Why was the Babylonian zodiac divided into 12 equal segments?

Question

Checking out the early history of the zodiac, it says the Babylonians were the first to divide it into an "equal" 12 parts, "by analogy to 12 schematic months of 30 days each". It also says "each sign contained 30° of celestial longitude, thus creating the first known celestial coordinate system" and "when the degrees of longitude were given, they were expressed with reference to the 30° of the zodiacal sign, i.e., not with a reference to the continuous 360° ecliptic."

I understand the babylonians used a base 60 number system, but why is that too?

How did they get 30, 360, and 12? Why was the sky or celestial sphere divided into 12? Did they literal invent the idea of 360 degrees?

Answer 1

I take the liberty of referring you to my answer to a similar question in the History of Science forum.

https://hsm.stackexchange.com/questions/1884/origin-of-360-degrees/1885#1885

Answer 2

When asking "Why was the Babylonian zodiac divided into 12 equal segments?" you are posing the question of their rationale/motive, which is not easily answerable all this time later.

• One theory is based upon the fact that the Babylonian calendar had 12 months, each 30 days long. It therefore stands to reason that as a matter of concurrence, the Babylonians divided the ecliptic also into 12 sectors of 30° each. Subsequently, in each resulting sector, they would identify a prominent group of stars and designate it as a zodiacal constellation.

• In theory, the reverse would also be conceivable: Babylonian astronomers could have first identified prominent star figures (the constellations) along the ecliptic, which so happened to amount to 12 by coincidence. And then, they could have fixed equidistant boundaries around the constellations, thus allocating 12 sectors to the 12 constellations: Based on my research though, this reverse approach is not what actually took place because the ancient Babylonian star catalogues reveal 18 zodiac constellations initially (not 12). The number was later reduced to 12 for unspecified reasons:

  The path of the Moon as given in MUL.APIN consists of 17 or 18 stations, recognizable as the direct predecessors of the 12 sign zodiac. […]
  Somewhere around the 5th century BCE, Babylonian astronomical texts began to describe the positions of the Sun, Moon, and planets in terms of 12 equally-spaced signs, each one associated with a zodiacal constellation, each divided into 30 degrees (uš). This normalized zodiac is fixed to the stars and totals 360 degrees. [source]