天干 Tian Gan - Heavenly Stems

What are the 10 Heavenly Stems (TianGan 天干)?

十天干 Shi Tian Gan - The celestial stems refer in the older system to the uneven division of the five phases (24 hour/5 phases of a day), where each division covers a set of asterisms depicted with the Yang characters and the sub-division; half of each division refers to the opposite side of each phase, depicted with Yin characters, while two of the stems point to the ancient central star. 

The later version refers to ten phases, each transitioning a specific asterism along the elliptic. These asterisms as markers are known in Chinese as 天干 TianGan, commonly known as the Ten Celestial or Heavenly Stems.

Why is it called the ten stems (天干 TianGan)?

The system of the ten heavenly stems and the twelve earthly branches combined to (干支 GanZi) the sixty-unit cycle predates any verifiable Chinese history; therefore, looking at the system from China's neighbors’ perspective may shed some light to grasp the original meaning of the (八字 BāZì) eight-character system.

Before the sun (was) born, the star* (was) born, meaning that the star was visible before the sun rose. *Star = god, king, ruler, something, or someone with impact or authority.

"On Ok (ock)" means ten arrows, ten directions, ten stems as tribes, ten suns, ten days, or ten gods. Unlike in English, in Chinese, one sun means one day; hence, ten suns are ten days. In short, the ten arrows (pointers) interchangeably called stems (tribes) are used to indicate ten suns or stars.

The ten stem characters 甲 乙 丙 丁 戊 己 庚 辛 壬 癸 are used to count units in the same way as with characters 1 2 3 or A B C. Although the day and year stems are different types of units and have their own astronomical names, the ten stem characters are reused for the hour, day, month, and year, as well as other cycles.

What are these 10 characters of the heavenly stems?

Whatever led to the development of the initial characters, the ten ancient characters are nothing more than visual resemblances of ancient asterisms inside an astronomical constellation. These ten asterisms or characters are part of the four constellations (四象 Sì Xiàng), Turtle, Bird, Tiger, and Dragon, which are used to determine the season’s phases.

天干 10 Heavenly Stems
春 Spring		夏 Summer		第五季 5. Season		秋 Autumn		冬 Winter
Yang Wood	Yin Wood	Yang Fire	Yin Fire	Yang Earth	Yin Earth	Yang Metal	Yin Metal	Yang Water	Yin Water
甲 Jia	乙 Yi	丙 Bing	丁 Ding	戊 Wu	己 Ji	庚 Geng	辛 Xin	壬 Ren	癸 Gui