The zodiac is a band of twelve constellations seemingly wrapped around the sky in the apparent annual path of the Sun through the heavens.
The root of the word Zodiac is that for zoo, because the zodiacal constellations, like Hydra or Leo, are mainly fancied to be animals.
In fact, a million years from now, the constellation of Leo will look less like a lion than it does today and perhaps our remote descendants will call it the constellation of the radio telescope – although we can guess that a million years from now the radio telescope will have become more obsolete than the stone spear is now. Anyway, constellations were given the names of the signs and asterisms could be connected in a way that would resemble the sign's name.
Although the zodiac remains the basis of the ecliptic coordinate system in use in astronomy besides the equatorial one, the term and the names of the twelve signs are today mostly associated with horoscopic astrology.
The Zodiac constellations are known to have been in use by the Roman era based on concepts inherited by Hellenistic astronomy from Babylonian astronomy of the Chaldean period (mid-1st millennium BC). Besides the construction of the zodiac is described in Ptolemy's Almagest (2nd century AD).
Babylonian astronomers at some stage during the early 1st millennium BC divided the ecliptic into twelve equal zones of celestial longitude to create the first known celestial coordinate system: a coordinate system that boasts some advantages over modern systems (such as equatorial coordinate system). The Babylonian calendar assigned each month to a sign, beginning with the position of the Sun at vernal equinox, which, at the time, was depicted as the Aries constellation, for which reason the first sign is still called "Aries" even after the vernal equinox has moved away from the Aries constellation due to the slow precession of the Earth's axis of rotation.
Because the division was made into equal arcs, 30º each, they constituted an ideal system of reference for making predictions about a planet's longitude. However, Babylonian techniques of observational measurements were in a rudimentary stage of evolution and it was probably beyond their capacity to define in a precise way the boundary lines between the zodiacal signs in the sky.
The Babylonian star catalogs entered Greek astronomy in the 4th century BC, via Eudoxus of Cnidus and others, but horoscopic astrology first appeared in Ptolemaic Egypt. The Dendera zodiac, a relief dating to ca. 50 BC, is the first known depiction of the classical zodiac of twelve signs.
Under the Greeks, and Ptolemy in particular, whose work Tetrabiblos laid the basis of the astrological traditions, the planets, Houses, and signs of the zodiac were rationalized and their function set down in a way that has changed little to the present day. Capricornus (Goat-horned - the Sea-Goat). Zodiac Fontain in the old city of Jaffa, Israël, photo by © Zvi Kaplan
Curiously enough, the Hindu zodiac signs and corresponding Greek signs sound very different, being in Sanskrit and Greek respectively, but their symbols are nearly identical. For example, dhanu means "bow" and corresponds to Sagittarius, the "archer", and kumbha means "water-pitcher" and corresponds to Aquarius, the "water-carrier". The correspondence of signs is taken to suggest the possibility of early interchange of cultural influences. Pisces (the Fishes). Zodiac Fontain in the old city of Jaffa, Israël, photo by © Zvi Kaplan
It is important to distinguish the zodiacal signs from the constellations associated with them, not only because of their drifting apart due to the precession of equinoxes but also because the physical constellations by nature of their varying shapes and forms take up varying widths of the ecliptic. Thus, Virgo takes up fully five times as much ecliptic longitude as Scorpius. Scorpio (the Scorpion). Zodiac Fontain in the old city of Jaffa, Israël, photo by © Zvi Kaplan
The zodiacal signs are an abstraction from the physical constellations designed to represent exactly one twelfth of the full circle each, or the longitude traversed by the Sun in about 30.4 days. Aquarius (the Water-Bearer). Zodiac Fontain in the old city of Jaffa, Israël, photo by © Zvi Kaplan
Due to the constellation boundaries being redefined in 1930 by the International Astronomical Union, the path of the ecliptic now officially passes through thirteen constellations: the twelve traditional zodiac constellations plus Ophiuchus, the bottom part of which interjects between Scorpio and Sagittarius. Ophiuchus is an anciently recognized constellation, catalogued along with many others in Ptolemy's Almagest, but not historically referred to as a zodiac constellation. Zodiac Fontain in the old city of Jaffa, Israël, photo by © Zvi Kaplan