《汉书·志·律历志上》
它在天上，用来辅佐北斗的璇玑星，斟酌斗柄所指的方向，用来协调曰、月、五星七政，所以叫玉衡。
右折见矩，其在天也，佐助旋机，斟酌建指，以齐七政，故曰玉衡。
It is in the sky, used to assist the Xuange star of the Big Dipper's, to consider the direction pointed by the Big Dipper, used to coordinate the sun, the moon, the five stars and the seven stars, so it is called Yuheng.