What did the ancients mean by the essence of the sun and the moon? Are there any related classics?

Take the aura of heaven and earth and gather the essence of the sun and the moon. Generally, it is said in Taoist classics. Rijing and Yuehua. The so-called moonlight is the brilliance of the sun and the moon, which is the most common and cheapest, but also the most direct and useful cosmic energy. It is inexhaustible and can continuously supplement the consumption of human energy.