Heavenly stems are a set of characters that were used to indicate order in ancient China. Heavenly stems were already in use during the Xia Dynasty at the latest, where the first heavenly stem 甲 was named after the emperor Kong Jia (孔甲) in the late Xia.
维基解释天干：The ten Celestial or Heavenly Stems (Chinese: 天干; pinyin: tiāngān) are the elements of an ancient Chinese cyclic character numeral system: Jia (甲), Yi (乙), Bing (丙), Ding (丁), Wu (戊), Ji (己), Geng (庚), Xin (辛), Ren (壬), Gui (癸). They were used for dates as early as the Shang Dynasty, and are now used with the twelve Earthly Branches in the Sexagenary cycle. They are associated with the concepts of yin and yang and the Five Elements.
Heavenly Stems 中文Chinese 拼音Pinyin 英文English
甲 Jiǎ First
乙 Yǐ Second
丙 Bǐng Third
丁 Dīng Fourth
戊 Wù Fifth
己 Jǐ Sixth
庚 Gēng Seventh
辛 Xīn Eigth
壬 Rén Ninth
癸 Guǐ Tenth
However, these ten symbols forming the heavenly stems should be used with the related earthly branches: the heavenly stems as the main trunk and the earth branches forming the branches. The ancient Chinese used a type of device like a zipper to combine the stems and branches. One one side the heavenly stems rotated and on the other the earthly brances rotated. Together they form a cycle with the least common multiple of 60.
There is no difference between simplified and traditional characters for the heavenly stems and earthly branches.