11. Calendars in the cradle of civilization

The ziggurat of Ur, built by the Neo-Sumerian king Ur-Nammu and his son and successor Shulgi in the 21st century BCE.  Image by Kaufingdude via Wikimedia Commons.

Today it’s called Iraq.  Its residents were the earliest known practitioners not only of astronomy but also writing and mathematics.  They might have even invented the wheel.

The southern region of Sumer came to prominence first.  Cities emerged along the Idigna and Buranuna rivers.  The Greeks would later refer to these rivers as the Tigris and Euphrates and the land as Mesopotamia which means “between rivers.” 

Sumerian cuneiform dates back to the 4th millennium BCE.  The third millennium BCE saw the rise of Akkad, a region to the North of Sumer inhabited by Semitic people.  As Sumerian and Akkadian kings competed for dominance, their languages merged.  Art, architecture, music, religion, philosophy, mathematics, astronomy, and medicine thrived.  Sumer collapsed around 2000 BCE due to ecological factors by which time there were two separate Akkadian nations, Assyria and Babylonia.  All of Mesopotamia came under the rule of the Babylonian king Hammurabi in the 18th century BCE and the city of Babylon became a major religious and cultural center.  

This 18 century BCE relief shows Hammurabi (standing) receiving his royal insignia from a Babylonian god.  Below the relief are Hammurabi’s code of laws.  Excavated by Jacques de Morgan in 1901-1902.  Louvre Museum, Mesopotamie, Room 3, 24656.  Image by Luestling via Wikimedia Commons.

Babylonian kings felt the need to appease the gods.  This is much easier to do if you know what the gods want.  Animals would be sacrificed and markings on their livers would be interpreted by priests as messages from the gods to the kings.

It occurred to someone at some point that patterns in the heavens could be a more sophisticated way for the gods to communicate with the king.

Astrology was born.  Astronomy became central to Babylonian religion.  Astronomer, astrologer, and priest were now one and the same.  As the one person who could interpret the will of the gods, he, or perhaps occasionally she, enjoyed a privileged position in the king’s court.  

Thousands of astrological omens are listed in a collection of cuneiform tablets on Anu and Enlil, the gods of the sky and the wind.  Tablet 63 in the series lists the rising and setting times of Venus over a period of 21 years during the reign of Ammisaduqa, the fourth Babylonian king after Hammurabi.  The more recent MUL.APIN is much more comprehensive, describing observations of the Sun, Moon, planets, stars, and constellations and associated astrological omens.  

Venus tablet of Ammisaduqa.  This tablet was made in the 7th century BCE but the text is believed to date back to the 17th century BCE. British Museum, Room 55, K.160.  Image by Fae via Wikimedia Commons.

The Babylonians, and to a large extent the Sumerians before them, had mastered the basics of observational astronomy.  Using a gnomon, they figured out the cardinal directions and the dates of the solstices and equinoxes.  They kept track of seasonal daylight variations.  They developed an early version of the zodiac after noticing that the Moon and planets only wander through these constellations.  And they noticed that the different celestial cycles do not fit together neatly.  

An astronomer-priest would announce the beginning of the month when he or she could see the waxing crescent moon around sunset.  Consequently, months would have either 29 or 30 days.  A year was twelve months or around 354 days.  Every few years, the king would announce the addition of a thirteenth month, at the advice of the astronomer-priests, to bring the year in line with the seasons.  Religious festivals were based both on the lunar cycle and the agricultural cycle which depended on seasons.  The king’s legitimacy rested, in part, on bringing these cycles into harmony.

Then something happened.  Someone conceived of an ideal world which is different from the natural world.  Months that vary between 29 and 30 days are not ideal.  Of the two, 30 seems preferable.  It is half of 60, a number so ideal the entire Babylonian numeral system was based on it.  Twelve ideal months would add up to 360 days.  Dividing the hour into 60 minutes and the circle into 360 degrees could have its roots in Babylonian idealism.

If it’s not clear to you how big a deal the new calendar was, let me illustrate with an example.  The following two observations are mentioned in the Venus tablet of Ammizaduqa:

“In Month XI 18th Day, Venus in the East became visible.”

“In Month VIII 11th Day, Venus disappeared in the East.”

How many days elapsed between the two observations?  

To answer to this question, you would have to know which calendar was used by the authors or translators.

If the dates had been converted to Gregorian dates, Month XI 18th Day would refer to November 18th and Month VIII 11th day would refer to August 11th.  If you assume the observations were made on consecutive years, you could look at last year’s and this year’s calendar and count the days from November 18th to August 11th.  If you don’t know which year the second observation was made in, your answer could be off by one day since February could have had either 28 or 29 days.

On the “ideal” Babylonian calendar, the calculation is much simpler since each month has exactly 30 days.

Now imagine doing the calculation if the length of each month was not predetermined but was instead either 29 or 30 days, depending on when the astronomer-priest saw the waxing crescent moon.  To calculate the elapsed time, we would have to look up how many days each month had in each of those years, assuming those records are even available.

The Babylonians did not abandon an observation-based lunar calendar once the 360 day calendar had been developed.  Both systems existed in parallel.  

This was a major precursor to the invention of science.  The ancient Babylonians did not practice science.  They were engaged in pseudoscience.  They thought they saw patterns between celestial events and worldly events and got carried away.  They did not yet possess the tools required to answer big questions regarding the nature of the universe.  But their practice of maintaining two separate calendars, one of which was a record of natural processes in their true complexity and the other an artificial creation that was easier to work with, bears a striking resemblance to modern astronomy.

Today there are two types of astronomers.  An observational astronomer examines the universe as it appears to be.  A theoretical astronomer, on the other hand, constructs models of the universe.  Each model is not designed to be a perfect replica of the real thing, just similar enough to serve a specific purpose.

A scientific model is a bit like a mannequin.  Imagine you have a month to design a dress for a celebrity.  You have no direct access to her and so you build a mannequin on which you’ll try out your creations.  Initially, you just want to see how different types of dresses will fit and so the mannequin just needs to have the same figure as the celebrity.  It doesn’t need eyelashes or even eyes.  Or maybe even a face.  But that will not do for your final product. So you look at as many pictures and videos of the celebrity that you can get your hands on.  You will use these to refine your mannequin.  You give it the same skin tone, hair color, and eye color as the celebrity.  But it still might not need toes.  And it certainly doesn’t need a nervous system.

To astronomers, the elusive celebrity is the universe itself.  Observers catch glimpses of it with their telescopes while theorists build models of it.  The first models are always simple.  You may even call them primitive.  But a theorist would call them ideal.

Where I went to college, there was a sign in the Society of Physics Students room that read: “You Might Be a Physics Major If…”  One of the items on the list was that you assume a horse is a sphere to make the math easier.

An ancient Babylonian astronomer would’ve got the joke.


Author: Bhasker Moorthy

I'm a Professor of Astronomy at Harper College in Palatine, IL, USA. My email address is bmoorthy [at] harpercollege.edu.

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