There's much to take in during Saturday morning's total lunar eclipse, including a rare Moon-galaxy pairing, the splendid summer Milky Way, and a chance to see your shadow reach all the way to the Moon.
When the Moon slips into Earth's shadow during Saturday morning's total lunar eclipse, the last thing you'll probably think about is turning away from the sight to look at something else.
Well, let me be that little devil on your shoulder. Can I coax you to spend a few minutes during the eclipse taking advantage of several unique opportunities? The first will still allow you to keep an eye on the Moon's progress.
If you live in the far western U.S. or Canada, the Moon passes unusually close to one of the brightest galaxies along its orbital path — NGC 4697, a 9th-magnitude elliptical 40-50 million light-years away in the constellation Virgo. Through my 10-inch scope in gibbous moonlight, the galaxy is easy to spot as an elongated hazy patch with a bright, extended nucleus.
When was the last time you saw the Moon and a distant galaxy side by side? Probably never, and you may not see it this time either, but it's worth a try. With the Moon's brilliance tamed by Earth's umbral shadow, there's a real possibility of spotting this rare pairing. Only two other galaxies of similar magnitude (and none brighter) lie along the Moon's weaving path — M74 and M96.
The Moon will likely be too low to glimpse this rare sight from the mountain states. Conditions are much better along the West Coast, where the galaxy lies 1/4° or 15′ west of the totally eclipsed Moon, and positively peachy in Hawaii. There, NGC 4697 glows 28′ or about lunar diameter away. Head down to Alice Springs, Australia and their separation increases to 1.5° at totality. That might be too expansive to call a close pair, but the galaxy should be easier to see.
Finding a galaxy lurking so close to the reddened Moon's rim might prove a great challenge, but you'll find our list of "other things to look for" is decidedly easier. When three-quarters or more of the Moon lies steeped in shadow, take a few minutes to look around. Remember how washed out the sky appeared before the eclipse? With the return of darkness, the sky's alive again with stars. Meanwhile, the landscape, once cheery with moonlight, has gone dark. The change is positively eerie and reminiscent of the temporary darkness during a total solar eclipse.
Because the eclipse happens during morning hours for observers in the western hemisphere, the summertime stars are all up, including the spectacular summer Milky Way. Take this rare shadow-time moment to appreciate its brief reappearance.
You may even want to try your eye on that nova in Sagittarius, which as of this writing, has re-brightened to magnitude +5.0, well within naked-eye range under a (surprise) moonless sky.
Another one of my favorite eclipse activities is tracing the curve of Earth's shadow over the lunar disk. Its arc reminds us we live on a sphere. Sure, we all know this in our heads, but there's nothing like the visceral sensation you get when you stare 240,000 miles out into space at Earth's curving outline.
Seen Comet Lovejoy in a dark sky lately? This steadfast friend still hovers around magnitude 6.5, making it an easy binocular object. If you have time, head over to Cassiopeia, low in the northern sky, and point your glass at 5th-magnitude Psi Cas. That shred of cotton candy 1° south of the star is the comet; telescopes will still show a tail.
We're almost done. I saved the easiest and more widely visible for last. Lunar eclipses occur when the full Moon passes directly through Earth's shadow. At the Moon's distance of 240,000 miles, the dark core or Earth's shadow or umbra subtends an angle of just 1.4°, or not quite three times the size of a Full Moon.
That's pretty tiny. Here on Earth, the planet's shadow spreads across half the sky. We see it every clear evening as a deep purplish band rising in the eastern sky after the Sun sets in the west. Likewise in the west shortly before sunrise.
On Saturday morning, whether you're watching the totally or partially eclipsed Moon in the brightening dawn sky, take note of Earth's vast shadow slowly descending toward the western horizon. Consider that you also make up a tiny part of this specter which shades the Moon 240,000 miles away. We're all in this eclipse together.
Get to know your Moon! Use the Sky & Telescope Moon Globe to plot your path from crater to valley, maria to moutain.