In less than a week, a "super blue blood moon" will grace the skies for the first time in more than 150 years. But what does that clumsy phrase really mean? Will the moon glow blue, red or even purple?
The clunky name refers to three separate lunar events that will coincide in the small hours next Wednesday. Depending on where you live, you might be able to see a supermoon, a blue moon and a lunar eclipse or "blood moon."
Newsweek spoke with NASA planetary geologist Sarah Noble about everything we need to know ahead of this astronomical treat on January 31.
What is a supermoon?
"Supermoon" is a popular term for what astronomers call a "perigee full moon." The moon orbits the Earth in an ellipse. That means it sometimes comes closer to us than usual. The moon's closest approach is called the "perigee," and when that coincides with a full moon it appears larger and brighter than average.
"In general, perigee full moons can be up to 14 percent bigger than apogee full moons, and up to 30 percent brighter," Noble explained to Newsweek. "The difference in size is very difficult for our eyes to discern, but the moon will probably be noticeably bright. That is mostly due to it being slightly closer to us, but also the moon always looks brighter in the winter."
On winter nights the Earth faces away from the center of the Milky Way and toward its bright spiral arm. The stars and moon appear clearer because we are looking out into deep space, rather than toward the galaxy's dusty, busy center.
The best time to look out for a supermoon is shortly after sunset, when the moon is low on the horizon. The moon tends to appear larger to the human eye at this point, although no one really knows why. The weather will also affect how big and bright the moon looks Wednesday, Noble noted.
What is a blue moon?
The term "blue moon" has nothing to do with color. The term refers to the second full moon in a calendar month, Noble explained. Traditionally, it signaled the third full moon in a season with four full moons.
On average, a blue moon occurs once every couple of years. This year, however, we'll be spoiled. "Interestingly, using the modern definition, we will actually have two blue moons this year, one in January and one in March, with no full moon in February," said Noble.
What is a blood moon?
"Blood moon" refers to a lunar eclipse, where the moon is bathed in a red glow.
"The sun, Earth and moon line up so perfectly that the Earth's shadow blocks the sun," Noble said. "The moon does indeed turn reddish during the full eclipse—an effect of some sunlight passing through Earth's atmosphere getting scattered and bent.
"You're basically seeing all the world's sunrises and sunsets at that moment reflected off the surface of the Moon, so it should be quite spectacular."
Where can I see the super blue blood moon?
A lunar eclipse can usually be observed by about half the globe, where nighttime coincides with the astronomical line-up, Noble explained. "For this eclipse, much of the U.S. will be racing against sunrise. Those who get up early in the west will have the opportunity to see the full eclipse, but for those of us on the east coast, we'll only see a partial eclipse—the moon will set before it reaches totality." You can find out what will happen in your city with this interactive eclipse map.
Of course, you can still get a great view of the moon even if you're not in an eclipse area. Both NASA and the Virtual Telescope Project will stream the eclipse live. NASA coverage will start at 5:30 a.m. Eastern time and end at 11:30 a.m. Eastern time.
If you don't want to get up early to watch the moon glow red, Noble won't judge. "The moon is beautiful almost every night in almost every phase, you don't have to wait for a 'supermoon' or a 'blue moon' to enjoy it," she said. "You don't even need any special equipment—just step outside and look up."
