Genesis 1:15

(15) To give light.—This was to be henceforward the permanent arrangement for the bestowal of that which is an essential condition for all life, vegetable and animal. As day and night began on the first day, it is evident that very soon there was a concentrating mass of light and heat outside the earth, and as the expanse grew clear its effects must have become more powerful. There was daylight, then, long before the fourth day; but it was only then that the sun and moon became fully formed and constituted as they are at present, and shone regularly and clearly in the bright sky.