If you do not like the seven-day week, blame it on Constantine the Great, first Christian emperor of the Holy Roman Empire from A.D. 306 to 337, who officially adopted it in 321.

The ancient Romans, during the Republic, used an eight-day week, which started to lose favor to the seven-day week after adoption of the Julian calendar in 46 B.C. Romans used both the seven- and eight-day weeks simultaneously over centuries as the seven-day week grew in popularity.

The origins of the seven-day week go back thousands of years to the beginnings of civilization when villages sprang up in about 5000 B.C. in Mesopotamia, the land between the Tigris and Euphrates rivers, mostly along the southern Euphrates close to the present site of Abadan, Iran.

By 3250 B.C. immigrants from the northeast had begun to intermarry with the local population. The newcomers and their offspring became Sumerians, who invented writing and navigation. A series of wars and conquests moved the seat of power northward from Sumer into Assyria, which included Babylon, a city near present-day Baghdad.

A series of cuneiform tablets known as the Enuma Anu Enlil recorded centuries of Babylonian observations of celestial phenomena. The series consists of 68 clay tablets dating from 1595 to 1157 B.C. that were in use for at least 1,000 years.

The first 13 tablets refer to the moonrise on various days of the month and its relation to planets and stars. Tablet 14 details a basic algorithm for predicting the visibility of the moon.

Babylonia briefly became the major power in the region after Hammurabi (1696-1654 B.C.) created a short-lived empire, which rapidly fell apart after his death. Neo-Babylon emerged in 619 B.C., after a thousand years of wars and dominion under Kassite and Assyrian rule, and comprised a massive empire that would last less than 90 years before Persians conquered it.

Persians remained in control with Babylon one of four capital cities until Alexander the Great conquered Babylon in 333 B.C. He died there in 323 B.C.

The Babylonian culture had begun developing empirical astronomy in the eighth and seventh centuries B.C. This would become the basis for later astronomies of Greece, India, the Middle East and our modern Western astronomy. Some historians have called this the first scientific revolution.

Seven days is nearly equal to one-quarter of a 29.5-day moon cycle. Babylonians knew this from their detailed observations of the moon in the Enuma Anu Enlil tablets.

Babylonians celebrated every seventh day as a holy day on which officials were prohibited from various activities and common people were forbidden to make a wish. Their lunar month of 29 or 30 days contained three seven-day weeks and a final week of eight or nine days as needed to be back in sync with the moon.

Conveniently, Babylonian astrologers dedicated one day for the sun, the moon, Mercury, Venus, Mars, Jupiter and Saturn. The Greeks inherited this tradition from the Persians and passed it on to the Romans, culminating with Constantine’s decree in A.D. 321.

Richard Brill is a professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.