CREATE TABLE table_203_839 (id DECIMAL, "station" TEXT, "type" TEXT, "latitude" TEXT, "longitude" TEXT, "el.  ( m ) " DECIMAL, "opened" DECIMAL, "closed" DECIMAL, "coordinates" TEXT)

when was the last recorded opening date of an observatory ?
SELECT MAX("opened") FROM table_203_839
SELECT MAX("opened") FROM "table_203_839"