Sky & Telescope - January 2020 - 53

LE AH TISCIONE / S&T

u Kordylewski made this isophote diagram by photoelectrically scanning negatives
obtained on March 6, 1961.
The contour lines of increasing brightness are centered
about 5° from the L5 point.

α Leo

5°
β Leo

densest at the center of
the image. To overcome
this source of error he
made four exposures
tic
lip
on every night with the
Ec
Lagrangian points at
different locations rela= Center
tive to the center of the
= Anti-solar point
photograph.
= L5 point
The negatives were
scanned with a microphotometer at the Wroclaw Observarecorded a faint glow around the L5
tory. This sensitive instrument measures point that was remarkably consistent
the density of the ﬁlms, allowing for the
with Kordylewski's observations six
accurate plotting of contour lines that
decades earlier.
delineate regions of density. DeformaThere has also been no shortage
tions of these contour lines on all of
of negative results, which led some
the photographs revealed the presence
astronomers to question the reality of
of two oval clouds near L5. Measuring
the cloud satellites. In 1991 the Japaabout 2° by 3° and separated by about
nese Hiten space probe failed to detect a
8°, the clouds were present in photosigniﬁcant increase in dust particle dengraphs of the constellation Leo taken on
sity when it passed directly through L4
March 6 but absent in photographs of
and L5. In 2010 astronomers Amanda
the same region taken two nights later
Lowry and Dwight Russell reported
when the Lagrangian point had moved
that they didn't record any reddening of
into the constellation Virgo.
the light of background stars in the L5
There has been no shortage of
region that would occur if the starlight
conﬁrming observations over the years.
had passed through even an exceedingly
In 1967 J. Wesley Simpson recorded
tenuous dust cloud.
the cloud satellites using instruments
The solution to this conundrum may
aboard NASA's Kuiper Airborne Obserbe that the Kordylewski Clouds are varivatory. Eight years later, J. R. Roach veri- able. The feeble pressure that sunlight
ﬁed their presence using data acquired
exerts will gradually sweep away any
during 16 successive lunations with the
sub-micron grains of dust at the L4 and
Orbiting Solar Observatory 6. During
L5 points, suggesting that the debris shed
the 1980s they were repeatedly captured by passing comets or ejected from lunar
on photographs taken with a battery of
impacts periodically replenishes them.
wide-ﬁeld cameras by the Polish astronIf you have access to an exceptionally
omer Maciej Winiarski from a dark site
dark observing site and modern digital
in the Carpathian Mountains.
imaging equipment, the Kordylewski
Last year a team of Hungarian
Clouds are well worth looking for. Just
astronomers reported the most conremember that you'll be straining to
vincing evidence yet. Reﬂected sunlight
record something ephemeral that may
is always polarized to some extent, so
not always be there!
by using a linear polarizing ﬁlter that
transmits only light with a particular
¢ This column marks Contributing Edidirection of oscillation attached to a
tor TOM DOBBINS' 50th article in Sky &
camera lens and CCD detector, they
Telescope in 33 years.
14

9
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12 0.0
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the narrow ﬁeld of a telescope. It would
be a classic case of being "too close to
the forest to see the trees."
A special combination of circumstances is required for such a cloud to
be observable. The Lagrangian point
of interest has to be located well above
the horizon to minimize the effects of
airglow and atmospheric extinction.
To provide a contrasting background, it
must lie clear of the Milky Way as well
as the band of the zodiacal light that
runs along the ecliptic. It would appear
brightest when nearly opposite the Sun,
but at such times the Moon - only 60°
away - is in a bright gibbous phase, so
sightings are only possible before the
Moon rises or after it sets to avoid a
bright background sky awash with scattered moonlight.
Kordylewski hoped to ﬁnd images
of cloud satellites in the thousands of
wide-angle patrol photographs taken at
the Sonneberg Observatory in Germany
over a period of three decades. Unfortunately, not a single one had recorded
the L4 or L5 points when the Moon was
below the horizon!
In October 1956 Kordylewski visited
the Skalnaté Pleso Observatory in the
Tatra mountains of Slovakia. From this
dark, remote location, he managed to
glimpse with his naked eye an exceedingly faint, diffuse patch of light about
four times larger than the full Moon
near the L5 point. It was one or two
magnitudes fainter than the notoriously difﬁcult gegenschein, the "counterglow" located directly opposite the Sun
within the band of zodiacal light that is
produced by the backscatter of sunlight
from motes of interplanetary dust. Its
changing location on successive nights
conﬁrmed that it was moving at the
same rate as the Moon.
On an expedition to nearby
Kasprowy Wierch mountain in the
late winter through early spring of
1961, Kordylewski took 11- to 13-minute photographic exposures of the L5
region using a wide-ﬁeld Leica camera
equipped with a 50-mm f/1.5 lens.
Although his ﬁlms covered an area of
25° by 37°, the optical system's vignetting imparted uneven sky fog that was

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