronomers believed the sky sent messages about the fate of the king and the land, so they watched it every night. In doing so they built up some of the most accurate astronomical records of the ancient world.

A lunisolar calendar

The Babylonian month began when the first thin crescent of the new Moon was seen in the evening sky. That made months either 29 or 30 days long, depending on what observers saw. Twelve such months fall about 11 days short of the solar year, so the Babylonians regularly added a thirteenth month to keep festivals and harvests in their proper seasons.

At first the extra month was added by royal decree, when officials judged it necessary. Over time the astronomers noticed a useful coincidence: 19 solar years are almost exactly equal to 235 lunar months. By the fourth century BC the Babylonians were using a fixed 19-year cycle with seven extra months. The same cycle was later named after the Greek astronomer Meton, and it still governs the Hebrew calendar today.

Special days and the number seven

The Babylonians gave particular attention to certain days of the lunar month, especially the 7th, 14th, 21st and 28th, which roughly mark the quarters of the Moon's cycle. Some texts treat these days as unlucky, with restrictions on what the king should do. A 28-day lunar cycle divides naturally into four parts of seven days, so it is easy to see how seven could become a meaningful unit.

The Babylonians also gave us the habit of dividing things into sixties. Their number system was based on 60, which is why we still have 60 minutes in an hour, 60 seconds in a minute and 360 degrees in a circle.

The Sabbath

The seven-day week as a continuous cycle, running on without regard to the Moon, is best known from Jewish tradition. The Hebrew Bible sets the Sabbath as a day of rest every seventh day, repeating endlessly regardless of months or years. Historians still debate how much the Babylonian lunar days influenced the Sabbath. Jewish communities lived in Babylon during the exile in the sixth century BC, and contact between the cultures was strong, but the Sabbath developed its own distinct meaning as a weekly day of rest.

Seven lights in the sky

The final ingredient came from astronomy. Ancient observers could see seven bright objects that moved against the fixed stars: the Sun, the Moon, Mercury, Venus, Mars, Jupiter and Saturn. In the Hellenistic world after Alexander the Great, astrologers assigned each hour of the day to one of these seven "planets" in a fixed order. The planet that ruled the first hour gave its name to the whole day.

That system spread through the Roman Empire. By the first century AD Romans were naming days after the planets, and in AD 321 the emperor Constantine made the day of the Sun an official day of rest.

Where our weekday names come from

Look at the names we still use and you can see all seven lights:

• Sunday and Monday are the days of the Sun and the Moon.

• Saturday is Saturn's day.

• Tuesday, Wednesday, Thursday and Friday come from the Germanic gods Tiw, Woden, Thor and Frigg, who replaced the Roman gods Mars, Mercury, Jupiter and Venus.

In French and Spanish the Roman planets are clearer: mardi and martes for Mars, mercredi and miércoles for Mercury, jeudi and jueves for Jupiter, vendredi and viernes for Venus.

A week that never stops

The seven-day week is a human invention that has run without a single break for around two thousand years. Calendar reforms have changed months, years and leap days, but the weekly cycle has survived them all. When you plan your week on a printed calendar, you are using a rhythm shaped by Babylonian astronomers, Jewish tradition and the seven wandering lights of the ancient sky.

Cover image: Cuneiform tablet- commentary on Enuma Anu Enlil, tablet 5 MET DP-442-002, CC0, via Wikimedia Commons.