Have you ever wondered why our week is divided into seven days? Unlike years, months, and days, which are based on natural cycles like the Earth’s orbit around the Sun or the Moon’s phases, the seven-day week doesn’t have an obvious connection to nature. Yet, almost every culture around the world uses it. Let’s explore why this is the case and how the number seven became so significant.
The number seven is quite unique. It’s a prime number, meaning it can’t be evenly divided into the 365 days of a year. Despite this, many cultures have considered seven to be special for thousands of years. Think about the Seven Wonders of the World, the Seven Deadly Sins, or the seven colors in a rainbow. But how did this number become linked to our week?
To understand the seven-day week, we need to look back at ancient astronomy. Long ago, people noticed seven bright objects in the sky that seemed to move independently from the stars. These were the Sun, the Moon, and the five planets visible to the naked eye: Mercury, Venus, Mars, Jupiter, and Saturn. Ancient cultures, like the Babylonians, Greeks, and Romans, associated these celestial bodies with gods and believed they influenced human life.
In ancient times, astrology was closely linked to astronomy. Greek astrologers in Egypt developed a system where each of these seven “planets” ruled over one hour of the day. With 24 hours in a day, this cycle repeated every seven days, giving us the planetary week. Each day was named after the planet that ruled its first hour, which is why we have names like Saturday (Saturn’s day) and Monday (Moon’s day).
As the Roman Empire expanded, it spread the seven-day week across Europe. When Christianity became widespread, the week was adapted to fit Christian traditions, with Sunday becoming a day of rest. In English, some day names were changed to honor Norse gods, like Thor for Thursday.
The seven-day week eventually reached India, China, and even indigenous American cultures through trade, conquest, and religious influence. While the names of the days might differ, the seven-day structure is nearly universal today.
But why is seven so special? One theory is linked to the Moon’s phases. A lunar cycle is about 29.5 days, and each quarter phase lasts roughly seven days. This connection to the Moon might have made seven a natural choice for dividing time.
Additionally, ancient mathematicians found seven intriguing because it was the first number that couldn’t be neatly divided into smaller parts in their base 60 system, making it seem mysterious and special.
While we may never know exactly who first decided on a seven-day week, it’s clear that the idea has deep roots in human history. It’s fascinating to see how ancient beliefs and observations have shaped something as everyday as our calendar. Now that you know the story behind the seven-day week, what will you do with this knowledge? Maybe start by appreciating the history behind each day!
Research and create a timeline that shows the development of the seven-day week from ancient times to its global adoption. Include key events and cultures that contributed to its spread. Present your timeline to the class and explain how each event influenced the week we use today.
Choose one of the seven celestial bodies (Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn) and create a poster that explains its significance in ancient astronomy and astrology. Include how it influenced the naming of the days of the week. Share your poster with your classmates and discuss the connections between astronomy and our calendar.
Explore the mathematical properties of the number seven. Work in groups to investigate why seven is considered a special number in mathematics. Present your findings, including any interesting patterns or properties, and discuss how these might have contributed to the cultural significance of the number seven.
In groups, role-play as ancient astronomers from different cultures (Babylonians, Greeks, Romans). Research how each culture viewed the seven celestial bodies and their influence on human life. Perform a short skit for the class, demonstrating how these beliefs might have led to the creation of the seven-day week.
Investigate how the names of the days of the week differ across various languages and cultures. Create a chart that compares these names and their origins. Present your chart to the class and discuss any patterns or interesting differences you find, highlighting the influence of different cultures on the seven-day week.
Here’s a sanitized version of the transcript:
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Hey, smart people! Joe here. I was looking at my calendar recently, and two questions popped into my head: One, why are llamas in pajamas so cute? And two, why do we have weeks? Why do we divide the calendar into seven-day chunks instead of eight or five? Most of the units we use to divide our calendar come from natural astronomical cycles. A year is the time it takes the Earth to orbit the Sun once, months are roughly the length of the moon’s full cycle of phases, and days come from the Earth’s rotation on its axis.
But the week? There’s no natural, earthly, or astronomical cycle that measures seven days. Despite that, almost every culture on Earth today divides its calendar this way. Some historians think the seven-day week is so old that it may be the oldest known human institution still functioning without a break. That suggests that the week is actually a pretty strong idea. But everything comes from somewhere; every invention has an inventor. So, why seven days?
