The strange saga of the Ashen Light of Venus.
On January 9, 1643, Giovanni Riccioli, a Jesuit priest and professor of theology at the University of Bologna, reported a telescopic observation of a faint metallic glow (golden or coppery) on the nightside of Venus. At the time, the visible side of the planet was 28 percent illuminated. Thus Venus appeared as a fat crescent and shone with dazzling brilliance. This observation marks the beginning of the strange career of Venus’s so-called Ashen Light, a phenomenon that evokes the well-known “new Moon in the old Moon’s arms,” in which Earthshine causes the Moon’s dark hemisphere to glow feebly. However, Venus is not the satellite of a bright planet, so it ought not to show such an effect.
The Ashen Light is one of the longest-lasting mysteries of solar system astronomy. Although what Riccioli saw must have been due to a scattering of light or chromatic aberration in his nonachromatic refractor, his observation served as the prototype of many to follow. In some observations, the Ashen Light appeared coppery; at other times, it was ashen or grey.
Many, but not all, visual observers of the planet have reported it. For instance, using a 9.4-inch silvered-mirror reflector on January 31, 1878, T. W. Webb, author of the 19th-century classic Celestial Objects for Common Telescopes, wrote: “After having many times looked for it in vain . . . I saw it . . . coming out at intervals rather paler and browner than the twilight sky, and equally visible when the bright crescent was hidden by a field bar.”
In more recent years, the Ashen Light has been seen by Richard Baum, Patrick Moore, Dale Cruikshank, Bill Hartmann and Tom Dobbins — all experienced visual planetary observers. On the other hand, the keen-eyed (and judicious) E. E. Barnard never succeeded in seeing it.
At one time, very little was known about Venus, and speculation about what conditions were like on the planet was rife. So — assuming the Ashen Light was real — it was easy to brainstorm possible explanations for it. The eccentric German astronomer Franz von Paula Gruithuisen, who used a 2-inch refractor, supposed the planet to be inhabited. He suggested that the Ashen Light might be due to a “general festival illumination” on the crowning of a new Venusian emperor or possibly to the burning of large stretches of jungle to produce new farmland. Geoffrey Sykes, who built the dome for the famous Clark refractor telescope at Lowell Observatory, speculated that it might be the faint reflection of light from Earth, the other planets and the stars from a perpetually frozen hemisphere of ice — obviously not likely on a planet now known to have a surface hot enough to melt lead!
More recent ideas put forward to explain the phenomenon have included Venusian auroras, lightning, emissions from oxygen atoms combining in the upper atmosphere of the planet and even perception of the infrared emission from the superheated surface. If the Ashen Light exists, it is clearly highly variable. There have been periodic surges of reports, as in 1940, 1953 and 1956-57 (during a sunspot maximum). More recently, there have been flurries of reports in 1988 and in the early 1990s. However — and this is the aspect of the phenomenon that leads us to compare it to the Loch Ness monster — these reports were all by visual observers. No convincing photograph or CCD image has ever been obtained — and, indeed, the most recent spate of reports occurred just at the dawn of the CCD era.
The authors made systematic observations of Venus’s dwindling crescent in the weeks leading up to the June 2012 transit of Venus. We observed visually in integrated light and with green, red and violet filters. At times, we had vague inklings of a feeble luminescence, which almost convinced us (though scattering of light by the Earth’s atmosphere, the colour filters or the optical system seemed more likely). On those occasions when the Ashen Light was most strongly suspected, we tried CCD imaging both with an 11-inch Schmidt-Cassegrain and with the famed 24-inch Clark refractor at Lowell Observatory. The images were combined in RegiStax and then manipulated in Photoshop to the maximum usable level. We also did comparison images of partially illuminated Styrofoam balls. Examples of our results are shown above. We can only say that we did not succeed in capturing the Ashen Light with these methods and at this observing window. Perhaps one of our readers will have better luck!
William Sheehan is, by vocation, director of a neurodevelopmental disorders program in Minnesota and, by avocation (and predilection), a student of planets, stars and galaxies. Klaus Brasch is a retired bioscientist, an avid amateur astronomer and a lecturer in the public program at Lowell Observatory, in Flagstaff, Arizona.