Why are there seven days in a week?

Babylonian astronomers tracked seven moving objects in the sky: the Sun, the Moon, Mercury, Venus, Mars, Jupiter and Saturn. Seven days also fits neatly into the Moon cycle of about 29.5 days, which divides into four phases of roughly seven days each. The Jewish calendar built its week around the Sabbath, and the Romans later named days after the seven planets, a pattern still visible in Saturday, Sunday and Monday. In AD 321 Emperor Constantine made Sunday an official day of rest, helping cement the week across the Roman world. Attempts to change it failed: revolutionary France tried a ten-day week from 1793, and the Soviet Union experimented with five- and six-day weeks between 1929 and 1940.