What Is a Blue Moon? Here’s Why This Rare Full Moon Is So Special

You’ve heard, and maybe even used, the expression “once in a blue moon.” But what is a blue moon—and what does it mean in astrology?

When do blue moons occur?

Simply put, a blue moon refers to a second full moon in a single calendar month.

What exactly is a blue moon?

At first glance that might sound ordinary. But a blue moon is much more special than you might think! This “bonus moon” means we have 13 full moons in a calendar year, rather than the typical 12 full moons.

And since full moons pull the tides and impact our moods (to name a few things), having an extra full moon can bring an extra wave of changes and surprising events.

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Why is a blue moon so rare?

Why are blue moons so rare?

Since the moon takes 29.5 days to complete a lunar cycle each calendar month it’s rare that two full moons can be squeezed into a single month. But every so often—or, to be exact, every 2.7 years—we get a month with two full moons in it!

Normally, there is only one new moon and one full moon per months—in other words, a total of 12 full moons per solar calendar year. In blue moon years, there are 13 full moons on the calendar, a “bonus moon” if you will.

This can only happen in a month that has 30, or usually 31, days. So we will never have a blue moon in February because there are only 28 or 29 days in that month.

In the Farmer’s Almanac, a blue moon means there are four full moons in a season, rather than three. Since this almanac was historically used for planting, hunting and fishing, it divides the year up by seasons.

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Blue moon explained

Is the blue moon actually blue in color?

Nope! “Blue moon” is a colloquial expression of uncertain origin that basically means “never.” (Once in a blue moon…when pigs fly…you get it.)

That said, the moon CAN actually appear blue at times due to atmospheric effects. For example, if there’s been a forest fire or volcano, it can impact the way light bends and refracts.

The smoke and ash can cause red light wavelengths to scatter, allow other colors to pass through the Earth’s atmosphere and be reflected on the surface of the moon. This can cause the moon to appear tinted with a blue, green or purple-ish hue.

However, this has nothing to do with the kind of blue moon that we’re talking about in astronomy and astrology.

One interesting note, NASA provides an explanation, but the web page devoted to the topic is in “archive” mode and hasn’t been updated in decades. You can view it here.

How is a blue moon different from a blood moon?

The blue moon is not to be confused with a “blood moon.” A blood moon happens at a lunar eclipse, when the Earth passes directly between the Sun and the moon, throwing shadows on the moon and plunging it into darkness.

As the sunlight moves through the Earth’s atmosphere it bends into short-wavelength shades of reddish-brown and orange that are reflected on the moon’s surface, resembling dried blood. Not exactly as romantic as it sounds, but that’s the science of it!

On January 31, 2018, the world experienced a full moon that was both a blue moon AND a blood moon. That night, the Leo lunar eclipse arrived as the second full moon of the month AND a supermoon—the first time all three of these events occurred simultaneously in 150 years!

Learn more about full moons here, and new moons here.


The AstroTwins

Identical twin sisters Ophira and Tali Edut, known as The AstroTwins, are the founders of Astrostyle.com and the authors of multiple bestselling astrology books. Their horoscopes reach millions here and through their resident astrologer column at ELLE Magazine.