《梦溪笔谈·杂志二》
要行船的人，夜间五更初刻就起来看天，见天上星月明亮皎洁，四周天际直到地面都无云气，便可出行，而到中午以前就停下来。
欲行船者，五鼓初起，视星月明洁，四际至地，皆无云气，便可行；至于巳时即止。
Boatmen get up at the beginning of the fifth watch at night to look at the sky. If the stars and moon in the sky are bright and clear and there is no cloud in the sky or on the ground, then they can go out, but they must stop before noon.