Firmament russian

• не́бо

  sky

  heaven, firmament

  air, airspace

• небосво́д

  firmament, the vault of heaved, dome of the sky

• твердь

  firmament, vault of heaven

  dry land, solid ground, earth

  fortress, stronghold

Examples

• В древних книгах небесная твердь казалась куполом

  In ancient books, the firmament appeared to be a dome.