Observers of Wednesday morning's total lunar eclipse might be able to catch sight of an extremely rare cosmic sight.
On Oct. 8, Interested skywatchers should attempt to see the total eclipse of the moon and
the rising sun simultaneously. The little-used name for this effect is
called a "selenelion," a phenomenon that celestial geometry says cannot
happen.
And indeed, during a
lunar eclipse, the sun and moon are exactly 180 degrees apart in the
sky. In a perfect alignment like this (called a "syzygy"), such an
observation would seem impossible. But thanks to Earth's atmosphere, the
images of both the sun and moon are apparently lifted above the horizon
by atmospheric refraction. This allows people on Earth to see the sun
for several extra minutes before it actually has risen and the moon for
several extra minutes after it has actually set.
As a consequence of this atmospheric trick, for many localities east of
the Mississippi River, watchers will have a chance to observe this
unusual sight firsthand. Weather permitting, you could have a short
window of roughly 2 to 9 minutes (depending on your location) with the
possibility of simultaneously seeing the sun rising in the east while
the eclipsed full moon is setting in the west.
Regions of visibility
From Newfoundland, the start of the partial stages of the total eclipse begins about 30 to 45 minutes before moonset.
A growing scallop of
darkness will appear on the upper left part of the moon when it sets as
the sun is coming up. Across eastern Nova Scotia, only the lowermost
portion of the moon will be in view as it drops below the western
horizon. Farther to the west and south along the Atlantic seaboard, the
moon will rise completely immersed in the Earth's shadow.
Now you see it … now you don't?
Then again, sighting a selenelion might be problematic feat.
Twenty-five years ago, in the August 1989 issue of Sky & Telescope,
Bradley Schaefer, an astronomer who extensively studied the visibility
of the moon when it was low in the sky, noted that the full moon only
becomes visible when it is about 2 degrees up and the sun is about 2
degrees below the horizon.
So, depending on the clarity of your sky, you might have up to roughly
10 to 15 minutes before sunrise for the sky to still be dark enough, and
the moon to be high enough above any horizon haze for it to be clearly
visible. And keep in mind that this holds only for the uneclipsed
portion of the moon. You might, however, be able to mitigate the effects of a brightening sky somewhat by using binoculars or a telescope.
If the moon is totally eclipsed
prior to sunrise, you probably are going to have to scan the western
horizon with binoculars as the twilight brightens in order to still
detect some semblance of the Moon, which will somewhat resemble a very
dim and eerily illuminated mottled softball.
A peculiar moonset
People who live in those portions of the United States and Canada that
are a few hundred miles inland from the Eastern Seaboard should have a
good view of the Moon's emergence from the umbra somewhat later. The
low, partially eclipsed Moon in deep-blue twilight should offer a wide
variety of interesting scenic possibilities for both artists and
astrophotographers. From Toronto and points south through the eastern
Ohio Valley and into the Piedmont to the Florida Gulf Coast, a
peculiar-looking, waxing crescent moon with its cusps pointing downward
will appear to set beyond the western horizon.
Farther west, across the western Great Lakes and down through the Deep
South to the Gulf of Mexico, the moon will appear to be notched on its
lower right side by the shadow.
Going still farther west, the Moon will go down "full," but if the
western horizon is haze-free, assiduous observers from much of
Minnesota, western Iowa, eastern portions of Nebraska and Kansas as well
as central sections of Oklahoma and Texas might still be able to detect
a faint penumbral stain on the moon's lower right limb.
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