Seven is an unusual choice for the number of days in a week, not just because it’s odd, but also because it’s a prime number and cannot be evenly divided into the 365 days of a year. However, seven has been regarded as a significant number by countless cultures for thousands of years in religion, mythology, superstition, and folklore. Think of the Seven Deadly Sins, the Seven Virtues, the seven days of Creation, the Seven Samurai, the Seven Heavens, the Seven Chakras, and the Mercury Seven. The Mercury Seven were the first astronauts selected for NASA’s human spaceflight program. Each spacecraft in Project Mercury had a name ending in the number seven, and Project Mercury laid the groundwork that led to humans landing on the moon.
Incidentally, counting Apollo 13, NASA originally planned seven missions to land on the moon. And the moon is where the story of the seven-day week begins. There are roughly, though not exactly, twelve moon cycles in a solar year, which was precise enough for a farmer in 5000 B.C. as a fundamental division of time. Twelve is a convenient number for a few reasons: it’s small, can be divided into two, three, four, or six parts, making it a good basis for measuring things like circles.
Once a month, ancient sky watchers observed the moon cycle through the twelve slices of the Zodiac, a seemingly unchanging backdrop of stars. The stars do rotate slowly once per day, but their positions don’t change relative to each other. However, like the moon, a few other bright objects visible to the naked eye seem to move on their own. These were known as wanderers or planets in Greek, including the moon, Mercury, Venus, Mars, Jupiter, and Saturn. If you add the sun, you get seven of them, originally thought to be embedded in rotating spheres with the Earth at their center.
Of these, not all are planets by today’s definition, and that’s not how the solar system works at all. But ancient sky watchers from the Babylonians to the Greeks and Romans, even in India and China, arranged the planets based on how fast they wandered through the sky, with Saturn being the slowest. This is where astronomy becomes astrology. In many ancient cultures, each planet represented a god whose position in the sky could influence the lives of people. Seven planets, seven gods, seven days—can you see where this is going?
Things weren’t quite in order yet. Around the third century B.C., Greek astrologers in Egypt decided that each god was in charge for one hour at a time, so Saturn would rule for an hour, then Jupiter, Mars, and so on. With 24 hours in a day and only seven planets, this cycle spilled into the following day, moving four planets down the list. After 168 hours, repeating on the eighth day, this became the planetary week. In this astrological system, each day was named for its first hour, leading to names like Saturday, Sunday, Monday, Mars Day, and Mercury Day.
However, Mars Day and Mercury Day don’t sound like Tuesday and Wednesday unless you speak Latin or any of the many languages descended from it. As the Greek astrological seven-day week was adopted by the Romans and spread across their empire, we can see the planetary roots in these languages. As Christianity spread throughout Europe, days were renamed to align with Christian traditions, and the first day of the working week was moved to Monday.
Now, you might wonder about English. Only three of these names sound like their Latin roots. Northern European cultures, including Germanic, Anglo-Saxon, and Norse ancestors, adopted the seven-day week but translated some of the day names and replaced others with their favorite local gods, perhaps as a way to send a message to their Roman conquerors. By 1500 years ago, the seven-day week had reached India and China, where the names for the days of the week in Hindi and some older Chinese dialects are named for the same astronomical bodies and gods as the Greek and Roman system, in the same order.
Even indigenous American cultures like the Navajo, with no native concept of a week, adopted the seven-day system following contact with the Spanish and adapted it to their local language. So, the seven-day week is used basically everywhere today. However, the naming system isn’t completely universal. A few unique day names remain. Icelandic, German, and Finnish call Wednesday literally “the middle of the week.” In the Gaelic languages of Ireland and Scotland, Wednesday through Friday are named for days of fasting. The Icelandic word for Saturday means “the day to bathe,” which sounds nice, though I try to bathe every day.
In some cultures, a seven-day week is used, but the names of the days are simply represented by numbers, which seems like a much easier way to do it. It’s amazing how quickly and widely the planetary seven-day week spread around the world through armies, trade, and religion. And we still use it today.
This doesn’t completely explain why the number seven is significant across so many disconnected cultures. For one thing, the planetary week isn’t the only seven-day week in history. The Jewish week dates back to at least 500 B.C. and is drawn not from the planets but from the seven days of creation in the Hebrew Bible, with the first six days numbered in order and the seventh day designated as a day of rest, called Shabbat, which was likely borrowed from the ancient Babylonian word for a festival of the full moon.
If we want to uncover more theories about why seven is significant, the moon is a good place to start. A month on our calendar traces its origin to the cycle of the moon from full to crescent and back to full. The moon is one of humanity’s oldest ways of tracking the passage of time, and it conveniently matches up with the time period between female populations and menstruation, which share the same word root as “measure,” as in measuring the moon’s phases.
