Genesis1:16

This word usually means “to rule, to govern, to have dominion over.” But how does the sun “rule” over the day? We are accustomed to the word “ruling” having to do with the enforcement of laws and the execution of judgment. But with the sun it is different, so something else must be going on. The word used here also means “power” and points to “the authority to govern” which is usually demonstrated in obvious ways so the populous will fear and obey the authority figure. In this context the idea of “power” makes more sense. Here it is a demonstration of the sun’s power which simultaneously provides the power we need on earth to sustain life. Every living thing that I know of gets energy in one way or another from the sun.

This is the same word used of the sun earlier in this verse, but its usage here is different because the object is different; the sun and the moon act differently. We know that the moon does not have any power of its own, it only reflects the sun’s light. The moon is not a source of energy through light and heat like the sun is; it is only a source of light which it borrows from the sun. Because of that this verb cannot mean that the moon powers the night, for that would be inaccurate, but it can mean that the moon’s demonstration of power, its light, provides what is needed for the night just like the demonstration of the sun’s power provides what is needed for the day.

This is added as a side-note, as if it were no big deal, as if to say, “By the way, you know those innumerable sources of light which are outside of your reach for proper study and analysis? Well, He made those too. It was easy.” The purpose of the stars is not given in this verse because the stars do not have a unique function of their own. Their purpose was revealed in the previous verse because it is the same general purpose as the sun and moon.