As long as clouds don’t get in the way, the view should be spectacular.

Look Up at the Super Blue Blood Full Moon Jan. 31 – Here’s What You’ll See and Why

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Shannon Schmoll /The Conversation

During the early hours of January 31 , there will be a full moon, a total lunar eclipse, a blue moon and a supermoon – all at the same time. None of these things is really all that unusual by itself. What is rare is that they’re happening all together on one day.

What Makes the Moon Look Full?

Like the Earth, half the moon is illuminated by the sun at any one time. The moon orbits around the Earth and as a result we see different amounts of the lit-up side.

The phases of the moon visible from Earth are related to its revolution around our planet.

The phases of the moon visible from Earth are related to its revolution around our planet. (Orion 8/ CC BY SA 4.0 )

A full moon is when we see its entire lit-up side. This occurs every 29.5 days, when the moon is directly opposite the sun relative to the 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 the 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 .

A lunar eclipse happens when the moon is completely in the Earth’s shadow.

A lunar eclipse happens when the moon is completely in the Earth’s shadow. (Tomruen/ CC BY SA 4.0 )

If that crossing corresponds to a full moon, the moon will pass into the Earth’s shadow, resulting in a total lunar eclipse . Since the moon needs to be behind the Earth, relative to the sun, a lunar eclipse can only happen on a full moon.

To see the phenomenon, you need to be on the night side of the 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 times a year. The next one visible in North America will be on Jan. 21, 2019 .

A Blue Moon that Looks Red

When a lunar eclipse happens, the moon appears to darken as it moves into the Earth’s shadow called the umbra. When the moon is all the way in shadow it doesn’t go completely dark; instead, it looks red due to 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 instead passes directly into your eyes while the blue light is scattered away from your line of sight.

A super blood moon tinted red by scattered light.

A super blood moon tinted red by scattered light. (GSFC/ CC BY 4.0 )

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 a 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. Since there are 29.5 days between two full moons, we usually only end up with one per month. With most months longer than 29.5 days, it occasionally works out that we have two full moons. We already had one on the first of this month and our second will be Jan. 31, making it a blue moon. With this definition our next blue moon is in March, leaving February with no full moon this year.

The second definition of a blue moon states it’s the third moon in a season in which there are four moons, which happens about every 2.7 years . We’ll only have 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.

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