What exactly do the Heavenly Stems refer to? What is the relationship between the Heavenly Stems and Earthly Branches and feng shui and fortune-telling?

  The Heavenly Stems are symbols used in China for calculating the calendar. The ten symbols of the Heavenly Stems are Jia (甲), Yi (乙), Bing (丙), Ding (丁), Wu (戊), Ji (己), Geng (庚), Xin (辛), Ren (壬), and Gui (癸). They embody the process of everything from sprouting to growing, flourishing, declining, and disappearing.

The original meanings of these ten characters are as follows:

Jia: Plants break through the soil and emerge.
Yi: Plants start to grow and sprout, with their branches and leaves soft, bending, and stretching.
Bing: The scorching sun shines on all things, and everything is bathed in bright sunlight.
Ding: Plants continue to grow and reach the stage of "able-bodied men" (a metaphor for a relatively mature growth stage).
Wu: Plants are luxuriant with branches and leaves.
Ji: All things rise up with a visible form that can be recorded.
Geng: It means the renewal of all things.
Xin: It means everything becomes new.
Ren: It means all things are nurtured.
Gui: It means all things sprout.

The Heavenly Stems are symbols used by the ancients to calculate years. So, how did the ancients calculate years using the Heavenly Stems?

Generally speaking, divide the year by 10. Ignore the quotient and look at the remainder. If the remainder minus 3 is a positive number, count down according to the order of the Heavenly Stems, and the resulting number is the Heavenly Stem number. If the remainder minus 3 is a negative number, add 10, and the resulting number is the Heavenly Stem number. If the remainder is 3, it corresponds to the last of the Heavenly Stems, that is, Gui.

For example, looking at the perpetual calendar, the year 2001 is the year of Xin Si. The "Xin" is calculated in this way.
2001 divided by 10 gives a quotient of 200 with a remainder of 1. 1 - 3 = -2, -2 + 10 = 8. Counting to the 8th according to the order of the Heavenly Stems, which are Jia, Yi, Bing, Ding, Wu, Ji, Geng, Xin, the 8th one is Xin.

The ten Heavenly Stems: Jia, Yi, Bing, Ding, Wu, Ji, Geng, Xin, Ren, Gui. The ancients used the Heavenly Stems to calculate years.