In January of 1980, the famous American film music composer John Williams was named conductor of the Boston Pops Orchestra, and on today’s date that year led the Pops in the premiere performance of a concert overture based on his score for the John Wayne film “The Cowboys.” Now, by 1980 Williams has scored dozens of classic American films, but not all that many westerns. “The Cowboys,” from 1971, for one, and “Missouri Breaks,” a quirky 1976 western starring Jack Nicholson and Marlon Brando, for another. If both “The Cowboys” and “Missouri Breaks” are somewhat unconventional samples of the western genre, Williams’ music is in the grand tradition of the classic film scores by Jerome Moross, who composed the music for “The Big Country,” Elmer Bernstein, who wrote the score for “The Magnificent Seven,” and Jerry Goldsmith, who has done that service for a number of other classic westerns. All these composers, however, owed a collective debt to an unlikely cowboy music composer: Brooklyn-born Aaron Copland, whose “Billy the Kid” and “Rodeo” ballet scores from the 1930s and '40s helped define the symphonic equivalent of the wide-open American landscape.