Complex sentence: Charles Sheffield (25 June 1935--2 November 2002), an English-born mathematician, physicist and science-fiction author, also served as a President of the Science Fiction and Fantasy Writers of America and of the American Astronautical Society.
Split into simpler sentences: Charles Sheffield (25 June 1935--2 November 2002), was an English-born mathematician, physicist and science fiction author. He was also a President of the Science Fiction and Fantasy Writers of America and of the American Astronautical Society.