Measuring the passage of time is extremely important for many things, and it is something that humans have been doing for a very long time and in many different ways.

Some measurements of time just make sense. For example, measuring a day is pretty easy since we all see the sun rise and set all the time.

Even measuring a year isn’t that hard, given the seasons and the positions of the stars. Of course, months are based on the cycle of the moon.

All of these measurements are based on natural phenomena of one type or another, even when their measurements weren’t always perfectly accurate.

Then you can look at certain things like minutes and hours, which aren’t based on anything found in nature. For those, it still makes sense in that dividing a day up into 24 hours makes it easy to segment things in a useful way.

24 hours gives you approximately 12 hours of day and night (depending on the time of year). It can also be divided into three 8-hour segments. Or any number of other ways based on need, since 24 can be evenly divided by many different numbers.

Same for minutes and seconds, in that 60 is a great number for dividing evenly.

But then we get to the week. Seven is not just an odd number, but a prime number, meaning that it can’t actually be evenly divided at all. There is also nothing in nature that it can be based on. It certainly doesn’t match up nicely with 30-day months or 365-day years, so what the heck?

First of all, it is important to note that 7-day weeks weren’t always the standard. There have been dozens of calendars over the millennia that have used anything from 3-day weeks to 10-day weeks, and many others as well.

The earliest known example of something like a 7-day week comes from the Babylonians. Their calendar had either nine or ten-day weeks, but they had a special name for every seventh day, regardless of where it fell in the actual week. This seventh day was known as a sapattu.

It is thought that this was chosen because the local rivers that were in the area had a roughly seven-day cycle, which does make it loosely based on nature.

That seven-day cycle seems to be where the early writers of the Bible got the term sabbath, which comes after 6 days, making for a seven-day week.

The seven-day week was really just one of many options for hundreds of years during that era, but then Christianity began and naturally adopted the seven-day week as well.

As the Roman Empire converted to Christianity, the seven-day week became the standard, and eventually it was adopted by nearly everyone else.

This adoption of the seven-day week took a long time, and it wasn’t exactly smooth in all locations, but in the end, it was accepted by nearly everyone in the world, and that isn’t likely to change anytime soon.

If you enjoyed this story, check out this post about how studies are showing that climate change is negatively affecting the mental health of young adults.