The twelve Earthly Branches or Terrestrial Branches are a Chinese ordering system used throughout East Asia in various contexts, including its ancient dating system, astrological traditions, zodiac and ordinals.
This system was built from observations of the orbit of Jupiter. Chinese astronomers divided the celestial circle into 12 sections to follow the orbit of 歲星 Suìxīng (Jupiter, the Year Star). Astronomers rounded the orbit of Suixing to 12 years (from 11.86). Suixing was associated with 攝提 Shètí (η Boötis) and sometimes called Sheti.
Earthly Branches
In correlative thinking, the 12 years of the Jupiter cycle also identify the 12 months of the year, 12 animals.
Chinese seasons are based on observations of the sun and stars. Many Chinese calendrical systems have started the new year on the second new moon after the winter solstice.
Take the year of 2016 for example, 2016 can be described as 'Bing Shen', a combination of the third sign in Heavenly Stems and the ninth one in the Earthly Branches. Then the year of 2017 will be 'Ding You' - the fourth sign and the tenth one's association.