Introduction to the Pyramid of the Sun: The Pyramid of the Sun is located on the east side of the Avenue of the Dead in Teotihuacan. It was built in the 2nd century AD. It is the third largest pyramid in the world and the largest building in Teotihuacan. The square base of the Pyramid of the Sun is 225 meters by 222 meters in size, and the five-story tower is 63 meters tall. The outer wall of the volcanic stone is carved with gorgeous patterns. The Pyramid of the Sun contains many relics, such as mirrors and other handicrafts in the inner chamber, sacrificial objects, and a complex drainage system composed of stone-carved water pipes. Because at noon every spring and autumn equinox, a gradually brightening straight shadow always appears on the first floor of the west face of the Sun Pyramid, and the light change time is always 66.6 seconds. Therefore, some people call it the eternal clock.