On the fourth day, God made the sun and moon, which replaced the light source of the first day. God tells us how these lights will serve us. They serve us as signs, for seasons, and for days, nights, and years.
I want to make a note here as to something I believe. I believe that God created the sun with age. In my mind, plant life required the sun to be so. I believe the animals were grown and able to bear young. Now, this answers the question of which came first, the chicken or the egg. I use to breed chickens and hatch eggs, so in my mind, I know which came first. Remember, the plants had seeds. Adam and Eve were mature.
God, with a word, created the heavens. Unbelievers, who follow astrological charts, to me slap the creator in the face. They worship and follow the creation and not the Creator.
Beloved, worship your Creator. He knew that you would be alive at this time before He created the earth. God knew He would have to die for you before He created the world. So, Beloved of God, love Him with all your being.