What is a blue supermoon and how rare is the Aug. 30 event?

partially-eclipsed super blue blood moon over the golden gate bridge
The partially-eclipsed Super Blue Blood Moon makes its way past the Golden Gate Bridge as seen from the San Francisco Yacht Club in San Francisco, Calif., on Wednesday, January 31, 2018. Carlos Avila Gonzalez/San Francisco Chronicle via AP

There’s a blue supermoon in the astronomical forecast for Wednesday, Aug. 30.

Northeastern Global News interviewed Jacqueline McCleary, an assistant physics professor and observational cosmologist at Northeastern, about what makes this the year’s biggest and brightest full moon.

Why is it called a blue supermoon?

headshot of Jacqueling McCleary
Jacqueline McCleary, associate professor in the College of Science, will stay up to watch transit of Wednesday, Aug. 30 blue supermoon. Photo by Matthew Modoono/Northeastern University

A blue moon and a supermoon are two distinct phenomena; this Wednesday’s full moon is noteworthy because it will be both. Allow me to begin by professor-splaining.
A complete lunar cycle — from new moon to new moon or full moon to full moon — spans 29.5 days. If you want to impress your friends, the technical term for this cycle is a synodic month. 

Not coincidentally, our calendar months, which have 28-31 days, roughly align with these lunar cycles. So, most of the time, you will observe only one full moon per calendar month.

However, because calendar months are a few days longer than a lunar cycle, there are instances where you can see a full moon both at the beginning and at the end of the month. This second full moon is commonly referred to as a “blue moon.”  

Will it look blue?

Unfortunately, the moon does not assume a pretty, blue hue during a blue moon. Generally, blue moons look just like any other full moon.

Why is it called a blue moon?

Beats me! As far as I can tell, this usage dates to the Maine Farmer’s almanac, which followed the tropical year (winter solstice to winter solstice).

They named every full moon, and for some reason, chose “blue” to refer to the 13th full moon in a year.

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How big will it appear compared to other full moons?

The moon’s orbit is slightly elliptical, deviating from a perfect circle by about 5.5%. At the farthest point in its orbit, or apogee, the moon is 405,500 kilometers away from the Earth. At the nearest point, or perigee, the moon is 363,300 kilometers away. When the moon is at perigee, it appears about 7% larger than average and 14% larger than at apogee. A full moon occurring when the moon is near perigee is called a “supermoon.”

When is the best time to view the blue supermoon?

Though factors like latitude and daylight saving will affect the exact time, full moons generally reach their highest point in the sky (or “transit”) around midnight.

To see it, just look up!

However, the best time to see Wednesday’s supermoon is actually around moonrise, 7:03 p.m. You will more easily notice just how much larger it looks compared to objects on the horizon.

Is a blue supermoon a super rare event?

Yes. Because the moon has a relatively short orbital period, supermoons are common, occurring about four to six times a year.

Blue moons are rarer, only occurring every 2.5 to three years. 

But having both a supermoon and a blue only happens once every 10 to 20 years. If you miss Wednesday’s, or you get “weathered out” as astronomers sometimes say, you will have to wait until 2037 to see the next one! 

Fingers crossed for clear skies. 

Will you stay up to watch the moon’s transit?

Of course, I will stay up to watch it! I’m an astronomer, so I’m a night owl anyway. But even if I weren’t, seeing objects around you casting shadows under a bright moon is something special. 

Cynthia McCormick Hibbert is a Northeastern Global News reporter. Email her at c.hibbert@northeastern.edu or contact her on Twitter @HibbertCynthia.