the fourth day

The next in the orderly series of days in which God created.

Let there be lights in the sky

God created the sun, moon, and stars by speaking a command.

Shining objects in the sky now provided light for the earth.

God created a different light to mark each portion of time from small to large, and set them to repeat continually until the end of time.

This word is used here in the sense of making something out of nothing.