Monday, January 12, 2009
Sometimes It Is Seen...Sometimes Not
643: Italian astronomer Giovanni Riccioli discovers a faint glow on the night side of the planet Venus. Other astronomers over the centuries since will also observe the Ashen Light, but one of the longest-running mysteries of astronomy still defies conclusive explanations.
Riccioli was an astronomer of some repute. Working in the first generation after Galileo, he discovered that Mizar (the middle star in the handle of the Big Dipper) is actually a double star — the first one known. He also discovered satellite shadows on Jupiter and published a map of our moon's surface. The names he assigned (e.g., Sea of Tranquility, Sea of Storms) are still used today.
The faint luminescence Riccioli saw 366 years ago has been seen many times since, by professionals and amateurs alike. It's also not been seen by many who were looking for it. Its apparent intermittence and the lack of a satisfactory explanation has led some to chalk it up to observer error, distortion caused by Earth's atmosphere and/or artifacts induced by telescope optics.
But, still: 366 years of similar observations? Those who've seen the Ashen Light of Venus report it looks a lot like the reflected "Earthshine" that sometimes casts a dull glow on the moon, but not even that bright. It's most easily sighted when the dusk edge of the sunlight on Venus faces Earth.