If the weather cooperates, Triangle residents getting ready for school or work Wednesday morning may see a reddish full moon, the tell-tale sign of a full lunar eclipse.
And those who pay a little more attention, and happen to have a really good view of the eastern and western horizons, may see something more unusual: Near the end of the full eclipse, the sun and moon will be visible at the same time, a phenomenon known as a “selenelion.”
A total lunar eclipse occurs when the Earth comes directly between a full moon and the sun. When that happens, the moon isn’t totally obscured; instead, light from the sun reaches the moon indirectly, by being bent as it passes through the Earth’s atmosphere.
The moon appears red because that’s the wavelength of light that gets through, said Patrick Treuthardt, assistant director of the astronomy lab at the N.C. Museum of Natural Sciences in Raleigh. It’s the same reason that sunsets often appear red, Treuthardt said.
On Wednesday morning, the partial lunar eclipse will begin about 5:15 a.m., as the curved shadow of the Earth begins to move across the moon’s surface. The totally eclipse will begin about 6:25 a.m., and unlike a meteor shower or other astronomical phenomenon, you won’t need to be out in the country where it’s dark to see the eclipse, low on the western horizon, Treuthardt said.
“As long as you can see the moon, you should be able to see it,” he said.
The total lunar eclipse will end about an hour after it begins, several minutes after sunrise. It’s that bit of timing that produces the selenelion here, as the sun rises and the moon sets.
A total lunar eclipse only occurs when the sun and moon are on opposite sides of the Earth, which means they shouldn’t be visible at the same time. But the refraction or bending of light that passes through the Earth’s atmosphere causes the sun and moon to appear higher in the sky than they actually are – an optical illusion of sorts.
The selenelion is not going to be easy to see, says Amy Sayle, an educator at the Morehead Planetarium and Science Center in Chapel Hill. As Sayle prepared to write a blog post about the lunar eclipse, she wondered whether to even mention the selenelion, since the moon and sun will be so close to the horizon that almost no one will see them at the same time.
“It’s going to be really tough around here because of all the trees,” she said. “You really need a true horizon to the east and the west, which is just really hard to pull off here.”
But Sayle is excited about the lunar eclipse, coming as it does at a time when many people will be awake. Total lunar eclipses don’t happen frequently – the next one won’t happen until April 4 – and often they happen in the middle of the night or behind the clouds. So far, the forecast for Wednesday morning looks promising.