The phrase ‘under the sun’ means ‘in the world’. Even the very latest novelty is only something revived. As the French paradox has it : ‘There is nothing new except that which has become antiquated.’
The source is Ecclesiastes, i, 9-10:
The thing that hath been, it is that which shall be; and that which is done is that which shall be done: and there is no new thing under the sun.
Is there any thing whereof it may be said, See this is new? It hath been already of old time, which was before us.