Sky and Telescope - September 2015 - 52

OBSERVING

Exploring the Solar System

The Ashen Light Redivivus
Modern research breathes new life into an enduring Venusian mystery.
For centuries, telescopic observers of the planet Venus
have occasionally reported an elusive, if not illusive, glow
on the nightside of the planet. This "ashen light," as it
became known, resembles the faint Earthshine seen on
the darkened portion of a nearly new Moon (S&T: July
2014, p. 54).
The literature on the subject is confusing. First,
observations of the ashen light proper, in which the
nightside appears brighter than the dark background sky,
are often lumped with those where the nightside appears
darker than the background sky. (The latter is a contrast
eﬀect.) Moreover, with an object as diﬃcult to observe as
Venus, illusions and optical eﬀects abound: for instance,
there is our well-known proclivity to "connect the dots"
into complete ﬁgures, as well as the scattering of light
by optical systems. (Observations made with reﬂecting
telescopes are especially suspect, because of diﬀ raction
by secondary-mirror supports.)
Nevertheless, a handful of observations cannot be so
easily dismissed, as Thomas Dobbins explored not long
ago in this column's pages (S&T: March 2012, p. 50).
Assuming that a real Venusian phenomenon might be
involved, what can the physical explanation be?
In the case of the glowing dark side of the Moon,
Earth's reﬂected sunlight is the explanation, as Leonardo
da Vinci realized long ago. But Venus does not have a
satellite, and while Earth would no doubt appear brilliant
from Venus, its reﬂected light is far too feeble to produce
such an eﬀect. Many other explanations have been proposed over the years, some more ingenious than plau-

sible - among them the notions that the inhabitants of
Venus had set ﬁres to clear forests or erected festal lights
hailing new Venusian emperors!
Less far-fetched was an 1872 proposal by Pierre de
Heen suggesting that, since Venus is much closer to the
Sun, auroral activity there must have intensity "the likes
of which we can hardly imagine" and that this might
account for the ashen light. His idea won support well
into the 20th century, prompting Patrick Moore, in his
1956 classic The Planet Venus, to conclude that the theory
"seems to be the best explanation."

Credible Witnesses
At about that time, high-school students Dale Cruikshank
and Alan Binder were working as summer assistants at
Yerkes Observatory in Wisconsin. It was a dream job for
anyone interested in the planets, and they enjoyed the
opportunity to observe with the great 40-inch refractor,
sometimes at night but regularly during the daytime
when it wasn't needed for other research. Then Mercury
and Venus proved to be the most rewarding objects.
They often noticed tantalizing surface detail on
Mercury, as well as variable dusky bands on Venus,
resembling those ﬁrst photographed in the ultraviolet in
1927 by Frank Ross at Mount Wilson Observatory. The
young students routinely observed Venus at 550× and
1200× through a Schott GG-14 yellow ﬁlter to reduce the
telescope's stray (extra-focal) blue light.
On July 11, 1959, Cruikshank noted an unusual "halo"
extending beyond the planet's cusps. This very faint
In May 2012 the authors photographed a simulated Venus
through a green ﬁlter (far
left) to mimic how it should
appear telescopically - and
then photographed Venus
itself with the same camera
and ﬁlter using an 11-inch
telescope. The resulting
stack of three images (left)
reveals a subtle glow on the
planet's nightside - but it's
an artifact due to light leakage in the ﬁlter and extreme
contrast enhancement.

September 2015 sky & telescope

Sky and Telescope - September 2015

Table of Contents for the Digital Edition of Sky and Telescope - September 2015