The original eight trigrams used by the early people, known as the pre-Heaven arrangement, came from the ancient map of the Milky Way.
One Spiritual Reality – a Guide to Universal Harmony by Hua Ching Ni and Mao Shing Ni
The Pre-Heaven order of the Bagua is expressed by the eight trigrams stably positioned in a cyclic order. To distinguish the pre-Heaven order is by the top and central trigram which is Heaven or light energy. Opposite Heaven in the pre-Heaven order is Earth or heavy energy… the pre-Heaven refers to the unmanifest (Heavenly) realm. This realm is
invariable. Deep within the core of one’s spirit and in the reality of the universe, is the primordial integrity of
“pre-Heaven” and “pre-Earth” which is beyond all time.
I Ching: The Book of Changes and the Unchangeable Truth by Hua-Ching Ni
… the octagon, a symbol of spiritual protection, represents the highest spiritual power, which comes from the integration of eight natural energies: sky, earth, water, fire, thunder, rain, wind, and mountain.
The Way, the Truth and the Light by Hua-Ching Ni