18 ASTRONOMY • DECEMBER 2019
SECRET SKY
On the night of July 16/17, 2019, I watched
— for the first time in decades — a partial
lunar eclipse through an 8-inch telescope.
Usually, I have monitored the visual phenomenon with
either my naked eyes or binoculars. But the change cast
some new light onto my impressions of Earth’s shadow.Penumbral differences
I was particularly surprised by how difficult
it was to perceive Earth’s penumbra (its
lighter outer shadow) during the early stages
of this 65-percent partial eclipse. The bril-
liance of the Full Moon is formidable
through an 8-inch telescope without a filter.
And although I knew some penumbral
phases of partial eclipses go unseen, the
shadow’s smoky yellow presence made itself obvious
only about 25 minutes prior to the appearance of Earth’s
umbra (its darker inner shadow), or about 70 percent
into the penumbral phase.
While this is within the norm, I found the situation
during the shadow’s egress different. Namely, I found
it much easier to follow the penumbra’s departure
almost to the Moon’s edge. My guess is that it is easier
to follow something dark becoming lighter (because
your eye and brain can gradually accustom to the
brightening shadow) than something overpoweringly
bright becoming partially dimmed.
However, I also noticed something different in myobserving behavior: During penumbral ingress, I
focused my attention only on the section of the Moon
that was to receive the shadow — just as one would
while awaiting the first nibble out of a solar eclipse.
During egress, however, my eye did not stop comparing
the shadowed region with the opposing bright lunar
limb — so I was continually evaluating the shifting
relative brightness of the two regions. Image 2 shows
how the penumbral shading was apparent in images
through my 3-inch refractor 50 percent into the pen-
umbral phase, and other images show it earlier still.Umbral irregularities
For the first five minutes after Earth’s umbral shadow
slipped onto the lunar disk, it appeared as a small semi-
c i rc u la r d a rk pat h su r rou nded by a d a rk g r ay au re ole. Five
minutes later, the central dark patch and aureole reversed
intensities, with the core appearing brighter
than its surrounding ring. Another five min-
utes into the event, and the entire umbral
shadow was a uniform dark gray surrounded
by an equally uniform collar of lighter gray.
The reverse-Life Saver effect was a mystery,
though I believe it to be a contrast effect
between the bright highlands and the dark
maria as the shadow moved over the region.
What was most apparent as the eclipse deepened was
that the leading edge of the umbra appeared bluer than
the deeper reddish umbral section. This is typical of an
eclipse where the Danjon value (a scale that evaluates
the color of total lunar eclipses) equals 4, which occurs
when E a r t h ’s at mosphere i s cle a r of cont a m i na nt s (such
as volcanic aerosols). The blue collar, as shown in
Image 3, is caused by light passing through the clear
upper stratosphere and penetrating the ozone layer,
which absorbs red light, thus making that section of the
shadow appe a r bluer. C lose i nspec t ion a l so re ve a led t he
deepest penumbral shadow causing a yellow band to
appear beyond the blue one.A lot more was going on than you’d think.
Thoughts on a partial
lunar eclipse
- This artistic
composite image
shows the main
phases of the
July 16/17, 2019, partial
lunar eclipse (running
right to left). Here,
they are being
“uplifted” by yoga
students on the
Makgadikgadi Pans
in Botswana, Africa.
ALL PHOTOS: STEPHEN JAMES
O’MEARA - Three high-contrast
images show the
subtle effects of the
penumbra. South
is up.
The reverse-
Life Saver
effect was
a mystery.
BY STEPHEN
JAMES O’MEARA
Stephen is a globe-
trotting observer who
is always looking
for the next great
celestial event.1
2