Sky and Telescope - January 2017 - 53

DIE TER GERDES (2)

calculated and observed times of Venus'
dichotomy. In August 1793 Schröter
found that the difference amounted to
8 days. Later, the famous German lunar
mappers Wilhelm Beer and Johann
Mädler, who were always merciless in
their criticisms of Schröter's lunar work,
confirmed the discrepancy in Venus'
dichotomy and found that it averaged
6 days. Since then, every observer to
make a serious study of the planet has
confirmed the "Schröter effect."
However, the discrepancy varies.
Henry McEwen, the British Astronomical Association's longtime Mercury and
Venus Section recorder, observed that it
was a fortnight in 1927. Another leading
British amateur, M.B.B. Heath, found it
to be generally only 2 days, but sometimes as much as 6 or 7 days, based on
observations from 1927 to 1958.
In part owing to the wide success
enjoyed by Patrick Moore's The Planet
Venus, first published in 1956, the late
1950s and 1960s were the heyday of
amateur studies of Venus and the other
planets. (It was Moore, by the way, who
coined the phrase "Schröter effect.")
Back then Venus was a planet of mystery, and anything that could add to our
knowledge of it was of value.
Estimating the time of dichotomy was
thus a particularly attractive project, as
it could be pursued without advanced
equipment, and many amateurs of note,

Sunlight

Scattered
light seen
from Earth

Thin
upper
atmosphere
Day-night
terminator

p Here's how Venus looked at greatest elongation on March 20, 2012. Note the shading near the
terminator, telltale evidence for a dense atmosphere, and the fact that its disk does not appear
exactly half lit. Right: Sunlight scatters as it passes through the upper atmosphere of Venus. As
seen from Earth this scattered light is most pronounced near the planet's poles, creating a slight
brightening on the night side and causing the planet's terminator to appear slightly curved.

including John Westfall, Dale Cruikshank, William Hartmann, and Alan
Binder of the Association of Lunar and
Planetary Observers and Klaus Brasch
and Geoffrey Gaherty of the Royal
Astronomical Society of Canada, were
keen students of the Schröter effect.
(Even yours truly, as an 11-year-old
armed with a standard-issue, 60-mm
refractor, made an estimate of the
Schröter effect at Venus' eastern elongation in November 1965 and found that
dichotomy occurred a week before the
calculated time.)
Mariner 2's historic flyby of Venus
in December 1962 revealed that this
cloud-covered planet was not the kind
of world expected by many astronomers. It is indeed surrounded by a dense
atmosphere - but one that consists

p In another painting by Vogt-Vilseck, Schröter bows as he welcomes Prince Adolph (Duke of
Cambridge and Viceroy of Hannover) to Lilienthal, Germany, in September 1800. In the background
is Schröter's largest telescope, which had a 20-inch mirror and 27-foot focal length.

mostly of carbon dioxide. The resulting
extreme greenhouse effect makes the
surface hot enough to melt lead.
The Schröter effect clearly had
something to do with the Venusian
atmosphere. However, a quantitative
model was not forthcoming until 1996,
when Anthony Mallama published an
elegant investigation in the Journal of
the British Astronomical Association. He
noted that the thin atmosphere above
the cloud deck should scatter sunlight
passing through it. The longer the light's
path through this layer, the greater the
scattering. So, from Earth's perspective,
the brightness of this scattered light
increases poleward along the terminator, thus producing the apparent concavity first noted by Schröter so long ago.
Although the Schröter effect was
never more than a minor mystery and
has now been fully explained, you can
see this phenomenon firsthand in the
days leading up to January's eastern
elongation of Venus. Carefully note
whether the terminator is a curve or
a straight line, along with the instruments you used and seeing conditions.
Such estimates, involving this simplest
of simple observations, might provide
information about variability in the
planet's upper atmosphere.
Also, by noting this timing, you'll
pay tribute to the enthusiastic, almost
forgotten 18th-century observer for
whom the Schröter effect is named.
¢ WILLIAM SHEEHAN looks forward to
seeing his 32nd greatest eastern elongation of Venus from the Sun in January.

s k y a n d t e l e s c o p e .c o m

* JANUARY 2017

53


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Sky and Telescope - January 2017

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