What does Genesis 1:18 mean?

As with verse 17, these words repeat information given previously. This is part of a poetic structure known as a chiasm, which arranges ideas around a central theme. This verse repeats the intended purposes of the sun, moon, and the stars as lights in the expanse of the sky. They were made to rule over the day and to rule over the night, to separate the light from the darkness.

The fact that these are said to be appointed by God, in order to "rule over" the day and night is also important. Genesis 1 is very explicit that these lights are not persons and they are not gods. They are not to be worshipped in hopes of earning their favor or avoiding their anger. These are created things, subject to the one and only God.

In other words, these things "rule" only in the sense that without their light, all productive life on the earth would come to a halt. Directly or indirectly, all life on earth has become dependent on the sun's light during the day, and the light of moon and stars at night. Even now, in the era of artificial light, the rule of sun, moon, and stars is absolute in that they mark the passing of time. We often speak of running things "by the clock" even though clocks themselves only show time passing. In the same sense, the movement of the sun, moon, and stars display the passage of time, though they themselves don't make time itself operate.

Following the same pattern as the other days of this creation "week," God recognizes His creation as "good." He approves of the sun, moon, and stars and their dominant place in the life of our planet. In doing so, He declared their glory and His own.