Why Is a Week Seven Days Long?

A day is one turn of the Earth, a year one trip round the Sun, a month roughly one cycle of the Moon. The week is different: nothing in the sky takes seven days. So where did it come from?

Seven wandering lights

Ancient sky-watchers saw seven bright objects moving against the fixed stars: the Sun, the Moon, Mercury, Venus, Mars, Jupiter and Saturn. Babylonian astronomers gave seven special importance, and a quarter of a lunar cycle (about 7.4 days) made seven a handy count too.

Sabbath and Rome

The Jewish week of six working days and a Sabbath spread with Judaism and then Christianity. Separately, a seven-day “planetary week” with each day ruled by one of the seven lights became popular in the Roman world. In 321 CE, Emperor Constantine made Sunday a day of rest, fixing the seven-day week in the Roman calendar that most of the world now uses.

India has the same planets

The Indian names match the same seven objects, in the same order:

The planetary week reached India through astronomical contact with the Hellenistic world, and the order matches across languages that never otherwise met.

Attempts to change it

Revolutionary France tried a 10-day week in the 1790s, and the Soviet Union experimented with 5- and 6-day weeks around 1930. Both returned to seven.