Early on the morning of Jan. 31, 2018, millions of onlookers will get to watch a total eclipse of the moon. And not just any total eclipse. This one's going to be a spectacle of historic proportions; the kind that happens once every, well, blue moon.
The phrase "blue moon" has multiple meanings, though in this context it has nothing to do with color. Merriam-Webster defines it as "a second full moon in a calendar month." These aren't rare per se, but they're not too common either. Full moons appear in the sky on 29.53-day intervals. In other words, a blue moon shows up once every two-and-a-half years (on average).
In 2018, the moon was already full on Jan. 1, and it will be again during the last night of the month. Anyone who looks at it then will, accordingly, be seeing a blue moon. But they'll get to observe something else, too.
As we mentioned earlier, the moon also will be undergoing a total lunar eclipse the morning of Jan. 31. This takes place when a full moon enters the umbra, which is the darkest part of Earth's shadow. In the process, it adopts an ominous new color scheme.
With the exception of the "new moon" stage, Earth's satellite receives direct sunlight during all its stages. In a total lunar eclipse, though, Earth blocks out the sun's rays.
Yet, during a total lunar eclipse, light that represents the reddish part of the color spectrum gets bent around our atmosphere. This refracted light does illuminate the moon, often giving it a rusty red color.
That's how it'll appear at the end of this month. On Jan. 31, for the first time in more than 150 years, a total lunar eclipse will take place on the night of a blue moon. According to our historical records, such a thing hasn't happened since March 31, 1866.
And get this: In addition to everything else, NASA tells us that our celestial neighbor is going to look around 14 percent bigger — and 30 percent brighter — than usual that night. That's because when Jan. 31 arrives, the moon will be approaching its perigee. This is the point in its elliptical orbit at which it is closest to Earth.
Depending on where you live, you may need to take a road trip to see this remarkable display for yourself. Fortunately, earthsky.org has posted a handy schedule that can tell you when — or if — the show will be visible in your neck of the woods. Happy moongazing.