Verse 18 repeats and expands the purpose set forth in verse 16. Now, however, the purpose has been carried out.

To rule over the day and over the night: see verse 16.

And to separate the light from the darkness: see verse 4.

And God saw that it was good: see verse 4 and verse 10.

Quoted with permission from Reyburn, William D. and Fry, Euan McG. A Handbook on Genesis. (UBS Helps for Translators). New York: UBS, 1997. For this and other handbooks for translators see here .