The Origin and Evolution of the Seven-Day Week

The Origin and Evolution of the Seven-Day Week

Have you ever wondered why the week is divided into seven days? While most of our calendar units come from natural astronomical cycles, there's no natural or astronomical cycle that measures seven days. Despite that, almost every culture on Earth today divides its calendar this way. So, where does the seven-day week come from, and why is it a fundamental division of time?

In this article, we'll explore the history and evolution of the seven-day week, its connection to astrology, and how it spread across different cultures and languages.

The Story of the Week

Let's start with the basics. A year is the time it takes the Earth to orbit the Sun once, and a day is roughly the length of the Earth's rotation on its axis. But the week? It has no natural, earthly, or astronomical cycle that measures seven days. So why seven days?

Seven has been regarded as a significant number by countless cultures for thousands of years, in religion, mythology, superstition, and folklore. The Seven Deadly Sins, the Seven Virtues, the seven days of Creation, the Seven Samurai, the Seven Heavens, the Seven Chakras, the Seven Lucky Gods, and the Mercury Seven are just a few examples. But why seven, and not six, eight, or any other number?

The Moon Connection

The answer lies in the moon. There are roughly, though not exactly, twelve moon cycles in a solar year, and that was precise enough for a farmer in five thousand B.C. as a fundamental division of time. Twelve is a convenient number for a few reasons. For one, it's pretty small. You can probably even count to it. It can also be divided into two parts, three parts, four parts, or six parts, which makes it a good basis for measuring things like circles.

Let's draw a circle with a radius and from the edge, mark it intersecting with a circle with the same radius. And again on the other side, these points of intersection divide a circle into six equal portions. To subdivide each of those in half, you can split a circle into twelve equal parts without complicated measuring tools, which is handy for dividing the sky into signs of the Zodiac.

Once a month, ancient sky watchers watched the moon cycle through the twelve slices of the Zodiac, a seemingly unchanging backdrop of stars. It's an observer on Earth. The stars do rotate slowly once per day, but their positions don't change relative to each other. But like the moon, a few other objects bright enough to be seen with the naked eye do seem to move on their own, not following the background stars.

These were known as wanderers or planets in Greek, and they were the moon, Mercury, Venus, Mars, Jupiter, and Saturn. And if you add the biggest, brightest wanderer, the sun, you get seven of them, originally thought to be each embedded in rotating spheres with the earth at their center. Of these, not all planets by today's definition, and that's not how the solar system works at all. But ancient sky watchers from the Babylonians to the Greeks and Romans, even India and China, put the planets in this order based on how fast they wandered through the sky, with Saturn, the planet with the longest cycle, at the top. And this is where astronomy becomes astrology.

The Astrology Connection

In many ancient cultures, each planet represented a god whose position in the sky could influence the lives of mortals. Seven planets, seven gods, seven days. Can you see where this is going? Around the third century B.C., Greek astrologers writing horoscopes in Egypt decided that each god was only in charge for one hour at a time.