The Main Mark Tower is 23.26 meters high. The number is the same with the latitude of the Tropic of Cancer. On the top of Main Mark Tower, there is a "Sun Observing Hole". On the noon of the summer solstice every year, sunlight passes through the "Sun Observing Hole" to reach the small copper sheet in the Tower, and thus tourists can observe the astronomical wonder that "a pole erected casts no shadow".