Simplify the following text for a Grade 7 native English-speaking person.
Original: The term Middle-earth can be also applied as a nickname of the entirety of Tolkien 's creation , instead of the more appropriate , but less known terms Arda which refers to Tolkien 's world ( including celestial bodies ) , and EÃ , which refers to the universe . This is seen also in the title of books such as The Complete Guide to Middle-earth , The Road to Middle-earth , The Atlas of Middle-earth , and in particular the series The History of Middle-earth , all of which cover areas outside of the strict geographical definition of the term Middle-earth .
Simplified: Tolkien created and worked on Middle-earth through most of his life , and it is this creation for which he is most famous .