“And to govern the day and the night, and to separate the light from the darkness; and God saw that it was good.” (Genesis 1:18, NASB)
As with verse 17, verse 18 continues to show the fulfillment of “it was so” (v 15). The governing of the day and night show the fulfillment of verse 16. Interestingly, we are brought back to day one (verse 4) where God separated the light from the darkness. In verse 18 God gives the purpose or duty of separation to the bodies He created – sun, moon and stars. Again, God establishes not only His power of creation but also His authority over all created things – the ability to enforce obligation.
As with the creation of light, the separation of the dry land and water and the creation of the vegetation God sees that the creation of the bodies of light and purpose assigned to them is good.