The Sun traverses the full length around the Zodiac once each year, following exactly the same path against the background of fixed stars each time. Eventually, astronomers coined the term "ecliptic" to designate this invariable path of the Sun as it shifts from point to point within the Zodiac. The zodiac is the band of constellations that contain the ecliptic. The ecliptic line thus bisects the zodiac into two rings, like a central zipper holding everything together all the way around.
Using Babylonian sexagessimal units for angular measurement (360 degrees = a full circle), Seleukid era astronomers divided the twelve zodiacal constellations equally into angular longitudinal arcs of 30 degrees (360 degrees ÷ 12 = 30 degrees of arc per constellation). If the Sun moved at constant speed, it would enter a new constellation about every 30 days (365 days ÷ 12 constellations = roughly 30 days per constellation).
The circle of the Earth's equator (defined by the cardinal directions east and west) can be projected onto the "sphere of fixed stars" (as the Greeks conceived it) to form the celestial equator.
The ecliptic (defined by the Sun's positions throughout a year) is inclined by about 23.5 degrees to the east-west running celestial equator.
The distinction between the celestial equator and the ecliptic, implicit in many archaeological structures, was explicitly articulated by the Pythagoreans in the fifth century B.C., and referred to by Plato in his dialog, The Timaios. The celestial equator and the circle of the ecliptic intersect at two points, the vernal and autumnal equinoxes, the two positions in which the Sun lies either due east or due west at its rising and setting.
At the vernal equinox (in March), the Sun crosses the celestial equator, heading northward, and appearing higher in the sky.
At the autumnal equinox (in September), the Sun descends across the celestial equator, heading southward, and appearing lower in the sky.
The extreme northerly or southerly points of the ecliptic, measured from the celestial equator, are the solstices, which the Sun occupies around June 21 and December 21.
If the ecliptic is mapped onto the (spherical) Earth, its northernmost and southernmost points lie on the Tropic of Cancer and the Tropic of Capricorn respectively, defined by Eratosthenes in Alexandria, third century B.C., as two boundaries of seven terrestrial "klimata." At that time the solstices were located in the constellations of Cancer and Capricorn.
One consequence of the Sun's shifting position is that the Sun will move into a new constellation from time to time. The constellation containing the Sun at a given time would be overhead at noon, of course, invisible in the daytime sky. Conversely, the constellation crossing overhead (transiting the meridian) at midnight will differ from time to time throughout the year.
According to R.H. Allen, the Akkadians recognized a band of constellations they called the Furrow of Heaven, ploughed by the Bull of Heaven (which is mentioned in the Epic of Gilgamesh). From the fourth to the second millenia B.C., Taurus the Bull, one of the most ancient of constellations, was the constellation containing the Sun on the first day of spring.
No allusion to the zodiac is made by the early Greek poets, Homer and Hesiod; it seems not to have entered Greece until around the fifth century B.C.; a century later Aristotle alluded to the zodiac (literally "Circle of animals").
The temple of Denderah in Hellenistic Egypt portrays the zodiac in a spectacular ceiling representation.
Josephus, the Jewish historian of the Roman period, speculated that the twelve stones in the breastplate of the high priest represented the twelve signs of the zodiac.
Perhaps with the warning of 2 Kings 23: 5 in mind, Bede substituted a figure of John the Baptist for Aquarius in the eighth century, and filled in the other constellations with the eleven apostles.
The Rig Veda of India spoke of a "twelve-spoked wheel" of heaven.
In China, an independently-derived "Yellow Way" began with a Rat (Aquarius), and numbered twelve constellations in a direction opposite that of the Babylonian zodiac.
A Mayan lintel frieze from Chichén Itzá (Mexico) depicts at least six animals in a 24-section series, with the symbol for Venus either occupying its own section or being integrated into the representations of the six animals (pig, bird, turtle, scorpion, vulture, snake). The whole question of a "Mayan zodiac" is controversial (see Aveni, 199ff).
To refer to the above-mentioned Akkadian, Chinese, Indian, and Mayan constellation bands as "zodiacs" strictly speaking is confusing, perhaps anachronistic, in that the meaning of "zodiac" may not have always been the same. What we call the zodiac represents the path of the Sun and planets, not the constellations along the celestial equator.
In contrast to the planets, the fixed stars themselves change from season to season in a more regular manner: those fixed stars overhead at midnight in midwinter will be overhead at midday in midsummer (invisible except during solar eclipses). A fixed star returns to the same position in the sky at a given time of day after exactly one year.
The Greek poet Hesiod admonished the farmer (Works and Days): "Forget not, when Orion first appears, To make your servants thresh the sacred ears..." The morning rising of Orion occurred at the beginning of summer; its midnight rising heralded the grape harvest; and sailors feared its evening rising as signaling the onset of winter storms: "...then the winds war aloud, And veil the ocean with a sable cloud: Then round the bark, already haul'd on shore, Lay stones, to fix her when the tempests roar..."