KentuckyFC writes "One of the puzzles of Earth's climate history is an apparent 140-million-year cycle in the climate record. Various astronomers think this can be explained by the passage of the Sun through the spiral arms of the Milky Way, which also seems to have had a period of about 140 million years. The thinking is that in regions of denser star populations, supernovas would have been more common, bathing the Earth in cosmic rays more often. These cosmic rays would then have seeded the formation of clouds that cool the planet. But in recent years, astronomers have mapped out the structure of the galaxy in much more detail. And now a pair of US astronomers have reanalyzed this climate change idea in light of the new evidence. Their conclusion is that the climate change cycle cannot possibly have coincided with the movement of the Sun through the spiral arms. So whatever caused the 140-million-year climate change cycle on Earth, it wasn't the Sun's passage through the galaxy."