WWW.ASTRONOMY.COM 17
views show a broken arc (or necklace) of illuminated
peaks all along the lunar limb that Saber also imaged
when the Moon was extremely thin.
The Moon in striped pajamas
The lunar blackdrop effect is different. When the Moon
is less than one day from New, an observer can see
multiple blackdrop effects
along the slender lunar crescent
under imperfect seeing condi-
tions. I first realized this on the
night of September 29, 2019,
when the 21-hour-young lunar
crescent appeared only 4 ̊ above
the western horizon.
Unlike Saber’s beads, these
dark lunar “pajama stripes”
were illusions caused primarily
by diffraction — the bending
and interfering of light waves
— which was intensified by the
blurring effects of Earth’satmosphere. Undoubtedly, the Moon’s complex inter-
play of light and shadow along the thin crescent
enhanced the phenomenon.
The Moon was only about one day from perigee (its
closest point to Earth during any orbit), which helped
make the phenomenon readily apparent to the unaided
eyes. However, the truth came forth through 8x42 bin-
oculars, which showed the blackdrop effect coming and
going depending on the seeing, especially as the
Moon sank ever closer to the horizon.
When crisp, the Moon was an unbroken smile;
under heat turbulence rising from the surrounding
desert sands, the Moon fractured into a toothy
Cheshi re-cat smi le, w it h t he da rk gaps wa x i ng a nd
waning in the unstable air.
Have you also seen this effect? If so, send your
impressions to [email protected].When the
Moon is less
than one day
from New,
an observer
can see
multiple
blackdrop
effects.
BROWSE THE “SECRET SKY” ARCHIVE AT
http://www.Astronomy.com/OMeara- The blackdrop effect
due to diffraction appears
at different locations in the
irregular spacing between
two fingers that are not
touching. The lower finger
represents the dark sky,
the top finger represents
the dark lunar terminator,
and the space between the
fingers represents the thin
lunar crescent. (The
fingers are intentionally
placed out of focus to
represent the blurring
effects due to poor
atmospheric seeing.) - Stephen Saber’s image
of a young Moon shows a
thin crescent displaying
dark areas between lunar
brightness peaks, giving
one the impression of
Bailey’s beads as seen
during a total solar
eclipse. STEPHEN SABER - As Galileo first spied
through his telescope in
1609, the cusp of the
Moon’s limb can appear
segmented with bright
spots appearing through
the darkness. This image
shows that phenomenon
beyond the southern cusp
on an approximately two-
day-old Moon. - Blackdrop effects (both
thick and thin) appear
on a 21-hour-young
Moon that stood only
4° above the western
horizon. These black
“pajama stripes” waxed
and waned with the
steadiness of Earth’s
atmosphere, becoming
more prominent under
less-than-perfect
conditions.
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