caelum

Caelum, caeli is second declension neuter. Classical Latin keeps it in the singular for the sky as a whole; the plural caeli belongs to Christian Latin, where it renders a Hebrew plural, which is why the Lord's Prayer has in caelis and Cicero would not.

The spelling is the practical trap. This corpus writes coelum roughly as often as caelum. That oe is a late-antique respelling, encouraged by a mistaken link to Greek koilos, "hollow", and it is not a classical variant, but you will meet it constantly in manuscripts and early printed editions, so read coeli as caeli without hesitating. Keep it apart from caelum, "chisel", a different word from caedo, and from caelebs, "unmarried". Family: caelestis, caelicola. French ciel, Spanish cielo, Italian cielo; English celestial and, through the church, the whole vocabulary of heaven.