Albert Barnes 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 - (Genesis 1:16). Margin, as in Hebrew, for the ruling of the day. That is, to control, as it were, the day; to determine its length—its beginning—its ending—to make it what it is.

For his mercy ... - By all the blessings of day as distinguished from night and darkness—by all that the sun in his daily course does to diffuse life, joy, peace, comfort, happiness on the earth—by all that are warmed by its beams, cheered by its light, guided in labor, guarded from dangers—do we derive an argument for the mercy of God; by all this there is laid a foundation for his praise.