"14Then God said, "Let there be lights in the sky to separate the day from the night. These lights will be used for signs, seasons, days and years. 15They will be in the sky to give light to the earth." And so it happened.
16So God made the two large lights. He made the brighter light to rule the day and the smaller light to rule the night. He also made the stars. 17God put all these in the sky to shine on the earth, 18to rule over the day and over the night, and to separate the light from the darkness. God saw that all these things were good. 19Evening passed, and morning came. This was the fourth day."
When reading about day three I always forget that there is no light at that point. The image in my head is sunny and warm. God however did not forget. He was aware that there was no light, and he was also aware that life on earth would struggle without the warmth from the sun to keep it going. Therefore he created the sun which gave us warmth, the ability to grow things and the things needed to stay alive. He also created the moon, the body just beyond our planet which creates the seasons and the tides.