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.
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:
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.
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!
In collaboration with Astronomer Joe Rao.