Axial Regression Line

Three generations of keepers of the Tropic of Cancer symbol tower have been there to record the shadowless spectacle at noon on the summer solstice. The new geographer is shocked: nearly a century's worth of shadow length data hides the Big Dipper array, pointing to the secret room under the tower. They pried open the floor tiles and found the diary of the tower keeper's great-grandfather - this man was actually a Qin Tianjian who avoided trouble in the late Qing Dynasty and used the sun shadow data to encrypt the routes of the great powers' ships. Tonight coincides with the arrival of the descendants of the fleet. The old tower guard lit the ancestral lamp array, and a light track trap suddenly appeared on the river: "There is no shadow on the summer solstice, but the eyes of our ancestors have always looked at you." In the morning light, the geographer took the copper key: "It turns out that the Tropic of Cancer returns the loyal souls in the blood."