Zodiac, from the greek zodiakós'(" circle with pictures of animals ") is the area of the celestial sphere bounded by two circles parallel to the ecliptic, having as total width of 18 degrees. This path is divided into 12 "signs" of 30 ° each, corresponding to the zodiacal constellations.
Already in the fourteenth century BC, the Babylonian astronomers drew celestial maps, dividing the apparent path of the Sun along the ecliptic into 12 signs named as follows:
The Greeks became aware of the ecliptic and the Zodiac much later, perhaps with Cleostrato of Tenedos, in the second half of the sixth century BC.
When the foundations of Greek astronomy were outlined in the fourth century. a. C., the sun passed through the constellations of the zodiac starting from the Ram, where at that time was the spring equinox, i.e. the point of intersection of the ecliptic with the celestial equator.
As centuries went by, that location varied because the Earth's axis changes its orientation continuously, with a conical motion called "precession of the terrestrial axis". The axis, in a period of 25,730 years, describes a cone around the pole of the ecliptic, whose half-opening is 23 ° 27 '. Jointly with the axis of diurnal rotation moves the whole of the Earth and, with it, the equator. Thus the line of intersection of the equator with the ecliptic recedes, as the axis of the Earth does, by the same angular width, amounting in one year to 50 ", 37.
The movement of the ascending node, also called " &Upsilon point;" or "first point of Aries", along the ecliptic, is called "precession of the equinoxes", and occurs clockwise towards the Sun.
The cycle just described is also known as "year Platonic" or "great year", or "cosmic year".
Not only the celestial North Pole proceeds with a continuous and very slow motion among the stars, but also the Υ point - which in Hipparchus' time was in the constellation of Aries - has since "retreated" by more than 30 degrees and is now in the constellation of Pisces (the Two Fish) and will continue to proceed toward Aquarius, Capricorn and so on.
In other words, while the subdivision into constellations is immutable because it is based on the fixed stars, the signs of the zodiac, instead, are bound to theΥ point and move constantly, as this point passes from one constellation to the other because of the precession.
The Sun takes a month to go through a sign, while the Υ Point requires 2000 years to transit from one constellation to another.
The tables are from Atlas Celeste / de Flamsteed / Publié en 1776 par J. Fortin / Ingénieur - mécanicien pour les Globes and Spheres / Troisième édition / revue, et corrigée augmentée / par MM. Delalande et Méchain / A Paris / Chez Delamarche, éditeur et géopraphe, successeur de Robert de / Vaugondy, rue du Jardinet, n ° 15 (quartier St.-Andre des Arcs) (Genoa,
the Navy Hydrographic Institute)
Text by Ilio Foschini