[Kim Long is a Denver-based author and illustrator. He created the Moon Calendar in 1980, the first publication to show accurate depictions of the moon phases for every day of the year.]

Tonight marks that rarest of events, not just the opening of a new year and a new decade but also the heralding of those events by a blue moon.

A blue moon is defined as a full moon that occurs twice in the same calendar month. When the date for one full moon falls on or near the beginning of a calendar month, the following full moon—always about 29.5 days later—comes before the end of the month. February has only 29 days (except for leap years), so there is never a blue moon in this month. Blue moons occur approximately seven times every 19 years, an average of once every 33 months, or 37 per century.

Why are they called “blue moons”? According to research by Philip Hiscock, a professional folklorist at the Memorial University of Newfoundland, the first use of the phrase dates to the 1500s, when it was used to describe something that could never happen. Later, in the mid-1800s, atmospheric pollution from forest fires and volcanic eruptions was linked to a rare, but real event, when the Moon actually appeared to turn a shade of blue. About the middle of the 1900s, the phrase began to be used to describe the second full moon in a month—another “rare” but real event—but it didn’t catch on until the 1980s.

Blue moons have no real significance but arouse interest because they don’t appear very often. Intriguingly, those months that do have blue moons are the same periods recognized as “leap months” in two older calendar cultures. The traditional Chinese and Hindu calendars are based on a lunisolar system, balancing the cycles of both the Moon and the Sun. In order not to get out of step, these calendars must periodically adjust dates and the adjustment periods are the same months in which there are blue moons.

Blue moons may sometimes cause confusion because of differences in local time zones. A full moon that falls on the first day of the month but only a few hours after midnight, for example, will produce a second full moon in that month. But a few time zones to the west, the local time of the first full moon will actually fall on the last day of the preceding month, making the blue moon a month earlier.

Less noticeable than blue moons but equally rare are months where there are two new moons, but the rarest calendar phenomenon of all is a month when there are no full moons. This can only happen in February and the event will only happen four times in the 1900s, the last in February 1999. In the twenty-first century, this kind of full moonless month will also occur four times, in 2018, 2037, 2067, and 2094. On each of these occasions, the month before and the month after will both have blue moons, because one event cannot happen without the other.

Even though a blue moon as we now know it is not colored blue, unusual atmospheric conditions can change lunar color. In some cases, the Moon may appear to be tinted blue. A blue-colored moon or one with a greenish color is most likely seen just before sunrise or just after sunset if there is a large quantity of dust or smoke particles high in the sky. These particles can filter out the colors of longer wavelengths such as red and yellow, leaving only green and blue. In 1883, after the Krakatoa volcano in Indonesia erupted, observers around the world noted that wildly colored sunsets were common for most of the following two years. At the same time and for the same reason, the Moon appeared blue.

Vital Statistics

On average, blue moons occur:

Once every 2.7 years

Once every 33 months

Once every 33 full moons

7 times every 19 years

37 times every century