You just referred to it.
Isa_13:10 For the stars of heaven and the constellations thereof shall not give their light: the sun shall be darkened in his going forth, and the moon shall not cause her light to shine.
If the sun stops shining, the same happens to the moon. We have been to the moon. It cannot produce light but it can reflect light from the sun. No surprise, the moon is bright only when the sun can shine upon it.
Isa_30:26 Moreover the light of the moon shall be as the light of the sun, and the light of the sun shall be sevenfold, as the light of seven days, in the day that the LORD bindeth up the breach of his people, and healeth the stroke of their wound.
Here if the suns brightness shall be sevenfold, the same goes for the brightness of the moon! That's because the moon reflects the light of the sun.