The comparison means that it's sort of like a golden coin that someone had dropped down a sink drain on an accident, but someone dropped it down the drain in heaven and it has now landed in the sky of the Earth.
I think the author chose to make this comparison because the sun is yellow and it is sort of a golden color and it is sort of common for things to be dropped down the drain. Therefore, it is like a golden coin going down the drain of heaven and into the sky.
