STATEN ISLAND, N.Y. – It only happens, well, “once in a blue moon,” so a glance toward the sky on Saturday night will yield a unique sight.
An atypical type of blue moon, a full flower moon will be observable, though it probably won’t give off the bluish color you might expect from the name. It only looks blue if there are ash or dust particles in the air, but it’ll still look cool.
So, what makes it a blue moon?
It’s all about the lunar calendar. When two full moons occur during the same calendar month, based on the timing of the lunar cycle, the second full moon is commonly described as a “blue moon” or a “monthly blue moon.” That’s the official definition of a blue moon, astronomy experts say. A full moon in May is known as a flower moon to signify the flowers that bloom during this month.
May 2019 is a little different, because there’s only one full moon appearing this month. So why is it still called a blue moon?
According to Space.com, the answer is found in the World Almanac from August 1937: If four full moons -- instead of three -- occur during one astronomical season (spring, summer, fall or winter), the third full moon of the season is called a “blue moon.”
That explanation says that the moon "usually comes full 12 times in a year, three times for each season." Occasionally, however, there will come a year when there are 13 full moons during a year, not the usual 12. "This was considered a very unfortunate circumstance," the almanac said, "and it upset the regular arrangement of church festivals. For this reason, 13 came to be considered an unlucky number."
It would stand to reason that the fourth full moon of a season would be known as the blue moon. But, when a particular season has four moons, the third is called a seasonal blue moon, so that the fourth and final one could continue to be called the late moon, according to Space.com.
And this year, the May 18 full moon happens to be the third of four full moons appearing during the spring season of 2019, so it’s a “seasonal blue moon.” This spring already had full moons on March 20 and April 19, and two more are coming, one on May 18 and one on June 17.
The Saturday moon will officially be full at 4:11 p.m. Eastern time, so it will look nearly full on Friday night, May 17, completely full on Saturday night, and nearly full on Sunday night, May 19.
Also visible in the sky will be the bright star Antares in the constellation Scorpius, and the planet Jupiter, follow the moon into the sky by early to mid-evening, according to earthsky.org.
The moon will start rising in the eastern sky over New York City at about 6:50 p.m. on Friday, about 8 p.m. on Saturday and about 9:05 p.m. on Sunday. It will look full on all three nights to the casual observer.
According to Space.com, the last time we had a blue moon based on the “four full moons in a season’’ rule was in May 2016.