What does it mean that thousands of feet suspects that the Milky Way has set for nine days?

"Flying down to thousands of feet for three days, it is suspected that the Milky Way has fallen for nine days" means that the water in the waterfall falls from a very high place, as if the Milky Way had fallen from nine days. This poem is from Two Waterfalls of Lushan Mountain written by Li Bai, a poet in the Tang Dynasty. This is a landscape poem written by Li Bai when he lived in seclusion in Lushan Mountain at about fifty years old. This poem vividly depicts the magnificent scenery of Lushan Waterfall and reflects the poet's infinite love for the great rivers and mountains of the motherland. The expanded information "shed three thousands of feet" is eloquent and forceful. The word "fly" vividly depicts the scene of the waterfall spewing out; "Straight down" not only describes the steep slope of the mountain, but also indicates the urgency of the water flow. The sky is falling and unstoppable. However, the poet was not satisfied enough, and wrote another sentence, "It is suspected that the Milky Way has set for nine days". He really wants to fall from the sky and surprise his soul. Li Bai likes to enter famous mountains and swim in the beautiful mountains and rivers of Lushan Mountain all his life, which shows the aura of the poet's name. He has a rich imagination, fantastic ideas, great momentum and unrestrained feelings. The aesthetic characteristics of his poems are the beauty of nature, frankness and freedom.