Du Mu, a poet of the Tang Dynasty, wrote in the famous poem "Autumn Xi":"Silver candles and autumn lights draw cold screens, and light and small fans catch the fireflies. The night at Tianjie is as cool as water, and I lie down and watch the morning glory star Vega." At the turn of summer and autumn, in the clear night sky, the Milky Way hangs obliquely across the sky like a veil, and the Cowherd and Weaver Girls look at each other across the river. There is a small star on each side of Altair (Alpha Aquili). It is a pair of children on their shoulders. It was called the "Three Stars of the River Drum" in ancient times. The star Vega (Alpha Lyra), brighter than Cowherd, stands alone on the other side. The cross-shaped constellation Cygnus flies in the center of the Milky Way with its wings. There is a bright star on its tail feather. The Chinese name is Tianjin IV (Alpha Cygnus). Tianjin generally refers to a ferry, but the nine stars that make up the ancient China star official "Tianjin" are like a large ship, spanning the center of the Milky Way, of which the fourth is the brightest. The large triangle formed by Tianjin Four, Cowherd and Vega is close to isosceles, called the Summer Triangle, and is easy to find.
Some versions of the last sentence of Du Mu's poem read "sit and watch the morning glory star Vega", but in fact "lie down and watch" is more reasonable. Because "Autumn Xi" refers to the seventh day of the seventh month of the lunar calendar, which is about the first and middle of August in the Gregorian calendar, around the beginning of autumn solar terms, for the Central Plains of China, the summer triangle is located directly above the top of the head."Sitting and watching" or standing and watching will make people's necks sour and difficult to persist. At this time, the heat had not subsided, and people were outdoors to cool off at night. The boys and girls who had just played fireflies and were tired of playing, lying on the temporarily built cold beds and "lying and watching the morning glory and vega", thinking about the romantic and poignant fairy tale about the meeting of the cowherd and the vega girl on the seventh day. It should be a very natural and pleasant enjoyment.