firmament(vault of the heavens, see also: heavens; sky) noun [UK: ˈfɜː.mə.mənt] [US: ˈfɝː.mə.mənt]
welkin(the sky which appears to an observer on the Earth as a dome in which celestial bodies are visible, see also: firmament; sky) noun [UK: ˈwelkɪn] [US: ˈwelkɪn]