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

blue moon full moon what does it mean

by The AstroTwins

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?

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

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.

So what about new moons? Don’t they get to be part of this blue moon cobalt coalition?

Here’s the deal: a blue moon can ONLY happen during a double header of full moons, not with new moons. When there is a second new moon in a calendar month, it’s referred to as a black moon. (More on that in a bit!)

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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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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.

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!

Can a new moon be a blue moon? Introducing…the black moon!

Simply put, no — a month with two new moons does NOT call the second moon a blue moon.

However, there IS a name for this phenomenon! The second new moon in a single calendar month goes by the term “black moon.”

A black moon can occur in one of three situations:
1. There are 2 new moons in a single (solar) calendar month — such as April 2022 having a new moon on both April 1 and April 30

2. A February occurs with NO new moons, leaving January or March to pick up the slack and host a black moon. Why does this happen? Well, a lunar month (a complete cycle of the eight moon phases) is 29.5 days long, and February is only 28-29 days long. So that means a new moon can get skipped.

3. An astronomical season, which is three months long, has four new moons instead of the usual three.

The lunisolar calendar: a lunar vs. a solar month

lunisolar calendar is a calendar in many cultures whose date indicates both the moon phase and the time of the solar year.

In a lunisolar calendar, the month begins at the new moon and is 29.5 days long. That creates a 354-day year, which is 11 days shorter than a 365-day year in the Gregorian or solar calendar. Various cultures deal with this differently, some adding a 13th month periodically, others having long and short months.

This 28-to-31-day month accounts for the discrepancy of blue moons and black moons in Gregorian calendar, which has been used by Western cultures since the late 16th century.


Photo by Jason Blackeye on Unsplash