Complex sentence: The lower the sun is to the horizon, the more of the circle we see--right at sunset, a full semicircle of the rainbow with the top of the arch 42 degrees above the horizon is visible.
Split into simpler sentences: The lower the sun is to the horizon, the more of a circle we can see. At sunset, a semicircle of the rainbow with the top of the arch 42 degrees above the horizon is visible.