nieboskłon is a "surface" of sky. (the lower part)-http://sjp.pwn.pl/szukaj/nieboskłon.html

widnokręg is literal perceived line of sky meating with eath/sea http://sjp.pwn.pl/slowniki/widnokrąg.html

horyzont has also figurative meanings http://sjp.pwn.pl/szukaj/horyzont.html