Why Are There Seven Days in a Week?

Why are there seven days in the week?

No, this isn’t a question that a curious 10-year old asked me. Instead it was the late-night group musing on a recent work trip. After an extended back and forth, we collectively chose the answer provided by an astrology fan among us:

The days of the week parallel the seven celestial bodies you can see with your naked eye – the Sun, the Moon, Mars, Mercury, Jupiter, Venus, and Saturn.

The idea seemed compelling late at night but, on the plane back home, I began to think it was unlikely. Based on some mid-flight Web searching, I discovered this was a common theory which originated either with the Babylonians or the Greeks.

This would have been the end of my curiosity and you wouldn’t be reading this blog if I hadn’t stumbled on this passage from Alex Bellos’ book, The Grapes of Math.

Seven was already the most mystical of numbers by the time the Jews declared that God took six days to make the world, and rested the day after.
[…]
They decreed that the Sabbath come every seven days ad infinitum, irrespective of planetary positions. The continuous seven-day cycle was a significant step forward for humanity. It emancipated us from consistent compliance with Nature, placing numerical regularity at the heart of religious practice and social organization, and since then the seven-day week has become the world’s longest-running uninterrupted calendrical tradition.

Well, that makes total sense. A religious group chose the number 7 for the length of the week because the number is mystical. Case closed.

Hold on. What makes 7 special?

According to Bellos, there are many theories:

• There are seven openings in the head: the ears, eyes, nostrils and mouth
• Physiologically and biblically speaking, six is the optimal number of days to work before you need a day of rest
• Seven (plus or minus two) is the number of things a person can remember simultaneously

Bellos doesn’t believe any of these theories and calls them coincidences. Instead, he claims seven is mystical due to its mathematical properties. It’s the only Mersenne safe prime. (Yes, I had to look it up too.)

Regardless of whether seven is mystical or not, I definitely will be reading the rest of The Grapes of Math. It seems to be full of interesting stories about numbers.

Now if I could only get an eighth day of the week so I had time to read the book.