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

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