"The moon is not a god, but a great rock, and the sun is a hot rock." _Anaxagorus 500-428 b.c.e.
The idea that the sun is a disk made of gold or bronze was a common notion, because that is what it looks like. The sun has many vital attributes: the fact that it produces heat is evident, and it was thought to be a fire, or on fire. These explanations follow directly from human sense information, and for most of human history, this was good enough.
The sun was an obvious candidate for the most important object in man's environment: it moved across the sky, defining east and west, day and night and supported life. An explanation for this motion was wanted. If you were a people that used horses, chariots, and carts for travel the answer seemed obvious: the sun was carried by one of these conveyances. After gods were invented, the sun became a god who drove the chariot. This thought process, although leading to incorrect magical conclusions, would eventually lead to rational problem solving. The spark that was missing was an individual or group for whom a magical explanation was not good enough.