The question
Why seven days, when the week matches no natural cycle exactly?
The short answer
The seven-day week grew from Jewish tradition and from Hellenistic astrology, which tied each day to one of the seven lights ancient people saw move across the sky.
What science knows
Historical FactAncient astronomers counted seven moving “planets”: the Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn. By the 1st century CE the planetary week was widespread in the Roman world, and in 321 CE Constantine made Sunday a day of rest.
The Jewish week, ending on the Sabbath, is older and was counted independently of the planets.
The pattern
Established MathematicsAstrologers assigned each hour of the day to a planet in the order Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon. A day has 24 hours, so each new day starts three planets further on, giving Saturn, Sun, Moon, Mars, Mercury, Jupiter, Venus: Saturday through Friday. The historian Cassius Dio described this scheme around 200 CE.
What is still uncertain
Mixed EvidenceA lunar quarter lasts about 7.4 days, and some historians think this helped seven feel natural. How exactly the Jewish week and the planetary week merged is debated.
Why humans find it interesting
Cultural TraditionEnglish replaced some Roman gods with Norse ones: Tuesday is Tiw’s day (Mars), Wednesday Woden’s (Mercury), Thursday Thor’s (Jupiter), Friday Frigg’s (Venus). French kept the Roman names: mardi, mercredi, jeudi, vendredi. Attempts to replace the week, such as revolutionary France’s ten-day week, did not last.
CUE perspective
CUE PerspectiveEvery week is a small piece of ancient astrology, hidden in plain sight in the names of the days.
CUE reads timing through numerology rather than planetary hours, but it shares the old intuition that days can have different characters.