The ancient Chinese saw how the cycles of the seasons profoundly impact everything from weather to plants and crops, animals and even human mental well-being.
They surmised that to each season corresponds a specific energy that influences all life forms. They concluded that in addition to the cyclical movement of yin and yang, there must also be changing energies causing, and therefore related to, seasonal changes.
The ancient Chinese subdivided yin and yang into five phases, also called elements which they named: Wood, Fire, Earth, Metal and Water. But, why five rather than four to correlate with the four seasons?
The answer is symptomatic of Chinese thinking: How could there be any sudden shift from one seasonal energy to the next? Would such a significant change take place in the one minute before midnight on the last day of one season and immediately after, reappear as the energy of the next season?
This could not be, as it is contrary to the yin/yang principle. It led them to surmise that there must necessarily be a period of transition between seasons, when the energy of one season wanes and becomes transformed into the energy of the next season.
Since all these energies were seen by the Chinese as coming from the earth, they surmised that at the end of each season the energy goes back to the earth for its transformation before appearing as the energy of the next season.
This transition period necessarily repeated four times a year became known as the Earth season. A little arithmetic made it clear that each energetic season is approximately seventy-two days and that Earth season also occurs approximately for eighteen days four times a year (for a total 72 days out of 365).
The next "Between Seasons" Earth element period is April 18 to May 5.
This blog post has been updated April 9, 2019 for freshness and comprehensiveness.