What type of radiation is scattered to produce the blue color of cloud-free skies?

The answer is Solar radiation.  This is scattered to produce the blue color of cloud-free skies.  A clear cloudless day-time sky is blue because molecules in the air scatter blue light from the sun more than they scatter red light. 

Neptune is the eighth planet from the sun. it was the first planet to get its existence predicted by  mathematical calculations  before it was actually seen through a telescope on sept. 23, 1846. irregularities in the orbit of  uranus  led french astronomer alexis bouvard to suggest that the gravitational pull from another celestial body might be responsible. german astronomer johann galle then relied on subsequent calculations to   spot neptune via telescope. previously, astronomer galileo galilei sketched the planet, but he mistook it for a star due to its slow motion. in accordance with all the other planets seen in the sky, this new world was given a name from greek and roman mythology — neptune, the roman god of the sea.