Complex sentence: In 1962, Slipher's monumental work, "A Photographic History of Mars (1905-1961)," was published, marking a major advance in the planetary science of Mars.
Split into simpler sentences: In 1962, Slipher's monumental work, "A Photographic History of Mars (1905-1961)," was published. The work was a major advance in the planetary science of Mars.