Why do we say "There's nothing new under the sun"?

Meaning:

Many things/ideas that appear new aren't really new at all.

Background:

'What has been will be again,
what has been done will be done again;
there is nothing new under the sun.
--Ecclesiastes 1.9. Bible: New International Version.

According to various books of proverbs and websites, the first English language recording of this expression is in Chaucer's Knight's Tale (1386), but we could not find a reference. If somebody can point us to it, we'll gladly correct this entry!

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