When we got musing over the place of the number nine in mythology, we had to laugh. After all, we counted up nine muses, sister goddesses who preside over song and poetry, the arts and sciences. But our musing led us to Urania the muse of the heavens and astronomy…and whose name means "The heavenly." (来源：英语学习门户网站EnglishCN.com)

When you put nine plus the heavens together, you land on "cloud nine." To be on cloud nine is to experience a feeling of extreme elation. Lexicographers believe cloud nine may have developed that sense after the 9th and highest heaven in Dante’s Paradise, whose inhabitants are most blissful because they are nearest to God.

Other names for heaven include the Empyrean and welkin. Like the muses, the word empyrean has a link to Greek. Second century Alexandrian astronomer Ptolemy identified the 5th heaven—the seat of the deity—as pure elemental fire, and gave it a name derived from empuros, Greek for "fiery."

But most of us still think of heaven as more cloudlike than fiery. That’s where welkin fits in. That Middle English word for "cloud" entered is almost as old as English itself; in early Middle English, it came to mean "heaven; the celestial abode of God or the gods;" and, by the late Middle English period, also denoted "the upper atmosphere."