The names of constellations were being recorded on clay tablets as far back as the time of the Sumerians, around 3000 bc. Some of these names are familiar to us today: the bull (Taurus), the lion (Leo) and the scorpion (Scorpius). This interest in forming constellations may have reflected their desire to organize the sky in a mythologically meaningful manner, particularly the area through which traveled the Sun, Moon, and planets, which we now call the ecliptic. In this way, a reference point for describing the location of these heavenly bodies was made, and this information was useful in preparing calendars for agricultural and social purposes, improving navigation at sea, and making astrological predictions.
Since the calendar consisted of 12 months by the time of the first Babylonian period (around 1800 bc), it seemed reasonable to divide this area into a like number of parts. By 1100 bc, a system had been created where three groups of 12 stars were arranged in three paths across the sky, each of which was related to a creator god. These are described in the Mul Apin clay tablets, which were produced early in the 1st Millennium bc and contained a catalog of important stars and some 60 constellations, along with their rising and setting times. The middle path was roughly plus or minus 17 degrees from the ecliptic line and was related to Anu. The path north of this area was named for Enlil, and the path south for Ea. Based on his review of the appearances and locations of the constellations in the sky taken from the Mul Apin clay tables and other sources, astronomy historian Bradley Schaefer has concluded that the bulk of the Mesopotamian constellations were developed between 1300 and 1100 bc by Assyrian observers in the northern part of the region. The influence of this system spread widely into India, China, Egypt, and Greece.
Paralleling this development was the creation of 18 "constellations" that were easily observed at night to be in the path of the Moon. These included not only star groups more or less similar to our own, but also some asterisms that we do not recognize as constellations. Historian Nicholas Campion (2000) gives a list of these 18 groups from the Mul Apin, and this list includes a number of familiar names: the bull (Taurus), the twins (Gemini), the crab (Cancer), the lion (Leo), the scales (Libra), the scorpion (Scorpius), and the goat fish (Capricornus). Thus, a lunar zodiac was created that was based primarily on star groupings, and this soon took on astrological meaning. For example, the Enuma Anu Enlil contains omens whereby the positions of the planets are described in relation to some of these constellations. It was perhaps inevitable that this 18-constellation lunar zodiac would evolve into a 12-constellation solar zodiac more similar to our own by the 5th Century bc. Although the constellations differed in size, they were given yjth of the ecliptic each, and in time this was transitioned into an astrological area of 30 degrees. This focus on the ecliptic and the zodiac (rather than on the celestial equator, such as happened in China) was transported to Greece, and this became the preferred orientation in the West for describing the positions of the heavenly bodies until the 18th Century ad.
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