On January 9, 1643, the Italian astronomer Giovanni Battista Riccioli pointed his telescope at Venus and noticed a faint glow on the planet's unlit side. Riccioli figured it was an artifact of his glass — the same trick that makes a prism throw rainbows. Almost four centuries later, astronomers are still arguing about what he saw. Wikipedia's entry on the persistent astronomical puzzle of ashen light traces sightings from Riccioli through Sir William Herschel, Sir Patrick Moore, and a string of 20th-century observers. The German priest Athanasius Kircher may still have noticed it five years earlier, during a 1638 visit to Sicily. The phenomenon got its name in the late 1800s, when astronomers compared it to earthshine — the dim ash-colored light reflected from Earth onto the dark side of the Moon. Theories have come and gone. Harold Urey suggested in 1957 that ions whipped up by solar ultraviolet light were doing it. A 1969 paper proposed aurorae.…