《礼记·月令》
于是命令太史遵守六典，执行八法；又命令太史推算日月星辰的运行，太阳所在的位置，月亮所经的地方，都要计算得丝毫不差，不得背离法度，要遵循旧章而不变。
乃命大史守典奉法，司天日月星辰之行，宿离不贷，毋失经纪，以初为常。
So he ordered the Grand Historian to abide by the Six Codes and execute the Eight Laws. He also ordered the Grand Historian to calculate the movement of the sun, moon, and stars, the location of the sun, and the place of the moon. He had to calculate them precisely, without deviating from the law, and to follow the old codes without changing them.