Why are there 12 signs of the Zodiac? Because the Babylonians decided that there should only be 12.
What are the signs of the zodiac? The signs of the zodiac are the symbols that represent the 12 different constellations that the sun passes in front of throughout a year. They are all found in a belt 8 degrees wide that the sun, moon, and all of the planets orbit through. If you draw an imaginary line from the Earth, through the sun, and off into space, then track that line for a year, that is the belt that the Zodiac constellations are found in. Each one of the 12 constellations is in a 30-degree section of the sky.
The zodiac constellations can also be used for navigation. By knowing the position of the constellations, certain stars, and the sun at certain times of the day, it was possible to roughly calculate longitude and latitude.
So, why are there 12 signs of the Zodiac? It comes from the Babylonians and their base 60 counting system. They divided the sky up into 12 neat sections. There are actually 13 different constellations that the sun passes in front of over the year, but the Babylonians couldn’t fit 13 into their system, so they ignored the last one.
People have always used the moon, the sun, and the stars to measure time and to find directions. It was natural and the movement of the stars through the year could easily be tracked. Different groups of people came up with differing systems of astronomy over time. The Egyptians tracked stars throughout the year. They divided the year up into 36 groups of stars and called each group a “decan”. One decan, for the Egyptians, lasted 10 days, giving them a 360 day year. They added 5 days at the end to make it fit.
The Babylonians divided the Zodiac belt up into 12 sections of 30 degrees. Babylonian math was based on a base 60 system, which is why we have 60 seconds in a minute, 60 minutes in an hour, 12 hours in a half day, and 12 months in a year. A base 60 counting system seems strange to us, but it is actually a good system for maths because 60 has more divisors than the base 10 system we use. 60 can be divided by 1,2,3,4,5,6,10,12,15,20,30,60. It is said that the Babylonians developed the base 60 because there were two tribes that came together. One tribe had a base 5 system they had developed because there are five fingers on one hand, and the other tribe had a base 12 system they had developed because there are 12 joints on the four fingers of one hand. These two tribes came together, multiplied 5 by 12 and ended up with the base 60 system.
The Babylonians gave names to each of the constellations. The names were suggested by what they thought each constellation looked like, such as GIR TAB, “scorpion”, and AL LUL, “crayfish”. They ignored the 13th because it didn’t fit. The Greeks inherited most of Babylonian math and astronomy. They changed the names of the zodiac to fit their own religion, but they kept the meaning behind the names mostly the same as the names the Babylonians had used. The Greeks also set down what effect they believed the positions of the planets and the constellations had on humans. In fact, the great astronomer Ptolemy wrote a book about astrology which set out principles of astrology that are still followed today. He believed that astrology was just as much a science as astronomy and these ideas were believed for centuries. It is only in the last two hundred years that people have realized that astrology is not a science.
A good example of how the study of astrology is not a science is because the signs of the zodiac no longer match the constellations they are named after. When the Babylonians came up with the 12 signs of the zodiac, they had the first one, Aries, starting with the spring equinox. The trouble is that the axis of the Earth ever so slowly changes as it orbits the sun. The stars, as viewed from Earth, move backwards by about 1 degree every 72 years. That means that since the invention of the signs of the zodiac, 2,500 years ago, the constellations have moved beyond the section allocated to them. To cope with this, astrologers have had to come up with something called the tropical zodiac system. This takes Aries to begin with the spring equinox and calculates from there. So, if you are Taurus, as I am, the constellation of Taurus will be visible between November and January, not during the month of Taurus, which is in May. And this is what I learned today.
Sources
https://en.wikipedia.org/wiki/Zodiac
https://www.britannica.com/topic/zodiac
https://en.wikipedia.org/wiki/Sidereal_and_tropical_astrology
https://www.bbc.com/news/magazine-12207811
https://www.thoughtco.com/why-we-still-use-babylonian-mathematics-116679
https://www.ox.ac.uk/news/arts-blog/ancient-knowledge-transfer-egyptian-astronomy-babylonian-methods
https://en.wikipedia.org/wiki/Babylonian_astronomy
https://en.wikipedia.org/wiki/Decan
https://time.com/5867647/nasa-zodiac-star/