A full moon occurs when we see its entire lit-up side. This occurs every 29.5 days, when the moon is directly opposite the sun relative to Earth. Jan. 31 will be our next full moon in the lunar cycle.
What's a lunar eclipse?
The moon's orbit is tilted by about 5 degrees relative to Earth's orbit. So, most of the time the moon ends up a little above or below the path Earth follows as it revolves around the sun. But twice in each lunar cycle, the moon does cross into our planet's orbital plane.
If that crossing corresponds to a full moon, the moon will pass into Earth's shadow, resulting in a total lunar eclipse. Since the moon needs to be behind Earth, relative to the sun, to become overshadowed, a lunar eclipse can happen only on a full moon.
To see the phenomenon, you need to be on the night side of Earth; this eclipse will be visible mostly in Asia, Australia, the Pacific and North America. But don't worry if you miss it: Lunar eclipses happen, on average, a couple of times a year. The next one visible in North America will occur on Jan. 21, 2019.
A blue moon that looks red
When a lunar eclipse happens, the moon appears to darken as it moves into Earth's shadow, which is called the umbra. When the moon is all the way in shadow, it doesn't go completely dark; instead, it looks red because of a process called Rayleigh scattering. The gas molecules of Earth's atmosphere scatter bluer wavelengths of light from the sun, while redder wavelengths pass straight through.
This is why we have blue skies and red sunrises and sunsets. When the sun is high in the sky, red light passes straight through to the ground while blue light is scattered in every direction, making it more likely to hit your eye when you look around. During a sunset, the angle of the sun is lower in the sky, and that red light passes directly into your eyes while the blue light is scattered away from your line of sight.
In the case of a lunar eclipse, the sunlight that makes it around Earth passes through our atmosphere and is refracted toward the moon. Blue light is filtered out, leaving the moon looking reddish during an eclipse.
On top of it all, the Jan. 31 full moon is also considered a blue moon.
There are two different definitions of blue moon. The first is any time a second full moon occurs in a single month. Because there are 29.5 days between full moons, we usually end up with only one per month. With most months being longer than 29.5 days, it occasionally works out that we have two full moons. We already had one on the first of January, and our second will be Jan. 31, making it a blue moon.
The second definition of a blue moon says it is the third full moon in a season in which there are four moons, which happens about every 2.7 years. We'll have only three this winter, so the Jan. 31 full moon won't be blue by this definition. Stargazers will need to wait until May 18, 2019, for a blue moon that fits this older, original definition.
A supersize supermoon
Finally, this will also be a supermoon.
The moon's orbit is not perfectly circular, which means its distance from Earth varies as it goes through a cycle. The closest point in its orbit is called the perigee. A full moon that happens near perigee is called a supermoon by some.
This happened with our full moon on Jan. 1, and it will happen again on Jan. 31.
Its proximity makes it seem a little bit bigger and brighter than usual, but that's the extent of its effects on Earth. The distinction is usually hard to notice unless you're looking at two pictures side by side.
There are long traditions of giving different moons names. With this being a bigger, brighter, reddish-looking blue moon, perhaps we should call it a super purple moon. The moon will not actually appear purple, nor will have it a cape — but Jan. 31 is a great time to gaze up and enjoy the night sky.
Schmoll is director of the Abrams Planetarium at Michigan State University. This article was originally published on theconversation.com.