It was believed in ancient China that a heavenly king ruled the sky. There are 28 "star houses" which run the circumference of the sky ("Star House" is the Chinese term for "constellation"), located on the plane of the ecliptic (i.e. The plane running between the sun and Earth as it traces its orbit.) The heavenly king divided the sky into the northern, southern, eastern, and western quadrants, with each quadrant ruled over by a beast god. This belief was later adopted by ancient Japan. The 28 constellations were assigned by the ancient Chinese which run around the plane of the ecliptic in the night sky. If you think of the sky being ruled by a heavenly king, there are 28 star houses located around him. They were originally born to point out the areas above the heavenly sphere of the moon.