Complex sentence: In April 1982, Gribbin and Plagemann published a lesser-selling book, "The Jupiter Effect Revisited", theorizing that the effect had actually taken place in 1980, despite a lack of alignment, and had triggered the eruption of Mount St. Helens.
Split into simpler sentences: In April 1982, Gribbin and Plagemann published a lesser-selling book, "The Jupiter Effect Revisited". In it they theorized that the effect had actually taken place in 1980, despite a lack of alignment, and had triggered the eruption of Mount St. Helens.