John Gill Commentary Psalms 136:8

"The sun to rule by day; For his lovingkindness [endureth] for ever;" — Psalms 136:8 (ASV)

The sun to rule by day
Which is the greater light, (Genesis 1:16) ;

for his mercy [endures] for ever :
this great light, so beneficial to the earth and the inhabitants of it, which was made near six thousand years ago, still continues, and will to the end of time; enlightening the world, warming the air and earth, imparting its light and heat to all creatures on it, and influencing the earth to bring forth a variety of things for the use and delight of mankind, called "the precious things [brought] forth by the sun", (Deuteronomy 33:14) ;

an emblem of a more beneficial light, in which the rich grace and mercy of God appear, and for which we have reason to give thanks; even the sun of righteousness, the Messiah, the light of the world, the author of the light of nature, grace, and glory; and who will continue as long as the sun, and when that shall be no more, (Malachi 4:2) (Psalms 72:17) .