A full moon lunar eclipse, also known as a "blood moon," over Koege, Denmark, on Sept. 7. (Mads Claus Rasmussen/AFP/Getty Images)
A total lunar eclipse is coming to the Americas on Tuesday morning, visible over the entire Lower 48, Alaska and Hawaii. For 58 minutes, the moon will turn a rusty red as it slips into the Earth's shadow. The celestial spectacle will also deliver another rare scene for folks on the East Coast -- a selenelion, when both the sun and eclipsed moon are simultaneously visible.
What is a lunar eclipse?
An eclipse happens when one planetary body blocks sunlight from reaching another.
Total solar eclipses are very rare. In the United States, there was last one on April 8, 2024. During a solar eclipse, the moon blocks sunlight from reaching a narrow sliver of Earth, transforming day into night. Any given location will experience a total solar eclipse only once every 375 years on average.
Lunar eclipses, on the contrary, are much more widely visible.
Most locales experience one every 2.5 years or so. During a lunar eclipse, the entire nighttime side of Earth gets to witness the show. Lunar eclipses happen when the Earth passes between the sun and the full moon. That blocks most (emphasis on most) of the sunlight from reaching the lunar disk. But during totality, something extra special happens.
What happens during totality?
The moon produces no natural light of its own; it's illuminated by sunlight. So during a total lunar eclipse, you would think the moon would disappear into the darkness of Earth's shadow.
But a tiny bit of refracted, or bent, sunlight reaches the moon. It’s the sunlight that skims along the edges of Earth and is filtered through our atmosphere. That gives the light a reddish tinge. It’s the same reason we see sunrises and sunsets as red; when light shines through Earth’s atmosphere at an angle, shorter wavelengths are scattered away. Only the longer wavelengths, which are preferentially red, can penetrate that much of the atmosphere.
So during a lunar eclipse, the moon is actually receiving the combined light of all of Earth’s sunrises and sunsets simultaneously. (If you were on the moon, you’d see the Earth blocking the sun, making a solar eclipse. The sun’s corona, the outermost layer of its atmosphere, would be radiantly emblazoned against a jet-black backdrop of deep space -- but it would be reddened.)
What does the color represent?
The color the moon turns is one indicator of how polluted Earth's atmosphere is.
Scientists use the Danjon Scale to ascribe a zero through 4 categorical value to the shading of a lunar eclipse. Colors can vary markedly between eclipses. Eclipses that are hardly visible earn a zero, like the Dec. 9, 1992, one that followed the 1991 eruption of Mount Pinatubo in the Philippines. By contrast, a level 4 eclipse would be bright red.
When should I look up?
The partial phase of the eclipse will be relatively unremarkable; that's just the point at which the Earth's shadow first nicks the moon, and then continues to take a progressively bigger and bigger bite out of the visible disk. Partiality begins at 4:50 a.m. Eastern time, or 1:50 a.m. Pacific time. (For Hawaii, this is a late Monday night into Tuesday morning eclipse given the time zones).
Totality is when the "blood moon" effect is most dramatic. That starts at 6:04 a.m. Eastern time, or 3:04 a.m. Pacific time. It lasts 58 minutes.
For Alaska, Hawaii and the western two thirds of the Lower 48, people will be able to see the entire duration of totality. But on the East Coast, the moon will set, and the sun will rise, as totality is ongoing. That makes for something called a selenelion.
What is a selenelion?
For most major East Coast cities, sunrise Tuesday morning is around 6:25 a.m. or 6:30 a.m. That’s also when the moon will set. (But the total phase of the eclipse will be ongoing until 7:02 a.m.)
That means the moon will dip below the western horizon mid-totality. That’s a bummer, right? Exactly the contrary! For a few fleeting moments before moonset, East Coasters will be treated to an ultrarare, special planetary alignment. They’ll briefly be able to see the sun rising and the totally-eclipsed moon setting on opposite horizons. That’s what the word selenelion means.
Given the sun, Earth and moon will be at syzygy -- or in a straight line -- the moon and sun will be exactly 180 degrees apart. So technically speaking, while you shouldn’t be able to have both celestial bodies visible above the horizon simultaneously, that’s where optics play a role.
Depending on the temperature profile of the lower atmosphere, the moon might appear about 0.5 degrees higher than it should since the light is being bent upward and then back toward the observer. That will allow a five-minute window during which the moon might be visible even as the sun is cresting over the horizon.
If you want to catch the selenelion, find a high-up location that allows a clear view of both the western and eastern horizons.