What is the year when sigourney weaver was nominated for best actress?
CREATE TABLE "nominees" (
  "year_ceremony" text,
  "actor_actress" text,
  "category" text,
  "film_title_used_in_nomination" text,
  "result" text
);
SELECT "year_ceremony" FROM "nominees" WHERE "category"='best actress' AND "actor_actress"='sigourney weaver';