Huai Nan Zi
The Huainanzi (The Master of Huainan) is a compendium of knowledge dating from early in China’s Han dynasty (206 B.C.E.-220 C.E.). It was compiled under the auspices of, and probably with the active participation of, Liu An (1797-122 B.C.E.), the king of Huainan. Liu An was an influential member of the imperial family who ruled a sizable kingdom within the Han Empire. He also was known as an essayist, critic, poet, and patron of learning.
Liu An in 139 B.C.E. traveled to the Han capital at Chang’an and presented his treatise on good government, the Huainanzi, to the imperial throne.
Records tell us that Emperor Wu was “delighted” with the book, but Liu An himself failed to gain much influence at court. He returned to Huainan and in 122 was accused of harboring imperial ambitions. He committed suicide rather than face prosecution, the kingdom of Huainan was abolished, and Liu An’s extensive library and other personal possessions were confiscated by the throne. Even though Liu An died in disgrace, the Huainanzi lived on.
Source, The essential Huainanzi / Liu An, King of Huainan ; translated, edited, and selected by John S. Major
Link The Essential Huainanzi : Liu An : Free Download, Borrow, and Streaming : Internet Archive
The Huan Nan Zi contains beautiful poems that clearly describe the holistic nature of ancient Chinese thought. The following text talks of Heaven, Earth, and the eternal Dao. I will first give you the whole text so you can get a feeling for the poem. Then I break it down into two parts, the first part is about Heaven, and the second part about Earth.
Huan Nan Zi
The Dao of heaven is round
The Dao of earth is square
The square controls the darkness
The round controls the brightness
Brightness is blowing qi
Therefore, fire is the outer shadow
Darkness is absorbing containing qi
Therefore, water is the internal shadow
The blowing qi is nourishing
The absorbing qi containing qi alchemically transforms
Therefore yang is giving,
and yin is transforming
We can gather the lines concerning the Heaven and Earth to get a clear view of the meaning in this poem.
Heaven
The Dao of heaven is round
The round controls the brightness
Brightness is blowing qi
Therefore, fire is the outer shadow
The blowing qi is nourishing
Therefore yang is giving
The Heaven is represented by roundness. Roundness is a circle and this corresponds with the circulatory nature of the Heavens. The climatic patterns of the season change in cycles. The Heavens revolve in the universe.
The Dao is often represented by a circle painted with a brush. It represents the Taiji, the Supreme Ultimate of the Dao.

The brightness that is being controlled by the Heavens is the Yang qi. Brightness belongs to the sun and Fire.
The Yang qi causes transformation in the form of moving wind called blowing qi.
The blowing qi causes transformation which nourishes change, this causes Yin to grow. Yang gives transformation to yin.
The shadow is referring to the mystery of the Dao. It is the secret of the Dao that Yang Fire that is giving.
The secret is also referring to the incomprehensible and inexhaustible nature of the Dao.
In the following text from the Dao De Jing the shadow is being called the mystery.
Dao De Jing
The Way that can be walked is not the eternal Way.
The name that can be named is not the eternal name.
The nameless is the beginning of Heaven and Earth.
The named is the mother of all things.
Therefore: Free from desire you see the mystery.
Full of desire you see the manifestations.
These two have the same origin but differ in name.
That is the secret,
The secret of secrets,
The gate to all mysteries.
Earth
The Dao of earth is square
The square controls the darkness
Darkness is absorbing or containing qi
Therefore, water is the internal shadow
The absorbing qi or containing qi alchemically transforms
Therefore yin is transforming
The Earth is represented as square. This squareness is comparable to a box. What is in the box, why Yang naturally. The function of Yin Earth is to store Yang. This is called Ling Potential.
That Earth controls the darkness is another way of saying that Earth controls Yin.
Yin contains Yang qi. It is the internal mystery of the Dao.
The Yin grows and changes, therefore it is transforming.