The Sumerians and later the Babylonians were the first known civilizations to use what we now recognize as a lunisolar calendar.
By the 21st century B.C. the Sumerians had come up with a solar year consisting of 360 days. It consisted of 12 lunar cycles (354 days) which were rounded up to 360, forming 12 months at 30 days. What differentiated the Sumerian calendar system from any other lunar calendars of this time, was the way they measured time. The Sumerian calculations are all heavily based on the numbers 6, 12 and 60, still used today. Our current year has 12 months and the day in many countries is structured as 12 hours am and 12 hours pm. The hour itself has 60 minutes and every minute has 60 seconds.
To bring the shortfall of these embellished lunar months into sync with the solar year, the equinoxes (where day equals night) and the solstices (longest and shortest day of the year), the Sumerian astronomers introduced an extra month every four years. This is what we now call a leap year.
The Sumerians also recorded ‘day qualities’. Enuma Anu Enlil is a collection of stone tables and oracles compiled by Sumerian and Assyrian scribes. The tables include information about lunar eclipses, weather events, the movement of the stars, planets and constellations. The most important part was the interpretations of all these cosmic movements and what they meant for life on earth. The dominant observations concerned the moon cycle and its relation to the other stars and the sun.
It is still unclear where all this knowledge came from. But what is certain that it helped to shape the later famous Babylonian calendar. Although these were humble beginnings, these scribes crucially laid down the foundation of the houses, the star signs and the creation of the zodiac (called the ‘circle of the animals). They named many star constellations in the sky and created the basic principals of Western astrology. The movements of the planets and stars were meticulously observed, recorded, and interconnected with symbolic meaning. These observations traveled to other countries, particularly to India in the 3rd century B.C.
From 499.B.C. the Sumerian calendar transformed into a proper lunisolar calendar. The shift came when it was recognized that 19 solar years equal exactly 235 lunations (moon cycles) and this formed the first proper re-occurring connection between the solar and the lunar cycle. This principle was formally described by the Greek astronomer Methon of Athens in 432 B.C. when he ‘discovered’ the Metonic cycle. But most probably he came across the Babylonian calendar and recalculated their calendar formula.
The Sumerian calendar month started at sunset with the first sighting of the new crescent moon (Waxing Moon) and ended with the last sighting of the descending crescent (Waning Moon). Once the Waning moon had vanished there followed a period of the ‘disappeared moon’ (New Moon) when no moon was visible in the sky.
This played a big part when planning to travel, especially crossing a desert. Due to the hot climate caravans preferred to travel at night. But with no moonlight to guide them, there was a great chance of getting lost. Therefore the New Moon became known as the time to stay at home and rest. A concept we still use in the LWTM life goal planner.
On the other hand, the moon’s opposite position in the lunar cycle – the Full Moon- was the time for gatherings and ceremonies. Then people could easily find their way home once it got dark.
The roots of the Sumerian lunisolar calendar still exist today, particularly in many religious calendar systems and associated practices.
The ‘Living With the Moon lifestyle calendar’ is also based on the Ancient Lunisolar Calendar. It is now believed that this calendar model was in far more widespread use than initially thought. For example, many believe that the Minoan civilization (3000-1450 B.C). followed this calendar system.