The importance of Chinese Numerology in Chinese astrology is just like the importance of mathematics in modern science. Mathematics is the tool and language that modern science uses to describe the motioning of objects etc, while the ancient Chinese numerology is the tool and language that Chinese astrology uses to describe human life and the relationship between humans.
In ancient China, people did not count time, day, and year using Arabian numbers of 1,2,3,¡, but used a different numeral system called "Heavenly Stems" and "Earthly Branches". The mapping process between the Five Elements and time mentioned above relies on this numeral system of "Heavenly Stems" and "Earthly Branches".
"Heavenly Stems" (Chinese: 天干; pinyin: tiān gān) include ten numerals(characters):
Jia (甲), Yi (乙), Bing (丙), Ding (丁), Wu (戊),
Ji (己), Geng (庚), Xin (辛), Ren (壬), Gui (癸).
"Earthly Branches" (Chinese: 地支; pinyin: dì zhī) include twelve numerals(characters):
By taking one character from "Heavenly Stems" as the first character and taking another character from "Earthly Branches" as the second, a two-digit "Stem-Branch" numeral system is formed. In "Stem-Branch" system, counting the next numeral is to advance both the Heavenly Stems character and the Earthly Branch character to the next one in their own sequence. Thus, the "Stem-Branch" sequence will be "Jia-Zi, Yi-Chou, Bing-Yin,etc". This forms a Sexagenary Cycle (60). For example, the current year 2008 is "Wu-Zi" year.
The Chinese Numerology of "Heavenly Stems" and "Earthly Branches" is very important in Chinese astrology, because they have close connection with the Five Elements. See the table below. The "+" mark means Yang, "-" means Yin.
After mapping the Stem-Branch expression into Five Elements, we can find out the nature and character manifested from the Stem-Branch expression. Further more, according to the creation/inhibition principles of Five Elements, we can analyze the mutual relationship between the elements. This is the fortune-telling process in Chinese Astrology.
Please refer to "Horoscope Sign Explanations" for details on Four Pillars of Destiny.