Week of May 24-30, 2026
When you hear the phrase "once in a blue moon," what do you think? If you're like most people, you think of something that doesn't happen often. So what exactly is a blue moon, and why is it so rare?
First off, a blue moon isn't all that rare. It occurs on average about once every 2.72 years.
Secondly, the moon doesn't appear blue at all; it's just a regular ol' full moon that happens to be the second one in any particular month. You know this phenomenon if you receive a paycheck every two weeks. You'll usually get two checks a month, but occasionally you'll receive a third. It's not that uncommon, and neither is a blue moon, despite the popular meaning of the term.
A second full moon in a month occurs occasionally because our calendar isn't synced with the lunar phases, even though the word "month" derives from the word "moon". What we call a month — approximately one-twelfth of a year — is longer than the moon's "synodic" period — the average time between one full moon and the next (29.53059 days).
So every once in a while — every blue moon, one might say — two full moons will occur during the same calendar month. We'll see the next one on Saturday night, May 30.
Be sure to watch as the moon rises in the southeast just after sunset that night. Chances are that it will first appear rather orange or red because its light must pass through a long column of air on its way to our eyes, and normal particulate matter suspended in the air scatters the moon's light and colors it orange.
Some have defined the blue moon differently. Most seasons of the year will experience three full moons, but occasionally a fourth will sneak in. In that case, the fourth of the season would be known as a "blue moon."
This seasonal definition may have originated from a 1943 question-and-answer column in Sky and Telescope magazine in which Laurence J. Lafleur traced the term to a 1937 "Maine Farmers' Almanac." Three years later, it seems, folklorist Philip Hiscock wrote in the same magazine an article that gave birth to the "second full moon in a month" idea.
Neither of these situations is particularly unusual, so where did the rarity idea originate? Occasionally, especially thick smoke or volcanic ash particles suspended in our atmosphere can remove enough red moonlight to make the moon appear bluish. It is said that, after the Krakatoa volcanic eruption in 1883, the moon appeared blue for more than a year. As you might imagine, this is extremely rare. In fact, one might say that it happens ... only once in a blue moon!
Now there's one more thing about the full moon that we'll see this coming weekend. It reaches its orbital "apogee" at 4:45 a.m. EDT / 1:45 a.m. PDT, its farthest point from the Earth (about 250,517 miles away). This means that it will appear as the smallest full moon of this year. You might hear it called a blue "mini moon," though most stargazers won't be able to see much difference from a normal full moon.
Visit Dennis Mammana at dennismammana.com. To read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at www.creators.com.
The next "blue moon," meaning the second full moon of a particular month, will appear on May 30.
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