What is a blue moon? Everything to know before the upcoming celestial event

You’ve probably heard the phrase “once in a blue moon.” Maybe you’ve even used it to describe an occasional occurrence in your life — the frequency with which you buy a new cellphone, for example.

But this weekend will bear witness to a true blue moon: the second full moon to appear in the sky in a single month.

The celestial event comes around seven times every 19 years or roughly every two to three years, according to EarthSky.

What is a blue moon?

The moon cycle has eight lunar phases, and it lasts 29.5 days — just shy of a typical month’s length. As a result of this mismatch, the occasional month will host two full moons. The second of these moons is called a blue moon — a calendrical blue moon, to be precise.

Sometimes a single year can even feature two blue moons. In 2018, January and March both had two full moons, while February lacked one. The next time this phenomenon will happen is in 2037, according to The Old Farmer’s Almanac.

Another type of blue moon is a seasonal blue moon. This occurs when four full moons, rather than the usual three, appear within a three-month season. Within the lineup, the third one is called the blue moon. The next seasonal blue moon will rise on May 20, 2027.

Is the moon actually blue?

Despite its colorful name, Earth’s satellite will appear its typical silvery white or golden hue.

The phrase “the moon is blue” dates back to the 1500s and was used to describe something thought to be impossible, which is where the term “blue moon” comes from, according to Brittanica.

However, the 1937 Maine Farmer’s Almanac was where the phrase was first documented, and it stuck, evolving over time to refer to the celestial event as we know it today, noted Dr. Pamela Gay, senior scientist at the Planetary Science Institute.

It is, however, possible for the moon to appear blue when our planet’s atmosphere contains smoke or dust particles that are a hair wider than 900 nanometers, although it’s a rare occurrence. In 1883, people reported observing the moon turning blue after the eruption of Indonesia’s Krakatoa volcano, according to Brittanica.

How to see the blue moon

May’s blue moon will appear fullest, or peak, at 4:45 a.m. ET on Sunday before the sun rises. This event will also feature a micromoon, which means it will be near its farthest point from Earth in its orbit, according to EarthSky.

Because the moon’s orbit around Earth isn’t a perfect circle, its apparent size varies by about 10% at different points in its orbit, Gay said. The nearest point is called the perigee and the farthest the apogee. A full moon around the time of apogee is a micromoon — and the upcoming event will be this year’s most distant full micromoon.

Sunday’s moon will be about 252,334 miles (406,093 kilometers) from Earth, compared with its average distance of about 238,855 miles (384,400 kilometers), according to NASA. However, the silvery orb won’t look noticeably smaller to the unaided human eye.

“If you’re used to comparing the moon to known things in your landscape — or holding your thumb up at arm’s distance to compare it to your thumb — you can see that 10% difference. But to the average person you’re not going to notice the difference at all,” Gay said.

The last calendar blue moon, which took place in 2023, was no micromoon. In fact, it was the opposite: a supermoon, when the full moon coincides with perigee — its closest approach to Earth. At perigee, the moon is about 226,000 miles (363,300 kilometers) away from Earth, according to NASA.

For the Americas, Africa and Europe, the moon will appear fullest the night of May 30. Australia, New Zealand and Asia will find the brightest full moon on the night of May 31.

“When that moon is rising, sometime between 5:30 and 6:30 in your time zone, and the sky is still this beautiful blue, there’s not a lot of weird settings to fuss with, your cellphone will take care of all the details for you,” Gay said. “This time of year is really kind of an awesome time to get those photos of the full moon.”

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