Nor has this thy whale sunwards turned his dying head, and then gone round again, without a lesson to me.

你这大鲸呀，以行将要死的头向着太阳，倒是。

It cuts right down the middle of the visible face of the Moon, so it's half lit, with the sunward side of the Moon visible and the other side dark.

它正好把月球可见的面分成两半，所以月亮半亮半暗，朝向太阳的半可见，另半黑暗。