Why 7 days in a week? - An ode to the mystical number “7”

By - Shravankumar Setlur

One of the first things that instilled curiosity in us as kids when we started to understand our Calendars was 7 days in a week. The fact that there are 365 or 366 (full) days in a year and 7 not being one of it’s prime-factors didn’t really seem satisfying. And then adulting happened to all of us while we were slowly getting used to hating Mondays. But again! —

• Why 7? Why not 5 or 10, which would make more sense?
• Are there any other Calendar systems in other civilizations that would follow a different cycle?
• Is there a cosmic connection? If yes, what is it?!
• If not, why is this system so consistent without superficial relevance?
• Why do we need Calendars at all? (just kidding, this question is about 3,000 years too late, go take a day off!)

Calendar — derived from the Latin calendarium, meaning “interest register” or “account book,” itself a derivation from calendae (or kalendae), the first day of the month in the Roman republican calendar.

Let’s brush up some things about cosmic cycles that we already know before we get all nerdy.

1 Day — time taken by the Earth to complete a rotation about it’s own axis
1 Month — time taken (roughly) by the Moon to orbit once around the Earth
1 Year — time taken by the Earth to orbit once around the Sun
1 Cosmic Year — time taken by our Solar System to orbit around the centre of our galaxy — around 250 million years (I just felt like giving this info)

The earliest known time-keeping was practiced in the Neolithic era (around 12,000 years ago) and later more accurate Calendars devised by Sumerians, Egyptians, Assyrian, Elamite, Indians, etc., were either Solar (Egyptian) or Luni-solar calendars with unique intercalation* patterns. — All these Calendars used the common astronomical cycles of Earth, Moon and the Sun as units.

A week had neither the significance nor the astronomical relevance till this point to be included in the Calendars. So, why do we week?

Moon & its phases —

While the rotation of our Earth was set to take 24 hours equalling “1 day” (that’s whole another blog material), the concept of “month” evolved when sky-gazing Babylonians observed the pattern in Lunar cycles. There are roughly “12 Lunar cycles” in a year (12.37). Our Moon takes 27.3 days to complete a revolution around the Earth (while also moving around the Sun). But it takes 29.5 days to change from new-moon to new-moon.

Talking about New-moon, across civilizations the Moon was observed to have 4 distinct Moon phases — New moon, First Quarter, Full Moon & Third Quarter.

Though there were other moving celestial objects on the sky that can be observed through naked eye, the Moon was the chosen one — the primary reason for tracking days was agriculture and survival back then. Understanding Seasons were critical to both and hence Moon cycles made more sense as it’s much easier to sync Moon and Seasonal cycles. (a Season roughly comprised of 3 Full Moons).

What is a BLUE MOON ?—
A seasonal Blue Moon is the third Full Moon of an astronomical season that has four Full Moons. A monthly Blue Moon is the second Full Moon in a calendar month with two Full Moons.
Either type of Blue Moon occurs roughly every two or three years.

12 Zodiac Signs and it’s useful uselessness —

12 was the most convenient number to measure time — it is small enough to be able to count to it plus it is divisible by 2, 3, 4 & 6. This later motivated to split a circle into 12 equal parts and assign a sign to each of them. — The sky is the circle and each equal part is a zodiac sign.
Ancient astrologers watched the Moon moving through every slice of the Zodiac each passing month.

But nope, 12 wasn’t yet a choice for splitting months as it still gives many residual days. Then why did you make me read this?
One interesting thing that emerged out of this exercise was that — for an observer from the Earth the Moon with it’s face locked towards us seems to move across this Zodiac map without change in it’s backdrop of “fixed stars”. There arose another group of celestial objects with similar properties.

Tidal locking —(synchronous rotation) occurs when an orbiting astronomical body always has the same face toward the object it is orbiting.
The tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner. Eg: Earth and our Moon.

