It is the earth, not heaven, that requires illumination provided by God's light. Likewise, modern temples are made to brilliantly shine so as to light their nighttime surroundings, while at the same time being furnished with opaque windows that restrict outside illumination (see Doctrine and Covenants 43:15). Thus, the temple's function is symbolically portrayed as giving light, not necessarily receiving it from elsewhere. The Bible says Solomon's Temple was constructed with "windows of narrow lights."
"The ancients said: 'Whoever builds windows in his house, makes them wide outside and narrow inside, that they should bring in the light,'" wrote the late Israeli geographer Zev Vilnay in his book "Sacred Land." "'Not so in the Temple; because there the light was within, and shone forth onto the whole world.' 'As oil gives light -- so the Temple gives light to the world.'"
And Elder John A. Widtsoe said, "Spiritual power is generated within temple walls, and sent out to bless the world ... Every home penetrated by the temple spirit enlightens, cheers, and comforts every member of the household. The peace we covet is found in such homes. Indeed, when temples are on earth, the whole world shares measurably in the issuing light; when absent, the hearts of men become heavy, as if they said, with the people of Enoch's day, 'Zion is fled'" (See Moses 7:69).