Caelum.

celestial (adj.)

late 14c., "pertaining to the sky or the visible heavens; pertaining to the Christian or pagan heaven," from Old French celestial "celestial, heavenly, sky-blue," from Latin caelestis"heavenly, pertaining to the sky," from caelum "heaven, sky; abode of the gods; climate," which is of uncertain origin; perhaps from PIE *kaid-slo-, perhaps from a root also found in Germanic and Baltic meaning "bright, clear" (compare Lithuanian skaidrus "shining, clear;" Old English hador, German heiter "clear, shining, cloudless," Old Norse heið "clear sky").

The Latin word is the source of the usual word for "sky" in most of the Romance languages, such as French ciel, Spanish cielo, Italian cielo. Transferred sense of "heavenly, very delightful" in English is from early 15c. Celestial Empire "China" is from 1808, translating native names.

Thank you, Online Etymology Dictionary.