This sentence comes from Guan Shanyue written by Li Bai in Tang Dynasty:
A bright moon rises from Qilian Mountain and crosses the vast sea of clouds.
The mighty wind blew Wan Li and crossed Yumen Pass.
At that time, Han soldiers pointed at the mountain road, and Tubo coveted the vast territory of Qinghai.
This is a land of wars in past dynasties, and few soldiers can survive.
The soldiers turned and looked at the border, thinking of home with longing eyes.
The soldier's wife looked at the tower and lamented when she would see her relatives far away.
"The bright moon rises from Tiantai Mountain" means that a bright moon rises from Qilian Mountain and crosses the vast sea of clouds.
In general literary works, the description of "the moon rises from the East China Sea" or "the moon rises from the East Mountain" is common, and the Tianshan Mountain in the west of China seems to be the place where the moon sets. How can you say "the bright moon rises from the mountain of heaven"? It turns out that this is from the perspective of recruiting people. Looking back at the east, you can see the bright moon rising from Tianshan Mountain. Although Tianshan Mountain is not close to the sea, it is opposite to the sea of clouds. The poet combines the vast sea of clouds and the majestic Tianshan Mountains, which seems to be more common only after crossing the sea, and it is fresh and spectacular.