The Circular Mound Altar
The Circular Mound Altar, which was built in 1530 and enlarged in 1740. There are two walls walls that encircle the altar. Both walls are painted red and surrounded by blue tiles. Each wall represents something different. The round inner wall represents heaven while the square outer wall represents earth.
After the Emperor signed the declaration that would be read to heaven outside the square wall, he proceeded to the terrace.
In the southeast corner, pine and cypress branches were burned in the green-tiled oven to welcome the gods from heaven. To bid farewell to the gods, human sacrifices were burned in the oven. Next to the oven is a special pit called the Pit of Hair and Blood because the hair and blood of the sacrificial victims were buried here.
In the southwest corner, a platform to hold a lantern pole can be seen. The pole was built in 1530 but its length changed several times. Originally, a golden dragon ona blue background was painted on the pole, later, the color of background was changed to red. The pole was last redecorated by Yuan Shikai, the warlord made the last sacrifice to Heaven.
The Circular Mound is comprised of three round white marble terraces. Each one is edged with a marble balustrade. The bottom marble terrace represents Hell, the middle terrace represents the Mortal World and the top terrace represents Heaven.
And a kind of ancient yardstick was used to measure the length in "Zhang" (one "Zhang" is a little less than 3.5 metres), the Upper terrace is 9 "Zhang" (30 metres) in diameter, the middle terrace is 15 "Zhang" (50 metres) in diameter and the bottom terrace 21 "Zhang" (70 metres) in diameter, and the numbers 1X9=9; 3X5=15; 3X7=21, include all the so called " Heavenly numbers" 1, 3, 5, 7, 9; and the total number of the three terraces is 45, the result of 9X5, which is in complete conformity with "the Supreme number of nine and five" in the "Book of Changes". The arrangement of putting number nine and five together was used exclusively in China by the ancient emperors, that's why it was used here.
In the center of the upper terrace is the Heaven's Heart Stone which is surrounded by concentrically arranged flag-stones. There are 9 stones in the first circle, 18 in the second, 27 in the third. It continues in this manner up to the ninth circle, which has 81 stones. The middle and bottom terraces also have 9 circles each. The total number of the marble flagstones on the surface is 3,402, and each terrace has four entrances and a flight of nine steps leading down in every direction.
The total number of the carved balustrades on these terraces is 360, which is also the multiplies of 9. It stands for the 360 degrees of circumference of heaven.
During each ceremony, the shrine of god was placed on the central Supreme Stone, which symbolized that god lived above the "nine heavens".
The highest terrace produces a curious acoustic effect. If you stand in the center of the upper terrace and speak, you will hear your voice echo back because the balustrades send the sound back to the center.
The altar has withstood the test of time for nearly 500 year and stands perfectly preserved without any cracking or sagging.