Temple of heaven - The 7 star stone group (but there are 8...)

These 7 stones, put here during the Ming dynasty, are engraved with patterns of mountain peaks to symbolize the 7 Taishan Mountain.
But why 8 ? When the Qing, a Manchu dynasty, seized power, emperor Qianlong had another stone installed in the north-east to symbolize the unification of the country by indicating that Manchuria is one of the nationalities of China.