For the Chinese Astrology, Earth means dirt and stone, not the planet.
The Chinese Zodiac is a cycle of 5 "Elements" of 五行 (Wu Xing - sometimes referred to as chi) and 12 Animals. When someone turns 60 on or after the Chinese New Year, it's the year of their birth again.
The elements are Wood (associated with Jupiter), Earth (associated with Saturn), Water (associated with Mercury, the planet), Fire (associated with Mars), and Metal (associated with Venus).
Wu Xing, as defined during the Han dynasty (not Han Solo - stop it), is part of Taoism and works in a way that the each element benefits another element and opposes another one. Wood creates Fire but breaks Earth. Fire becomes Earth but melts Metal. Earth gives Metal but blocks Water. Metal makes no sense to me, but Metal becomes Water but cuts Wood. Water gives life to Wood but douses Fire.
What's funny to me is that I learned of Wu Xing through an old video game called Prince of Qin (pronounced "Chin" which became the reason behind the Western name for China). Irony is that Wu Xing wasn't well-established until the Han Dynasty, but it was a core element of the game mechanics.
As for the 12 animals, Their compatibility is based on the aspects of trine, square, and opposition. Trine, 4 years apart, are most compatible. Square, 3 years apart, are far less compatible with opposition, 6 years apart, being very incompatible.
Combine the two and you have this strange mathematical formula for determining what and who work better with what and who. (Feng shui is heavily based on Wu Xing such as you never put the sink opposite of the stove in the kitchen with the Water opposing the Fire.)
(cross posted for great justice)