《汉书·志·律历志上》
凡是春分、秋分、冬至、夏至以及立春、立夏、立秋、立冬，一定要记下天象云气的变化，是为了备用的缘故 。
凡分、至、启、闭，必书云物，为备故也。 
For the spring equinox, autumn equinox, winter solstice, summer solstice and the beginning of spring, beginning of summer, beginning of autumn, beginning of winter, the changes in celestial phenomena and clouds must be recorded, in order to be prepared.