April’s full moon is traditionally called the Pink Moon -- after one of spring’s earliest-blooming flowers, wild phlox -- and in 2013 it might live up to its nickname (though it probably won’t).
On Thursday at about 4 p.m. Eastern time, a tiny sliver of the moon will slip behind the shadow of the Earth. This partial lunar eclipse is projected to involve just barely more than 1.5 percent of the moon’s diameter, so not exactly a celestial spectacle. And folks in the Western Hemisphere won’t even see it.
“If we were to rank a total eclipse of the moon as a first-rate event, then what is scheduled to be seen on Thursday for those living in the Eastern Hemisphere would almost certainly fall into the third- or even fourth-rate category. In fact, it might add new meaning to the term ‘underwhelming,’” Space.com columnist Joe Rao wrote.
The partial eclipse might result in a slightly blushing moon, but don’t bet on it. It’s far more likely that the moon will merely appear gray and slightly smudgy during its close encounter with our planet’s shadow. (We're just trying to manage expectations here, folks.)
But other, more spectacular eclipses can provide a better light show. Lunar eclipses can result in a black moon that just about disappears. Other eclipses turn the moon orange, red or brown. Skywatchers use something called the Danjon scale to rate the color of an eclipse: It runs from L0 (very dark, almost invisible) to L4 (very bright copper-red or orange).
There are two main ingredients that contribute to the color of an eclipsed moon. The first involves the umbra, which is where Earth’s shadow totally blocks the sun’s light from falling on the moon. The deeper the Moon falls into the center of the umbra, the darker the eclipse will be.
The other thing that comes into play is the condition of Earth's atmosphere. Our planet’s atmosphere scatters and bends the sunlight that hits the edges of the globe (from the Moon’s perspective) and bends it. (Atmospheric conditions can also turn the moon different colors at any time, not just during eclipses -- like the bright yellow or orange "harvest moon" you sometimes see in autumn).
“That red light you see on the Moon during a lunar eclipse comes from all the sunrises and sunsets around the Earth at the time,” Sky & Telescope Editor-in-Chief Robert Naeye said in a 2010 press release.
If the air is clear along the sunrise-sunset line, the eclipse will be very bright (usually red or orange). But clouds, pollution and volcanic eruptions can darken the moon’s appearance during the eclipse. Large or frequent volcanic eruptions often result in several years of very dark red eclipses, according to NASA.
“If you were an astronaut standing on the Moon and looking up into the sky, the situation would be obvious,” Naeye said. “You would see the Sun covered up by a dark Earth that was ringed all around with a thin, brilliant band of sunset- and sunrise-colored light, bright enough to dimly light the ground at your feet.”
To get a better picture, take a gander at this painting by 20th-century French artist Lucien Rudaux:
Even if Thursday's eclipse isn't a show-stopper, it should still be a nice bright full moon, so keep watching the skies.