The Wanderers —

The Moon along with the Sun, Mercury, Venus, Mars, Jupiter and Saturn were grouped as the “wandering stars” called planētēs (planets) in Greek, believed to be revolving around the Earth.

We now know this is not how the Solar system works at all and there are 3 more actual planets in the list, but for a human 3,000 years back this is what they could observe using just eyes and was sufficient knowledge to make peace with the routine, which now sounds ironic, but move on!

Astrology driving Astronomy —

This system was used by almost all major civilizations at the time from Babylonians, Egyptians to Indians, Chinese, Greeks and Romans.

The 7 planetes were each dedicated a day in the order based on how fast they wandered the sky (when viewed from Earth) — the order is
Saturn > Jupiter > Mars > Sun > Venus > Mercury > Moon.

This led to charting a planetary week with each hour of the day being dedicated to each God (planet) in the above order for 24 hours in a day. The name of the current day then was based on the name of the first hour of the day.

The number 7 was now fixed because it also made sense mathematically. The number closer to and less than 29 (Lunar cycle) which is divisible by multiple numbers is 28 (logic was the final step even back then). This meant a week for each moon-phase.

28 is a Perfect Number.
A number whose factors evenly divide the number and the sum of the factors is the number itself is Perfect Number.
Factors of 28 are 1, 2, 4, 7 and 14 which add upto 28.

The nomenclature however took some revisions across cultures —

• Latin, Spanish and French adoption of the Greek week-system resulted in inclusion of their translations of the same planet-based names of the days.
• Also with Christianity spreading across empires, some days like “Day of the Lord” was the role given to Sunday. And thus Moonday (Monday) became the first working day of the week.
• Ancestors of other cultures like Germanics, Anglo-Saxans, Norseman, etc — adopted their translated versions and preferred local Gods instead of Roman/Greek Gods. Eg: Thor’s-day became Thursday, etc.
• By 1,500 years ago 7 day week had also reached India and China as trade became more prevalent — the names in Hindi, Tamil and Chinese were the same astronomical bodies with no local influence.
  Eg: Mangalvar (Hindi) for Mars-day or present-day Tuesday, Sani-kizhamai (Tamil) for Saturn-day or present-day Saturday.
• In Hebrew, the 6 days are named after the number of each day (eg : rishon = First-day) and the 7th day was “Shabattu” meaning “rest-day” or festival of the Full moon in Babylonian.

Okay wait, so every culture just agreed upon a 7-day week?

Practically yes, but there is more. The number “7” itself is so significant across cultures and religions that civilizations and even some tribal groups couldn’t deny it’s usage.

• Moon cycles, based on which the length of the week is fixed, coincidentally syncs up with and hence was used to track time-period between Ovulation and Menstruation in human females.
  “Menses” (or Mēnsis) literally means a month in Latin.
• The rarity of Perfect numbers of which 28 is one (6, 28, 496, 8128, 33550336 and so on).
• Babylonians who used the Sexagesimal (base-60) number system at the time, knew that the fraction 1/7 results in an infinite sequence (0.8341783417…) around 4,000 years ago and 7 was the first such number whose fraction extended till infinity and hence it quickly became a superficial number in the culture.
• In addition to these — the 7 planetes, the 7 Heavens, 7 musical notes, the 7 Deadly Sins, the 7 virtues, 7 days of creation, the 7 Samurais, 7 Chakras, etc., also played their part in respective cultures in quick adoption of a week based on such a number.

How do I even conclude this?

There are numerous other calendar systems devised with other convenient base numbers used for weeks. Also, it’s much easier for computers to simply use the 10-day week calendars instead of 7-day weeks and the collateral inconsistencies.

Probably as we expand our base to the Moon, Mars and beyond we won’t repeat this mystical approach but rather a boring and straight-forward one.

Regardless of the number of days in a week, we all know at this point that the number of weekends should be almost equal to the weekdays. We missed the last time this was fixed but let’s ensure that it doesn’t happen again.