See a Selenelion at October's Lunar Eclipse
by Joe Rao on
Observers across the central and eastern United States and Canada should pay particular attention to the setting full Moon on the morning of October 8, for that morning’s lunar eclipse will still be in progress.
An interesting observation to attempt that morning would be to view the eclipsed setting 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° apart in the sky, so in a perfect alignment like this (called a “syzygy”) such an observation would seem impossible. But remember that thanks to our atmosphere, the images of both the Sun and Moon are apparently “lifted” above the horizon by atmospheric refraction. This allows us 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, there will be a chance to observe this unusual sight firsthand with October’s shadowy event; a short window of roughly 2 to 9 minutes (depending on your location) where there will be 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 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. See the table (below) on specific details for twenty-two selected U.S. and Canadian locations.
LOCAL CIRCUMSTANCES FOR THE CLOSING STAGES OF THE LUNAR ECLIPSE OF OCTOBER 8, 2014
|Location||Time Zone||Sunrise||Moonset||Moonset Mag*|
|St. John’s, NF||NDT||7:09 a.m.||7:11 a.m.||0.38|
|New York, NY||EDT||6:59||7:04||Total|
|New Orleans, LA||CDT||6:58||7:04||0.43|
|St. Louis, MO||CDT||7:03||7:11||0.33|
|Kansas City, MO||CDT||7:21||7:29||0.07|
*Moonset Mag: the fraction of the Moon’s diameter within the Earth’s umbral shadow at moonset rounded off to the nearest percent.
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 low in the sky, noted that the full Moon only becomes visible when it is about 2° up and the Sun is about 2° 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, somewhat resembling a very dim and eerily illuminated mottled softball.
A Peculiar Moonset
For those portions of the United States and Canada a few hundred miles inland from the Eastern Seaboard, the Moon’s emergence from the umbra somewhat later should be well seen. 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 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 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 if the western horizon is haze-free.