A super-rare double blue moon is almost here. 

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On Saturday, March 31 — just ahead of Easter and a day after Passover begins — a second blue moon will rise in the evening sky. This event, more commonly known as a blue moon, is the second of its kind this year (the last blue moon was just back in January). According to NASA, this super-rare double blue moon only occurs about four times in a century.

For novice astronomers and casual stargazers, here's everything you need to know about the blue moon on Saturday.

What Is a Blue Moon?

When two full moons fall during the same calendar month, the phenomenon is popularly called a blue moon. Unlike the January blue moon, which doubled as a Super Blue Blood Moon, the blue moon on March 31 is simply a calendar quirk. Astronomers, however, have a slightly different definition of a blue moon, defining it as a the third full moon of a season that has four full moons.

When Is the Best Time to See the Blue Moon?

Although the moon will be officially full at 08:36 a.m. EST on March 31, it will be just below the horizon by then. So the blue moon will be best viewed a bit earlier, just as it sinks in the western sky before setting at 07:03 a.m EST, or much later that same day around 07:37 p.m. EST, when it's rising almost full in the east.

Despite common misconceptions, the blue moon won't appear to be blue to observers, and it won't have any unusual effects on people's behavior.

 

How to Celebrate the Blue Moon Across the Country

Bars and restaurants all across the United States are celebrating this particular blue moon, which is the last one until 2020. Olive Garden restaurants, for example, are selling commemorative glasses for $2.99 nationwide to anyone who buys a 22-ounce Blue Moon Belgian White beer on Saturday.

Meanwhile, Denver’s Blue Moon RiNo brewery is marking the occasion with a special tapping of Orange Blossom Honey Kölsch beer. The Blue Moon Beach Grill in Hags Head, North Carolina will host live music and have kitchen and bar specials, while the Blue Moon Saloon in Lafayette, Louisiana is staging a night of zydeco music from Louisiana guitarist Lil Buck Sinegal.

Are There Other Names for This Blue Moon?

Full moons that occur in March were called both Sap Moons and Worm Moons by Native Americans and early colonists in North America. The former name marks the time of the year when maple sap begins to flow, while the latter is named for the seasonal return of earthworms to the ground in North America. Today, these names are still used to refer to full moons rising during the month of March.

Why Did March Have Two Full Moons?

The moon takes 29.5 days to orbit the Earth, so every 29.5 days there's a full moon. Most months have at least 30 days, so just occasionally there can be two full moons — or two new moons — in the same month. The exception to that is February, which has 28 days, or 29 days in a leap year (that happens next in 2020). For this reason, February can never have a blue moon.

And this February, there was no full moon at all.

It follows that when there isn't a full moon in February, both January and March get two full moons each. That's exactly what happened in 2018. A second full moon in March was therefore inevitable.

When Is the Next Blue Moon?

A single blue moon occurs, on average, once every 33 months. If you go for the modern day definition of "two full moons in one month," the next blue moon will arrive on October 31, 2020 (Halloween, as an added bonus). The following blue moons will rise on August 31, 2023, and then May 31, 2026. Astronomers using the seasonal blue moon explanation, however, will tell you that the next blue moon is on May 18, 2019.

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And the next double blue moon, like this one on March 31, won't happen again until January and March of 2037.