Science

How to Watch the Super Blue Blood Moon

It sounds extremely exciting, but it is really only moderately interesting.

Illustration of the super blue blood moon.
Photo illustration by Slate. Photos by NASA.

During the wee hours of Wednesday morning, the night skies will host not just a blue moon, nor just a supermoon, nor just a blood moon—but a super blue blood moon. It’s a rare triad of celestial hype that’s partially justified and partially unwarranted but still extremely exciting to talk about. What does it all mean? Glad you asked!

What’s lost in this triple moniker is that the main event Wednesday is a normal lunar eclipse, aka when the Earth, sun, and moon line up such that Earth casts a shadow on the moon. The eclipse takes place in two parts—the total eclipse, during which the moon will be eclipsed by the Earth’s shadow, or the umbra, and the partial eclipse, where some sunlight still peeks through the Earth’s penumbra. In neither case does the Earth’s shadow totally blot the moon, though—in fact, the moon takes on a fearsome red hue. That’s why it’s referred to as a “blood moon,” traditionally viewed as a harbinger for death and destruction (it’s not).

The redness happens because Earth’s atmosphere is filled with particulate matter that causes certain colors in the light to scatter around. During the eclipse, in the umbra, red and orange light (with longer wavelengths) make it through the atmosphere, while blue (with a shorter wavelength) gets tossed out. In every lunar eclipse, there’s a blood moon in the totality phase. During the penumbra, the moon will probably look a little brown and dim, certainly not as red or orange as a blood moon, because even a small amount of sunlight scrubs out the red color.

A lunar eclipse occurs about twice a year, so while it’s an uncommon phenomenon, it’s not an outright rarity. It lasts for about four hours, so you have a pretty wide window for making some time to go outside and gawk at a crimson moon for a few minutes. As Phil Plait once wrote for Slate, “If you were standing on the moon during the deepest times of the eclipse, from your view you’re seeing all the sunrises and sunsets on Earth at that moment.”

A lunar eclipse can only occur during a full moon, and a full moon also happens to be a prerequisite for both a blue moon and a supermoon. A blue moon occurs when there’s two full moons in the same calendar month, a rare occasion that is the source of the phrase “once in a blue moon.” It’s the second moon that’s the blue moon—and no, it’s not actually blue.

Meanwhile, a supermoon is basically just a full moon that occurs at the same time the moon’s orbit has reached its shortest distance from Earth (the perigee). It looks slightly bigger and brighter than it usually does, hence why we call it “super.” At its best, a supermoon can look roughly 14 percent bigger and 30 percent brighter, although some supermoons (like last December) only exhibit maybe half that increase in luminosity. Three to four full moons in a year qualify as supermoons. The average person, without binoculars or other visual aids, will hardly notice a difference between an average full moon and a supermoon.

Progression of the super blue blood moon.
Photo illustration by Slate. Photos by NASA.

So the upcoming super blue blood moon is the occasion of all three of these things happening at once. The trifecta is something North America hasn’t experienced since March 31, 1866.
That’s quite rare, and even if you’re just a casual viewer of the night sky, you might want to take some time to do some moon gazing for a few minutes come Wednesday. Of course, what you’re seeing is mostly just a slightly largely lunar eclipse, but it’s still cool.

You can catch the super blue blood moon at its best if you’re awake on the West Coast before dawn, where you’ll be able to view the lunar eclipse at every one of its stages, between 3:48 a.m. and 7:12 a.m. PST. The totality of the eclipse (the blood phase) will start around 4:51 a.m. and last until 6:08 a.m. PST. Sky and Telescope has suggestions for optimal viewing times for viewers in other parts of the continent, although East Coasters will undoubtedly face a narrow window that misses the total eclipse entirely.

You can also catch NASA’s livestream of the super blue blood moon beginning at 5:30 a.m. EST or the Griffith Observatory’s livestream at 5:45 a.m. PST. The agency says the next lunar eclipse (also a supermoon, but not a blue moon) visible from North America is scheduled for Jan. 21, 2019, so if you miss it, you’ll get another shot soon.