When we measure a lunar cycle, each of the four quarter moon phases is separated by about seven days. From new moon to full moon is approximately 14 days, or two sevens. The average time between one new or full moon is close to, but not exactly, four sevens or 28 days. This is a special set of numbers. The factors that evenly divide into 28 are 1, 2, 4, 7, and 14, and they add up to 28. Numbers whose factors add up to the number itself are called perfect numbers, which are rare. The ancient Greeks knew about four of them, and the next one is 33,550,336.
The precise length of an average lunar cycle is slightly more than 29 days, but seven is the nearest whole number of days between each of its quarter phases. The near-perfect number pattern centered around seven would have been known to mathematicians at least as far back as ancient Greece.
Speaking of math, we can trace the mystical nature of the number seven back to the earliest mathematics recorded on ancient clay tablets dating to nearly 2000 B.C. Unlike our modern decimal base ten system, the Babylonian and Sumerian scribes used a base 60 system. Each place in our base ten system tells us the number of hundreds, tens, and ones in a given number.
When any place fills up with ten, we roll it over to the next highest place. The number 2107.3 would be written as 2100 plus 0 tens plus 7 ones plus 3 tenths. In a base 60 system, the number 59 would be written using a single symbol. The number 60 would be written with a one in the 60s place and three ones.
What about fractions? In our decimal system, one-half simplifies to 0.5. In a base 60 system, it would be written as 0 ones and 30 sixtieths. One-third would be 0 ones and 20 sixtieths, and one-fourth would be 0 ones and 15 sixtieths.
When we try to divide one into seven parts in a base 60 system, it simplifies into a repeating decimal fraction. More than 4000 years ago, Sumerian and Babylonian mathematicians who invented the earliest mathematics would have seen seven as the first number whose fraction is infinite. Some scholars believe this helped give birth to the mystical and superstitious nature of the number, which is still associated with luck today.
Perhaps when a superstitious Sumerian astrologer noticed that there were seven heavenly bodies moving across the stars, he saw it as a sign. There are seven stars in the Big Dipper, which is a well-known pattern of stars visible to the naked eye within a larger constellation. The arrangements and brightness of these stars are coincidences due to Earth’s position in space, but our minds love to insert meaning into coincidences.
To certain ancient eyes, the universe seemed to be signaling that seven was special. Of course, we know that there are actually eight planets. I have to wrap this up, but by the time humans figured all of this out, there were already seven days in a week, and nobody felt much like changing it, if they ever wondered where it came from in the first place.
It’s one of those rare ideas that is simply so old that no records remain of exactly who first invented it or why. It shows us that even ideas we take for granted, which seem like they’ve always been there, came from somewhere. Now you know! What will you do with all this knowledge? Please get rid of Mondays! Stay curious!
As always, we’d like to thank everyone who supports the show on Patreon. We couldn’t do it without you. Videos like this require extensive research to collect all this essential information, and we couldn’t do that without your support. We’d like to keep making more videos just like this one, so check out our community. Thanks!
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This version removes any informal language, personal anecdotes, and repetitive phrases while maintaining the core content and ideas.
Seven – The number that comes after six and before eight; often used to describe a quantity or count. – There are seven main types of stars classified by astronomers based on their temperature and brightness.
Week – A period of seven days, often used to measure time in calendars. – In astronomy, a week can be used to track the phases of the moon as it changes from new to full.
Astronomy – The scientific study of celestial objects, space, and the universe as a whole. – Astronomy helps us understand the movements of planets and stars in our galaxy.
Planets – Celestial bodies that orbit a star, such as the Sun, and are massive enough to be rounded by their own gravity. – The solar system consists of eight planets, each with unique characteristics and orbits.
Lunar – Relating to the moon, especially its phases and cycles. – The lunar cycle is approximately 29.5 days, which is the time it takes for the moon to go through all its phases.
Culture – The beliefs, customs, and practices of a particular group of people, often influencing their understanding of the world, including astronomy. – Many ancient cultures used the stars to create calendars and navigate across oceans.
Number – A mathematical object used to count, measure, and label. – In mathematics, the number pi is used to calculate the circumference of a circle.
Ancient – Belonging to the very distant past and no longer in existence, often referring to early civilizations and their contributions to science. – Ancient astronomers were able to predict solar eclipses using their observations of the sky.
Mathematics – The abstract science of number, quantity, and space, used in various fields including astronomy. – Mathematics is essential for calculating the distances between stars and planets.
Cycle – A series of events that are regularly repeated in the same order, such as the phases of the moon. – The water cycle on Earth is influenced by the gravitational pull of the moon and the sun.
