Why the week has seven days
A day is one turn of the Earth, a month is roughly one cycle of the Moon, and a year is one trip around the Sun. The week is the odd one out. Nothing in the sky repeats every seven days, yet almost the whole world now lives by it.
The closest natural rhythm is the Moon. Its cycle lasts about 29.5 days, and its four phases come roughly seven days apart. The Babylonians gave special weight to certain seventh days of the lunar month.
The Jewish week broke free of the Moon. Six days of work and a seventh day of rest repeated without pause, whatever the Moon was doing, and that unbroken cycle passed into Christianity and Islam.
A second seven came from astronomy. Ancient observers counted seven moving lights in the sky: the Sun, the Moon, Mars, Mercury, Jupiter, Venus and Saturn. Astrologers in the Hellenistic world gave each day to one of them, and this planetary week spread through the Roman Empire.
The names still carry that history. Saturday is Saturn's day, and Sunday and Monday belong to the Sun and the Moon. In English, Germanic gods replaced the Roman ones: Tiw for Mars on Tuesday, Woden for Mercury on Wednesday, Thor for Jupiter on Thursday and Frigg for Venus on Friday.
Which English day still carries the name of a Roman god?
The seven-day week comes from the Moon, the Bible and the seven planets of ancient astronomy, and Rome spread it everywhere.
