When I was a doctoral student at York University’s “Institute for Space and Terrestrial Sciences” my supervisor was very interested in calendars. This is perfectly reasonable given the link between our work and geodesy. One thing I recall him telling me (I admit to not always paying attention) was that there was a mystery around why we had seven days per week. The length of a month was tied to the moon’s path and he argued that 360° came from the number of days in a year. He loved to talk about the “age of Aquarius” and the link to precession.
Seven days in a week? THAT was a mystery.
I will now present a theory of where the seven days came from.
Image 1: Celestial map, signs of the Zodiac and lunar mansions in the Zubdat-al Tawarikh, dedicated to the Ottoman Sultan Murad III in 1583 (wikipedia). Note that the blue outer ring represents the 28 lunar mansions and the inner ring (just outside the path of the planets) is the zodiac of 12 signs. The zodiacal signs each span 30°, and there is a point of alignment after sets of three zodiacal signs and seven lunar mansions.
In a previous post I mentioned the 27 or 28 “mansions” or “houses” that the ancients used to map out the moon’s path through one month. The moon appears in a different house each day, and after roughly 28 days it arrives back where it started against the background stars. The houses are similar to the zodiac, but are greater in number. The path through the houses is the same as the path through the zodiac.
If we look up the moon’s state we usually see four phases emphasized, full moon, new moon, and the two half moons – waxing and waning. Between these phases are the periods of waxing and waning, gibbous or crescent, which span the times between the four key phases. Let’s follow the moon around the cycle and assume 28 mansions.
I will cheat a bit to simplify matters: The lunar cycle (e.g. full moon to full moon) takes 29.5 days, but it moves through 360° on the background stars in 27.33 days. These cycles differ because of the (apparent) motion of the sun during this month. For now, let’s assume both are exactly 28 days.
We will also cheat with respect to the zodiac. Today we have 12 signs spread over 360° of sky, giving 30° per zodiac sign. If we go back far enough in history the number of signs may have been some other number (11, for example). And the separation between signs was not precise: they didn’t have computers with exact lines drawn on a virtual screen of stars.
We’ll start at a full moon in some part of the star field, say the first point of Aries (zero right ascension in modern terminology).
In seven days the moon will have waned to half illumination. It will also have moved through seven houses and will be 90° from the initial point in the sky. This 90° will be one-fourth of the way around the sky, and will correspond to precisely three signs of the zodiac. Seven days equals seven “houses”, 90°, and exactly three zodiacal signs.
This would be the first point where the moon has passed through an integer number of both zodiacal signs and lunar houses. The other points where this occurs are at days 14, 21 and 28. The half-moon that appears after the first seven days is the next in sequence of the most important phases of the moon.
There may be some significance to the fact that three zodiacal signs corresponds to a season. The first three signs correspond to the spring, the next three to the summer, then autumn and winter.
Seven days later, fourteen days after the start, the moon will be a New Moon, the next important phase. It will have passed another seven houses and precisely three zodiacal signs and will be 180° across the sky from where it began.
Similarly, after 21 days, the moon will be a waxing half-moon, 50% illuminated. It will be at 270° from the starting point, and will have passed an additional seven lunar houses and three zodiacal signs.
And finally, after another seven days, 28 in total, the moon will have returned to a full moon. It will have returned to the starting point, cycling through the entire zodiac and all the lunar houses.
The importance of the seven day subsets is clear. Only after 7, 14, 21 and 28 days will the moon have passed through even numbers of houses and zodiacal signs. These points correspond to 90, 180, 270 and 360 degrees of right ascension.
In reality from one full moon to the next (a period of 29.5 days) the moon will have moved against the background stars because of the motion of the sun. In one month this background movement will have been approximately 30°, or through a distance equaling the length of one zodiacal sign.
Simple astronomical time-keeping for the seasons was almost certainly associated with the moon rather than the sun for most of human history. One cannot look at the sun and see what constellation it is in because the stars are invisible. The sun would have been most useful to know what time of day it is. When we see the moon we can always locate what house and zodiacal sign it is in. Watching the cycle of the moon and identifying key points to break that down to a smaller lengths of time could easily lead to a seven day subset of the lunar period (month).
The nature of the ancient zodiac means that the clean logic of three signs = seven houses may be less persuasive than I have presented it here. Nevertheless, the zodiac existed in a well-developed form millennia ago, and the lunar houses were also known thousands of years ago. A linking of them at 90° spacings is clearly shown in the image above of the Arab Zodiac where the zodiacal signs span 30° precisely. We cannot go back 3,000 years and interview the local astronomers but this alignment and association between three signs and seven houses is more than plausible. Importantly by spacing the zodiacal signs evenly the ancients facilitated the division of the year into four seasons of even length. A season of three zodiacal signs that matched seven lunar houses...
We should now address the other simplification made earlier. The ancients were watching the moon and identifying what phase it had when it was in a given house. This means that they were well aware of the difference between the 27-day path through the houses and the 29-day lunar cycle. The difference between these provided the annual calendar! Without this difference they would not have been able to use the moon to determine when to plant and when to harvest.
Nevertheless, there would have been a need to simplify matters. Farmers watching for a waxing half-moon in a particular “house” would not have had great time-keeping accuracy. Observations would not be done in the daytime so we can expect errors on the orders of several hours to one day. Just as 365.24 days in a year is believed to have led to exactly 360° in a circle it seems plausible that these two cycles of 27 and 29 days would have been simplified to one 28-day cycle broken into four neat parts (“weeks”), with full awareness that it slipped slowly around as the sun followed its own path. Some evidence for this lies in the fact that most versions of the “lunar houses” include 28 houses rather than 27 which would be more accurate. 28 was a number that led to 7 day weeks, while 27 led to mathematical confusion.
We should also ask ourselves why 5 day weeks or 10 day weeks were not chosen (in spite of the best efforts of the French). If we start with a 28 day month and wanted a smaller length of time, we would consider 14, 7 or 4 day periods. 28 is not divisible by 10, 9, 8, 6, 5, or 3. 14 days is probably too long and 4 is probably too short. The lunar/astronomical significance of the 7 day patterns would make it an obvious choice.
A simplified 28-day month seems to have been a historical fact. Here is a quote from a "calendar truth" website (see link below):
"The 13-month, 28-day alternative has been in use on this planet for more than 6000 years. In prehistoric India and China, and throughout South America it was the standard time-keeping system. The Essenes, Egyptians, Polynesians, Maya, Inca, Lakota, and Cherokee used a 13-month, 28-day calendar. The Celtic knowledge of the Druids is based on the Tree Calendar, also a 13-month, 28-day calendar. Today many cultures are still using their traditional 13-month calendar system."
What does it all mean?
To sum up we must first emphasize the critical importance of the moon to the ancients. The moon is the brightest object that can be easily observed moving through the background stars. The moon's phases created our months and it is likely that the combination of lunar phase and location among the stars was used thousands of years ago as a guide to the seasons, including when to plant and when to sow. The combination of zodiacal signs and lunar houses created clean breaks after seven-day periods, lining up with the lengths of seasons, etc. The seven day week appears to be a perfectly logical subset of a 28-day month.
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