Luopan The reason for the discrepancy in position of the twenty-eight lunar mansions is due to the movement of the heavens. Two thousand years ago, the mid-points of the four Great Constellations, Dragon, Bird, Tiger and Tortoise marked the four compass directions. The diviner’s plate showed xu xiu, Mansion 11, at the North point and the Bird Star at the South point. As the heavens change their position by about one degree every century, the positions of these points is no longer true. Consequently, genuine luopans which have been made for actual computation will show the lunar mansions in their true positions. http://www.derekwalters.de/text2.htm