It was found by Charles T. Kowal at the Mount Palomar Observatory on September 14 , 1974 , after three nights ' worth of photographic plates had been taken ( September 11 through 13 ; Leda appears on all of them ) . It was discovered by Charles T. Kowal at the Mount Palomar Observatory on September 14 , 1974 , after three nights ' worth of photographic plates had been taken ( September 11 through 13 ; Leda appears on all of them ) .