Let us look at what is said about the “first day” and the “fourth day”:

“Then God said, ‘Let there be light’; and there was light. And God saw the light, that it was good; and God divided the light from the darkness. God called the light Day, and the darkness He called Night. So the evening and the morning were the first day” (Genesis 1:3-5).

“Then God said, ‘Let there be lights in the firmament of the heavens to divide the day from the night; and let them be for signs and seasons, and for days and years; and let them be for lights in the firmament of the heavens to give light on the earth’; and it was so. Then God made TWO GREAT LIGHTS: the GREATER LIGHT to rule the day, and the LESSER LIGHT to rule the night. He made the stars also. God set them in the firmament of the heavens to give light on the earth, and to rule over the day and over the night, and to divide the light from the darkness. And God saw that it was good. So the evening and the morning were the fourth day” (verses 14-19).

There are certainly SIMILARITIES in these two passages but there are also DIFFERENCES. For example, the LIGHT on the first day caused the division between “Day and Night.” The LIGHTS on the fourth day also divided the “Day from the Night.” That similarity has caused many to think that both passages must be speaking of the SUN, yet as you say the SUN (which we assume is the GREATER LIGHT to rule the day) doesn’t seem to appear till the fourth day. As far as differences, on day one we read of only ONE LIGHT, but on day four we read of TWO LIGHTS (which we assume speak of the SUN and the MOON). Also, on day four God said, “let them be for lights IN THE FIRMAMENT OF THE HEAVENS to give light on earth,” yet on day one there was “no firmament,” for the firmament came into being on day two. “Then God said, ‘Let there be a firmament in the midst of the waters, and let it divide the waters from the waters.’ Thus God MADE THE FIRMAMENT, and divided the waters which were under the firmament from the waters which were above the firmament; and it was so. And God CALLED THE FIRMAMENT Heaven. So the evening and the morning were the second day” (verses 6-8).

Are you confused? We may not be able to understand exactly what the light was on the first day till we get to glory, but there are two prominent views. 1) Some believe it was the SUN but that the sun wasn’t “set into PLACE (the ‘firmament’…heaven) until the fourth day” and at that time it officially “ruled the day.” This SEEMS to be ruled out by the words “God MADE the two great lights” on the fourth day. 2) Others believe the LIGHT on the first day was “God Himself.”  Those who espouse this view refer us to Revelation 21:23 and 22:5, “The city had NO NEED OF THE SUN or of the moon to shine in it, for THE GLORY OF GOD ILLUMINATED IT. THE LAMB IS ITS LIGHT. ….There shall be no night there: They need no lamp nor LIGHT OF THE SUN, for THE LORD GOD GIVES THEM LIGHT.” In these verses the heavenly city, the New Jerusalem, is being described and we learn here that no “natural lights” (a lamp or the SUN) will be needed, for God’s glory shall be the LIGHT of the heavenly city. This SEEMS more probable.

I believe if God wanted us to know the answer to this question, he would have told us exactly what the LIGHT was on the first day. Let us be content to simply BELIEVE what God has said without having to UNDERSTAND it intellectually. I know that takes FAITH and some people want to “figure everything out.” Yet by FAITH we can UNDERSTAND by simply taking God at His Word! “Through FAITH we UNDERSTAND that the worlds were framed by the word of God, so that things which are seen were not made of things which do appear” (Hebrews 11:3). God said it and I believe it!  (DO)  (608.1)