《宋书·志·卷十六》
况且祭天是极尊高的事，衹有天子一人可以举行而已，所以不是天子不可以祭天。
又郊天极尊，唯一而巳，故非天子不祀也。
Furthermore, sacrificing to heaven is an extremely majestic affair, only the emperor can conduct it, so it is not that the emperor is not allowed to sacrifice to heaven.