Chances are that NASA's own website would be the best. That's about as primary of a source as you would find, and would also be more up-to-date than A would be.
Well, decasyllabic just means consisting of ten syllables or phrases.
You would use a Dictionary.
Dictionaries show denotative meanings while thesauruses show synonyms. Atlases show positioning of something somewhere (such as the geographical or the anatomical atlas) and an encyclopaedia shows a detailed description of a certain part of reality or fiction.