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skyandtelescope.com • JANUARY 2020 53
the narrow fi eld of a telescope. It would
be a classic case of being “too close to
the forest to see the trees.”
A special combination of circum-
stances 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 scat-
tered moonlight.
Kordylewski hoped to fi nd 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. Unfor-
tunately, 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 exceed-
ingly 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 notori-
ously diffi cult gegenschein, the “counter-
glow” 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
confi rmed 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-min-
ute photographic exposures of the L5
region using a wide-fi eld Leica camera
equipped with a 50-mm f/1.5 lens.
Although his fi lms covered an area of
25° by 37°, the optical system’s vignett-
ing imparted uneven sky fog that wasdensest at the center of
the image. To overcome
this source of error he
made four exposures
on every night with the
Lagrangian points at
different locations rela-
tive to the center of the
photograph.
The negatives were
scanned with a micro-
photometer at the Wroclaw Observa-
tory. This sensitive instrument measures
the density of the fi lms, allowing for the
accurate plotting of contour lines that
delineate regions of density. Deforma-
tions of these contour lines on all of
the photographs revealed the presence
of two oval clouds near L5. Measuring
about 2° by 3° and separated by about
8°, the clouds were present in photo-
graphs of the constellation Leo taken on
March 6 but absent in photographs of
the same region taken two nights later
when the Lagrangian point had moved
into the constellation Virgo.
There has been no shortage of
confi rming observations over the years.
In 1967 J. Wesley Simpson recorded
the cloud satellites using instruments
aboard NASA’s Kuiper Airborne Obser-
vatory. Eight years later, J. R. Roach veri-
fi ed their presence using data acquired
during 16 successive lunations with the
Orbiting Solar Observatory 6. During
the 1980s they were repeatedly captured
on photographs taken with a battery of
wide-fi eld cameras by the Polish astron-
omer Maciej Winiarski from a dark site
in the Carpathian Mountains.
Last year a team of Hungarian
astronomers reported the most con-
vincing evidence yet. Refl ected sunlight
is always polarized to some extent, so
by using a linear polarizing fi lter that
transmits only light with a particular
direction of oscillation attached to a
camera lens and CCD detector, they14.(^10)
1 2.^0
(^90). 0
(^8). 0
.^0
7.^0(^56). 0
.^5
5 °α Leoβ Leo= Center
= Anti-solar point
= L 5 pointEcli
pticrecorded a faint glow around the L5
point that was remarkably consistent
with Kordylewski’s observations six
decades earlier.
There has also been no shortage
of negative results, which led some
astronomers to question the reality of
the cloud satellites. In 1991 the Japa-
nese Hiten space probe failed to detect a
signifi cant increase in dust particle den-
sity when it passed directly through L4
and L5. In 2010 astronomers Amanda
Lowry and Dwight Russell reported
that they didn’t record any reddening of
the light of background stars in the L5
region that would occur if the starlight
had passed through even an exceedingly
tenuous dust cloud.
The solution to this conundrum may
be that the Kordylewski Clouds are vari-
able. The feeble pressure that sunlight
exerts will gradually sweep away any
sub-micron grains of dust at the L4 and
L5 points, suggesting that the debris shed
by passing comets or ejected from lunar
impacts periodically replenishes them.
If you have access to an exceptionally
dark observing site and modern digital
imaging equipment, the Kordylewski
Clouds are well worth looking for. Just
remember that you’ll be straining to
record something ephemeral that may
not always be there!¢This column marks Contributing Edi-
tor TOM DOBBINS’ 50th article in Sky &
Telescope in 33 years.uKordylewski made this iso-
phote diagram by photoelec-
trically scanning negatives
obtained on March 6, 1961.
The contour lines of increas-
ing brightness are centered
about 5° from the L 5 point.