Why my zodiac sign is ophiuchus?

Zodiac signs and astrology. It is surprising how this heathen babilonic tradition has survived thousands of years, and, even in the rational society of nowadays it still deserve a section in most of the newspapers. It is supposed to predict what is going to happen to us based in the interpretation of the sky-night firmament by some kind of magicians and/or witches.

Although almost everybody knows its zodiac sign not to many people know the real origin of it. Why, for example, you are taurus and not gemini. And why, among all the 88 official recognized constellations (such as ursa major, orion,...) in which the sky is divided, 12 of them have some influence in our lives. And, moreover, why people is talking about the new 13th zodiac constellation: ophiuchus.

The origin of astrology is the motion of the planets and stars along the nights of the year. And there is very simple and interesting astronomy behind it. So learning a little of science  will enable us to do our own astrological predictions. Who said science was worthless?

Galileo, the father of science, proposed the heliocentric model  that we all accept today. All the planets orbit around the sun on the same plane (which is called the ecliptic). The stars, which are very far, form the celestial vault and look static from the sun viewpoint. Lets now imagine we are on the earth orbiting around the sun once a year. The sun will always be in front of one of the constellations in front of the ecliptic. In the image we see what i am explaining.

From earth we have the impression that the sun slowly moves day after day onto the background of fixed stars. It happens that ancient people noticed this fact.They noticed that, year after year, the same stars appear to be behind the sun during its raising and fall, and they cataloged those stars in 12 constellations. Your zodiac sign corresponds with the constellation which is "hidden" by the sun when you were born. That is the explanation, the sun position in , for example, august is on the leo constellation, but it is constantly moving.This from-earth apparent slow motion of the sun on the ring of zodiac costellations is repeated after 86.400 seconds (365,25 days).

But in a scale of thousands of years even stars move one from another and the whole system sun-planets rotates his axis respect the rest of the galaxy. The next image shows how the ursa major constellation looks in our nights and how it will look in 50.000 years as a prediction of some galactic evolution models.

Something similar happened with the ecliptic. It shifted a little during the 3.000 years that passed since the arbitrary identification of stars into zodiacal constellation was invented. It shifted a little, but a little enough for introducing the path of the sun into a piece of the sky that the International Astronomy Union assigns to a constellation wich was precisely bordering three zodiac signs, sagitarius, scorpius and libra. The following diagram represents the possitions of sun and planets on the ecliptic as observed in 30 of december of 2005. We observe that the ecliptic line clearly crosses the ophiuchus border. Between november 30th and december 18th the sun is technically into ophiuchus. And therefore the nickname of the 13th zodiac sign.