observatorier is the neuter plural indefinite of observatorium.

observatoriumN

Kven

tähtitorniN

Norwegian Bokmål – Kvendict-nob-fkv/src/N_nobfkv.xml:72136Edit on GitHub Suggest

observatoriaN

Norsk-nordsamisk ordbokdict-nob-sme/src/N_nobsme.xml:103039Edit on GitHub Suggest

Case	Sg	Pl
Indef	observatorium	observatorier
Def	observatoriet	observatoriene