Theoretically, each of the twelve months represents a sign, which in turn represents the constellations which are exerting power over the Earth at that point in time. These signs have been used for defining human traits as well as helping humans to understand their place in the universe. Many artists choose human characteristics for the zodiac signs, even choosing to paint them in human form, such as in the work 'Libra with Her Sparrow' by Sir Edmund John Poynter, rather than depict these works astrologically.
The symbolism of each sign is different in the zodiac and has different traits and gods associated with it. For example, the sign Gemini is frequently associated in art with Artemis and Apollo, the twin gods of the sun and moon. Likewise, Taurus is commonly associated with hotheaded Zeus, while Pisces is typically used to represent his brother Poseidon, the god of the sea.
The Chinese zodiac has a similar purpose as the Greek one, including twelve signs and personality indicators depending on what sign the person happens to be born under. However, rather than depicting months, the Chinese zodiac has a sign last for a year, at which point the new year, and new sign, are ushered in.
Modern art, in particular, has developed a fascination with the possibilities of interpretation and symbolism afforded by zodiac art. This has led to an emergence of fantasy, pop, abstract, and stylized depictions of these astrological phenomenons.