In this episode of Our Island Universe, why are there 7 days in the week?
Wikipedia Article on the 7 Days of the Week
Transcript:
A day measures how long it takes the Earth to rotate once on its axis. A year measures how long it takes the Earth to make 1 revolution around the Sun. A month is approximately how long it takes the Moon to go though all its phases. So why do we have 7 days in a week?
To answer that question, we need to associate the number 7 with something that would be observable to our ancestors. Looking out at their dark, starry night sky, they certainly saw a brilliant sky illuminated by stars and the bright fog of the Milky Way. While the night sky does change from month to month, there’s far too many stars too count! So what could have our ancestors also seen? what they also saw in their sky where objects that wandered from night to night, day to day. These wandering stars are of course not really stars but the planets Mercury, Venus, Mars, Jupiter and Saturn — the 5 planets you can see without a telescope. But there are 2 other objects you also see every day — the Moon and our Sun! Add them up and that gives you 7 objects.
In fact, the names of the days of the week give the answer away! Our weekend days, Saturday and Sunday, are named after Saturn and the Sun, respectively. Monday is named after our Moon. Our English names for Tuesday, Wednesday, Thursday and Friday are named after the Germanic gods Tiw, Woden, Thor and Fria. But in french, these days are called mardi, mercredi, jeudi and vendredi after the planets Mars, Mercury, Jupiter and Venus! The correspondence is no longer perfect but the pattern is clear. We have 7 days because our ancestors saw 7 objects move with respect to the stars from day to day and night to night.