Introduction

The zodiac
was in use by the Roman era, based on concepts inherited by Hellenistic
astronomy from Babylonian astronomy of the Chaldean period (mid-1st millennium
BC), which, in turn, derived from an earlier system of lists of stars along the
ecliptic. The construction of the zodiac is described in Ptolemy's vast 2nd century
AD work, the Almagest.

The term
zodiac derives from Latin zōdiacus,
which in its turn comes from the Greek ζῳδιακὸςκύκλος
(zōdiakos kyklos), meaning
"circle of animals", derived from ζῴδιον
(zōdion), the diminutive of ζῷον
(zōon) "animal". The
name is motivated by the fact that half of the signs of the classical Greek
zodiac are represented as animals (besides two mythological hybrids).

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 term "zodiac"
may also refer to the region of the celestial sphere encompassing the paths of
the planets corresponding to the band of about eight arc degrees above and
below the ecliptic. The zodiac of a given planet is the band that contains the
path of that particular body; e.g., the "zodiac of the Moon" is the
band of five degrees above and below the ecliptic. By extension, the
"zodiac of the comets" may refer to the band encompassing most
short-period comets.