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January 31 Blue Blood Moon: Not A Supermoon!

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January 31 Blue Blood Moon: Not A Supermoon!

Many stargazers and media outlets are referring to the January 31, 2018 Blue Blood Moon as a “supermoon.” But here at the Farmers’ Almanac, we understand that to be labeled as a “supermoon,” the full Moon has to meet specific criteria, and this upcoming full Moon does not meet these criteria.

A full Moon most commonly earns the moniker of “supermoon” when its “new” or “full” phase coincides with perigee, the Moon’s closest point to Earth in its orbit. But that distance to Earth can vary. The absolute closest the Moon can come is 221,429 miles from Earth, which rarely happens.

Why The Term “Supermoon”?

We’ve heard from many people on Facebook that they’re tired of hearing the term “supermoon.” It became popular in March 2011 when the Moon’s perigee brought it to less than 221,600 miles of Earth; one of the closest full Moons in decades.

Then, someone decided that for any year, a full Moon that coincides with perigee should always be called a “supermoon.” But we don’t like to water down the term, and believe that a full Moon is truly a supermoon when the full phase and perigee occur at approximately the same time, and the distance is very close; what is usually referred to as an “extreme perigean” supermoon.

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The Last Two Supermoons

The last two full Moons — December 3, 2017, and January 1, 2018 — earned their supermoon status because the moment each Moon reached its full phase and when it was at perigee was less than 24 hours apart. And you have to consider the distance to Earth on these occasions:

December 2017
3rd: The Moon was astronomically full at 10:47 a.m. EST
4th: The Moon was at perigee at 3:53 a.m. EST
The difference in time between the full Moon and perigee was 17 hours 6 minutes.
At perigee, the Moon’s distance was 222,132 miles from Earth.

January 2018
1st: Moon was at perigee at 4:56 p.m. EST.
1st: The Moon was astronomically full at 9:24 p.m. EST.
The difference in time between perigee and the full Moon is 4 hours 28 minutes.
At perigee, the Moon’s distance was 221,560 miles from Earth.

This was considered an “extreme perigean supermoon” because the Moon was only 131 miles farther out than the closest the Moon can ever get, and perigee occurred only 4 hours and 28 minutes prior to the Moon being 100% full!

Now, let’s take a look at these same parameters for the full Moon on January 31st:

30th: Moon is at perigee at 5:05 a.m. EST.
31st: The Moon is astronomically full at 8:27 a.m. EST.
The difference in time between when perigee occurs and the Moon reaches its 100% full phase is 1 day, 3 hours, 22 minutes.
At perigee, the Moon’s distance will be 223,074 miles from Earth.

While the January 31st full Moon will be close, it won’t be as close as the prior two full Moons. In fact, the moment it reaches its 100% full phase, it will be even farther away  — at 223,816 miles from Earth!  Still, there are some who want to stretch the “supermoon” moniker out to include the January 31st full Moon as well.

Supermoon Or No, This Full Moon Will Be Totally Eclipsed!

Whatever you call it, the full Moon on January 31 brings with it a total lunar eclipse, so it will be quite a show. There’s a good chance you’ll see the Moon turn red, which often happens during lunar eclipses. This is why you often hear it called a “Blood Moon.” It will also have another funny moniker attached to it: it’s considered a “Blue Moon” because it’s the second full Moon in the month. If you have a clear sky, get out and enjoy it!

See the timetable for this lunar eclipse here.

In collaboration with Astronomer Joe Rao.

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4 comments

1 Susan Higgins { 01.26.18 at 9:17 am }

Hi David, the term “Blue Moon” was designated as the third full Moon in a season, not the fourth, but you bring up a great question!

2 David Benson { 01.24.18 at 4:56 pm }

If that’s the case, why is it the third full moon and not the fourth?

3 Susan Higgins { 01.26.18 at 9:24 am }

Hi Arlis – You bring up a great point. We do have many articles on the topic, explaining how the original “mistake” of the term ended up becoming a popular term in our lexicon, but like you said, now that it’s mainstream, it’s hard to change it. Maybe it’s not too late for the term supermoon (but it looks like it’s taken hold). Take a look at these two articles, where we explain the 55-year-old mistake: https://www.farmersalmanac.com/astronomy/2009/08/24/what-is-a-blue-moon/ and https://www.farmersalmanac.com/astronomy/2016/05/16/may-2016-blue-moon/

4 Arlis Tyner { 01.24.18 at 9:44 am }

I’m glad to read you don’t like to water down the term ‘supermoon’. I wish you felt the same way about the term ‘Blue Moon’.

A Blue Moon is the *third* full moon in a season that has *four* full moons. This occurs about every 2.5 years, according to NASA. The idea that a Blue Moon is the second full moon within a single calendar month has gained popularity in recent years because of a misinterpretation of an almanac’s original definition. Now everybody says it, and everybody reports it, but that’s not what a blue moon is.

I realize that getting people to go back to using the actual definition would be like putting toothpaste back in a tube, but from the tone of your article I hoped you knew what a real blue moon is too, but then, right at the end of your article, you make the same ‘blue moon’ mistake everyone does.

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