"Once in a Blue Moon." You've probably heard the saying. You may even understand that it means something rare, something that doesn't happen frequently. This phrase was intriguing enough that respected composers Rodgers and Hart wrote a song about it in 1934, which has been covered many times over the years and is still a classic. "Once in a Very Blue Moon" is the title track of Nanci Griffith's third album, released in 1984, and it has also been covered by Dolly Parton, Mary Black, and other noteworthy singers. The mystery around these moons has captivated musicians, poets, writers, and artists over many years. But just what is a Blue Moon? Why is it rare—and does it really change color?

A Blue Moon is an astronomical event, and even though it's quite unusual, it can be roughly predicted. It's all a matter of math. It takes the Moon just over 29 days to go from new to full and back to new again. However, there are more than 29 days in a calendar month—in fact, the word "month" has its origins in the word "Moon." There are exceptions to every rule, and in this instance, the anomaly is February, which usually has only 28 days. The numbers alone mean that sooner or later we're going to get two Full Moons in the same month, although it will never be in February—even taking into account a leap year. The last time the Moon was full on February 29th occurred in 1972—but it was still the only Full Moon that month.

Modern Versus Traditional
There are three types of Blue Moons—and there's a lot of disagreement as to which one is the "true" Blue Moon. The first is a calendar Blue Moon as mentioned above, where there are two Full Moons in the same month. The second of these moons is called the Blue Moon. This is largely based on a misconception printed in the 1947 Maine Farmer's Almanac. In this article, James H. Pruett talked about the second Full Moon of the month being the Blue Moon, and for some reason, this seemed to gain popularity. As a result, many people today call the second of two Full Moons in a calendar month a blue one.

Looking at it this way, then it's February—being the shortest month of the year—that gets skipped if there are going to be two Full Moons in another month. That also makes it more probable that January or March will catch the extra Moon. The calendar Blue Moon can happen in any month, and the math makes this event more common in the longer, rather than the shorter, months. Calendar Blue Moons occur approximately every two and a half years, but this still doesn't guarantee that the second Full Moon in that month is going to be what is traditionally believed to be a Blue Moon.

These moons aren't always counted as being blue, because the calendar as we know it is a manmade concept. Even if they are noted, the difference in time zones means that one area of the globe might have a Blue Moon, while another one does not. Some people think that these inconsistencies make observing the seasonal Blue Moons a more viable idea.

Seasonal Blue Moons
Prior to 1947, most of the recognized Blue Moons were the seasonal ones. This happens when there are four Full Moons in the same season, and calculating when that occurs is both complicated and confusing. Not every culture or tradition keeps track of time in the same way, so determining exactly when the seasons start and end might be a bit subjective. Even now there are those who believe that the Moon cycle starts when it is full, while others mark the start of the period when the Moon is new.

When observing the seasonal Blue Moons, it's the third Moon in the quarter that is considered to be blue, not the last one. Many cultures referenced each Moon of the year by a unique name—like "harvest moon," "wolf moon," or "blood moon." To have the last Moon of the period be the Blue Moon throws that cycle out of harmony, so the first Moon keeps its customary name, as does the second and the last. It's the rogue third Moon that is considered to be blue. These deviations in the annual cycle caused problems for Catholicism, too, as their holy calendar was interrupted if this extra Moon happened during their festival of Lent.

Astrological Blue Moons
The third type of Blue Moon is astrological, and this is when there are two Full Moons occurring in one sign of the zodiac.

At first this might seem to be complicated, but once you start to work with lunar astrology, you might find that it makes the most sense—particularly if you plant your garden and do other things in sync with the phases and signs of the Moon. These calculations are not based on any man-made calendar; they're established instead by the sun's movement. It is the sun's reflected light that makes the Moon shine full. These Blue Moons only occur every three years or so—quite possibly making them the rarest of the Blue Moons we've discussed.

For those who follow lunar astrology, then this Blue Moon is thought to bring the most intensity. It's believed that the characteristics of that sign of the zodiac will get a "double dose" of the available lunar energy, with the second Moon having more clout than the first one. That goes for mundane tasks such as trimming a hedge to encourage growth, as well as for personal exploration.

Is It Really Blue?
Does the Moon really appear blue during these times? Not usually. The association between the Moon and the color blue seems to have been first recorded by King Henry VIII's advisor, Cardinal Wolsey, who wrote that his enemies would have him believe that the Moon was the color blue. The phrase is believed to have lost popularity until somewhere around the mid-1800s. It's quite probable that most people at that time didn't really understand what a Blue Moon was, they just knew it to mean something that happened infrequently, and was an exception to the norm.

Sometimes a misty aura will appear around the Moon, but that doesn't mean that it's blue. This is a weather phenomenon, happening when the cirrus clouds are high. Those thin clouds contain ice crystals, and when the frozen particles catch the Moon's light, they shine around it like a halo. Weather lore says that when you see this phenomenon, storms aren't far away—and it's usually right!

Upcoming
The Moon's phases have great influence on the planet. Our oceans ebb and flow because of the pull of the Moon; without it, we'd have less dramatic tides, although they'd still be there due to the Sun's gravitational force. Although the Sun is larger than the Moon, it's also farther away, so in this instance, the Moon is master of the tidal rhythms. The Moon also controls the Earth's natural wobble. Without the gravity of the Moon, the Earth would be much more unsteady on its axis. This would greatly affect the days and seasons, making their length and temperature completely unpredictable, which would have an impact on the planet's lifeforms. The Moon is more than just a pretty light in the sky.

While folks may disagree on just when a Blue Moon falls, it is accepted that the Moon has four main phases: new, first quarter, full, and last quarter. There are secondary phases halfway between those points, too. These are called waxing crescent, waxing gibbous, full, waning gibbous (or disseminating), and waning crescent (or balsamic). The Moon makes it easy to mark time, for on clear nights the different phases are easily visible to the naked eye.

Full moons are times of gathering together, of things coming to fruition and ideas coming into fulfillment. They are a time of action, when emotions can be fierce, and where enthusiasm can outweigh logic and reason. Intuitive or sensitive people often feel at their most powerful or vulnerable during this Moon phase. Full Moons help us to understand our shadow self, because the light of the Moon brings an awareness and allows us to see what still needs to be done. The Moon doesn't realize that it's blue—it's just going through its orbit and doing its thing; it's the importance that we put onto it that gives it the extra oomph in our lives, ceremonies, and practices.

Whether you choose to honor Blue Moons by calendar, season, or astrological sign is a matter of personal choice. Both January and March of 2018 have calendar Blue Moons on the 31st of each month. There is no Full Moon in February this year. The second Full Moon in January is in a different sign of the zodiac than the first, so it's not an astrological Blue Moon. The second March Full Moon is not in the same season or astrological sign as the others, o it also is just a calendar Blue Moon. The next astrological Blue Moon doesn't occur until May 2019, with the following one being August 2021. As infrequently as they happen, you could say they only come around once in a Blue Moon!

Excerpted from Llewellyn's 2018 Moon Sign Book.