Answer:
I regularly change shape yet remain a sphere. I am always there but not often seen. What am I?
You find me behind the stars; or in a sixth seventh or third. It takes something round a computer and me to make pie. I am bigger than anything you can think of.
By Moon or by Sun I shall be found. Yet I am undone if there's no light around.
You use me between your head and your toes the more I work the thinner I grow.
I am commonly abbreviated to H
Who makes it, has no need of it. Who buys it, has no use for it. Who uses it, can't see it. What is it?