What is eight-character fortune telling?

The earthly branches on the date of birth, together with the heavenly stems, will form a horoscope when a person is born. For example, this year is Geng Yin, this month is Shen Jiayue, today is Mao Yiri, and now is Xin Sinian. So, the horoscope of the child born now is: Geng Yin, Shen Jia, Yi Mao, Xin Si. These are the eight pillars.