Super Blue Blood Moon 2018: What It Is and When to See It

A so-called blood moon as seen in 2015. Patrik Stollarz/AFP/Getty Images

The full moon comes around like clockwork every 29 or so days—but each time is a little bit different. And in January, those differences will add up to a truly spectacular celestial event, a super blue blood moon, arriving on January 31. Let's pick apart that pile of adjectives to understand what's happening with this special moon, the coincidental alignment of three different types of moon cycles.

The designation "super moon" refers to what astronomers call a perigean full moon, one that occurs around when the moon is at perigee, the point in its orbit when it is closest to Earth. The super moon term itself has been in use since 1979, according to NASA, when it was coined by an astrologer, but it is met with mixed responses from astronomers. More recently, the opposite phenomenon, a full moon when the satellite is around its most distant point of orbit, has been dubbed a "micromoon," although that term has yet to really catch on.

With the boost from the moon's closeness, the satellite appears during a super moon phase just a smidge larger than it would at other times of the year. Precisely how much larger depends on how high the moon is in the sky and where it is in its orbit, but the super moon is never more than about 14 percent larger than any other moon.

The "blue" piece of the January 31's moon's title refers to a coincidence in how the lunar cycle aligns with our calendar. Because the moon waxes and wanes about every 29.5 days, typically there's only one full moon each month—but every so often, the cycles line up so that two full moons fit in the same calendar month, in which case the second is dubbed a blue moon.

That's what's happening in January, with a first full moon falling on January 1 and the second, or blue, full moon falling on January 31.

Read more: The August Solar Eclipse Left Fascinating Clues About Hidden Atmospheres and Vital Satellites

And "blood moon" is simply a gruesome nickname for what's actually a lunar eclipse, when the Earth happens to align precisely between the sun and the moon. That casts Earth's shadow on the moon (the same way the solar eclipse in August cast the moon's shadow on the Earth), dyeing the satellite a reddish tinge.

Lunar eclipses can only occur during a full moon, but they don't occur every single full moon, since the Earth's orbit around the sun and the moon's orbit around Earth have little wobbles in them that sometimes prevent a perfect alignment.

And add a perigean moon and a quirk of our calendar system to a lunar eclipse and you get a full moon that's truly something special.