Cielo vs. firmamento
In Italian, while both cielo and firmamento can be translated as sky, they are not interchangeable. Cielo is the general, everyday term, whereas firmamento is a more poetic and specific word used to describe the sky filled with stars.
Cielo
A1This is the most common and versatile word for sky. It refers to the atmosphere above the Earth that we see every day, whether it's blue, grey, or cloudy. Crucially, cielo also means heaven in a religious or spiritual sense.
Oggi il cielo è di un azzurro splendente.
(Today the sky is a brilliant blue.)
Le nuvole grigie coprono tutto il cielo.
(The grey clouds cover the entire sky.)
L'aereo sparì tra le nuvole nel cielo.
(The airplane disappeared among the clouds in the sky.)
Nella tradizione cristiana, le anime buone ascendono al cielo.
(In Christian tradition, good souls ascend to heaven.)
Grazie al cielo siete tutti salvi!
(Thank heaven you are all safe!)
Dopo aver vinto, mi sembrava di toccare il cielo con un dito.
(After winning, it felt like I was touching the sky with a finger (I was on cloud nine).)
Firmamento
B2This is a more literary, poetic, and elevated term. It specifically refers to the vault of the heavens, especially the night sky perceived as a sphere dotted with stars and celestial bodies. It is not used for the everyday, daytime sky. Figuratively, it can also refer to a group of famous people, or stars, in a particular field.
In una notte limpida, il firmamento è pieno di stelle.
(On a clear night, the firmament is full of stars.)
L'astronomo studia i misteri del firmamento.
(The astronomer studies the mysteries of the firmament.)
Le costellazioni sono disegni di stelle nel firmamento.
(The constellations are patterns of stars in the celestial sphere.)
Quell'attrice è una nuova stella nel firmamento di Hollywood.
(That actress is a new star in the Hollywood firmament.)
Il poeta guardava il firmamento cercando ispirazione.
(The poet looked at the starry sky searching for inspiration.)
Summary
In short, use cielo for the general sky you see every day (blue, cloudy) and when talking about heaven. Use firmamento for a more poetic or scientific description of the night sky, emphasizing its vastness and the presence of stars.