The sun is like fire and the moon like water. The fire gives out light and the water reflects it. Thus the moon's brightness is produced from the radiance of the sun, and the moon's darkness is due to (the light of) the sun being obstructed. The side that faces the sun is fully lit, and the side that is away from it is dark. The planets (as well as the moon) have the nature of water and reflect light. The light pouring forth from the sun does not always reach the moon owing to the obstruction of the earth itself—this is called 'anxu', a lunar eclipse. When (a similar effect) happens with a planet (we call it) an occultation (xingwei); when the moon passes across (the sun's path) then there is a solar eclipse (shi).