Most tourists who visit Beijing only know about the Sky Altar. Actually there are five of them, with the Gu Palace as the focal point, Sky in the south, Earth in the north, Sun in the east and Moon in the west, the Primordial Farmer in the southwest.
The ancients believe that according to the Xian Tian hexagram, Sky belongs to the south and Earth to the North, the Sky is round and the Earth, square. The Sky Altar was designed with a round exterior in accordance with the astrological signs of the Heavenly Stars's twenty-four sections. The Earth Altar was designed according to geography and the five elements, hence its square-ish design.
Some friends ask me, why the Sky and Earth Altars are so far away from the longitude of the Forbidden City's Gu Palace. Some say that the human emperor cannot be equal with Heaven and Earth, so there is a necessity for differentiation; others say that the Yuan dynasty's imperial Palace are on the same line with the Sky and Earth Altars, and that the Forbidden City was later relocated in the direction of the White Tiger (west), while the designs of the Sky and Earth Altars remained unchanged; yet others maintain that the ancients already knew that the earth's magnetic field was misaligned by 7.5 degrees. Each has their own version of the story, but why is there no change to the latitude of the Sun and Moon Altars?
Veritably, the ancient emperors held the key concept of respecting Heaven and Earth, which is why they worship Heaven, Earth, Sun, Moon, and Primordial Farmer annually for fair weather and bountiful harvest. As such, emperors seem to spend more time on participating in grand ceremonies and tours rather than officiating.